NIST Technical Note 1731

Helium Dispersion in an Attached 

Single-Car Residential Garage with 

and Without Vehicle

William M. Pitts

Jiann C. Yang

Marco G. Fernandez

Kuldeep Prasad

http://dx.doi.org/10.6028/NIST.TN.1731
NIST Technical Note 1731 

Helium Dispersion in an Attached 

Single-Car Residential Garage with 

and Without Vehicle

William M. Pitts

Jiann C. Yang

Marco G. Fernandez

Kuldeep Prasad

Engineering Laboratory

http://dx.doi.org/10.6028/NIST.TN.1731

September 2012

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Technical Note 1731

Natl. Inst. Stand. Technol. Tech. Note 1731, 98 pages (January 2012) 

http://dx.doi.org/10.6028/NIST.TN.1731

CODEN: NTNOEF
i

Abstract

The dispersion and loss of helium inside a single-car residential garage attached to a single-family house 

was experimentally characterized by recording time-resolved helium concentrations at multiple locations 

in the garage and at a single location in the house during and following helium releases near the floor of 

the garage. Helium served as a surrogate for hydrogen for safety reasons, and helium release rates were 

adjusted to provide the same constant volume flow rate as that required to release 5 kg of hydrogen over a 

four hour period. Supporting measurements included compartment leakage, temperature, and 

atmospheric wind conditions. Helium was released upwards either as momentum- or buoyancydominated flows. Experiments were performed with the garage empty or with one of two conventional 

mid-sized automobiles parked over the release location. Six tests with the garage naturally ventilated and 

six tests employing forced ventilation with a fan are described. A variety of parameters were used to 

characterize the mixing behavior. Conclusions emphasized include: a) the role of Froude number on 

helium mixing behavior, b) the development of upper and lower helium concentration layers in the garage 

during a release, c) the measurable, but limited, effects of atmospheric wind on the results, d) the 

relatively efficient transfer of helium from the garage into the house during the releases, e) the ability of a 

vehicle to trap a high helium concentration in the engine compartment and, particularly, the undercarriage 

during a helium release and the relatively rapid drop in these levels to those of the surrounding garage at 

the end of the release, f) the relatively slow buildup of helium in the passenger compartment and trunk of 

a vehicle during a helium release and subsequent slow decay following cessation of the flow, g) the 

effectiveness of active ventilation in reducing helium concentrations in the garage to levels below those 

corresponding to flammable concentrations of hydrogen, and h) the trapping of helium/air mixtures 

corresponding to highly flammable hydrogen mixtures inside the vehicles even when active garage 

ventilation was employed.

Keywords: attached residential garages, automobiles, buoyancy-dominated flow, forced ventilation, 

Froude number, helium concentration, hydrogen concentration, momentum-dominated flow, natural 

ventilation, wind effects
ii

Table of Contents

Abstract............................................................................................................................................ i

Table of Contents............................................................................................................................ ii

List of Tables .................................................................................................................................iii

List of Figures................................................................................................................................ iv

1. Introduction............................................................................................................................. 1

1.1 Overview......................................................................................................................... 1

1.2 Summary of Related NIST Studies................................................................................. 1

1.3 Related Literature on Hydrogen Dispersion in Residential Garages and Vehicles........ 2

1.4 Overview of Gas Exchange Rates in Partially Enclosed Space...................................... 4

1.5 Introduction to the Current Study ................................................................................... 6

2 Experimental Set-Up............................................................................................................... 6

2.1 Overview and Accident Scenario.................................................................................... 6

2.2 Test House and Garage ................................................................................................... 8

2.3 Helium Supply and Release Systems............................................................................ 12

2.4 Instrumentation ............................................................................................................. 14

2.4.1 Helium sensors...................................................................................................... 14

2.4.2 Pressure measurement........................................................................................... 16

2.4.3 Helium measurements inside the test house ......................................................... 16

2.4.4 Additional Temperature Measurements................................................................ 16

2.5 Experiments with a Vehicle Parked in the Garage ....................................................... 18

2.6 Experiments with Forced Ventilation ........................................................................... 18

3 Experimental Results............................................................................................................ 19

3.1 Experiments with Natural Ventilation of the Garage ................................................... 19

3.1.1 Helium release from a vertical tube without vehicle ............................................ 19

3.1.2 Helium release over large area without vehicle.................................................... 35

3.1.3 Helium release over large area with vehicle ......................................................... 44

3.2 Experiments with Forced Ventilation of the Garage .................................................... 55

4 Discussion............................................................................................................................. 77

4.1 Naturally Ventilated Garage Experiments.................................................................... 77

4.2 Garage Experiments with Forced Ventilation............................................................... 84

4.3 Buoyant Gas Trapping within Conventional Automobiles........................................... 85

5 Summary............................................................................................................................... 89

6 References............................................................................................................................. 93
iii

List of Tables

Table 1. Helium Sensor Locations for 2008 Tests...................................................................................15

Table 2. Helium Sensor Locations for 2010 Tests without Vehicle ........................................................15

Table 3. Locations of Sensors #9 to #14 inside Vehicle (when present) for 2010 Tests.........................16

Table 4. Summary of Natural-Ventilation Garage Experimental Flow Parameters ................................20

Table 5. Helium Concentration Parameters for Garage at the End of Helium Release ...........................24

Table 6. Garage Parameters for Post-Release Period...............................................................................26

Table 7. Helium Parameters in the House at the End of Helium Release in the Garage .........................28

Table 8. Summary of Forced Ventilation Garage Experimental Flow Parameters..................................55

Table 9. Helium Concentration Parameters for Forced-Ventilation Garage Experiments.......................58
iv

List of Figures

Figure 1 Photograph showing the Indoor Environment and Ventilation Test House and attached

garage at NIST. .........................................................................................................................8

Figure 2 This schematic shows a floor plan for the Indoor Environment and Ventilation Test House, 

Building 423..............................................................................................................................8

Figure 3 This schematic shows a floor plan for the garage attached to the Indoor Environment and 

Ventilation Test House, Building 423.......................................................................................11

Figure 4 An interior view of the garage looking toward the front in which the floor and garage door

can be seen. ...............................................................................................................................12

Figure 5. An interior view of the garage looking toward the rear is shown. The exterior doorway on

the rear wall can be seen. The helium release flow conditioner used for the 2010

experiments is visible on the floor in the foreground................................................................13

Figure 6. Photograph showing the test vehicle parked inside the garage. ................................................17

Figure 7. Photograph showing the OSB board with attached flange in place in the rear doorway of

the garage. The flange was connected to the Infiltec Model DL1-DM4-110 duct leakage

tester (visible at the base of the doorway) by flexible duct.......................................................19

Figure 8 Measured helium volume flow rate is plotted as a function of time for the 9/11/08 garage 

experiment.................................................................................................................................20

Figure 9. Helium volume percent is plotted as a function time for the eight sensor heights indicated

for the 9/11/08 experiment. .......................................................................................................21

Figure 10. Helium volume percent is plotted as a function time for the eight sensor heights indicated

for the initial 1000 s of the 9/11/08 experiment......................................................................21

Figure 11. Helium volume percent is plotted as a function time for the eight sensor heights indicated

over the period from 6300 s to just after the end of the helium release for the 9/11/08 

experiment. The color codes for the sensor locations are the same as in Figure 10..............22

Figure 12. Time variations of wind velocity and the differential pressure between the garage interior

and outside ambient are plotted over the time period from 6300 s to 14400 s for the 

9/11/08 experiment. ................................................................................................................23

Figure 13. Experimental helium volume percents measured along the vertical array at the end of the 

9/11/08 release are shown along with extrapolated values at the floor and ceiling. The

solid lines are approximations of the data that are used to estimate the average helium 

concentration along the vertical direction...............................................................................24

Figure 14. Helium volume percent is plotted as a function time at the eight sensor heights indicated 

for the period from 43 200 s to 90 000 s in the 9/11/08 experiment.......................................25
v

Figure 15. Helium volume percent recorded by Sensor #4 is plotted as a function time for the

9/11/08 experiment. The red line is the result of fitting the data to an exponential decay

using a nonlinear least squares curve fit. ................................................................................26

Figure 16 Temperatures measured using thermocouples (symbols) and thermistors (lines) are 

plotted as a function of time for measurements recorded on 9/11/08 in the family room, 

garage, and outside. ................................................................................................................27

Figure 17. The helium volume percent measured in the family room of the house is shown as a 

function of time for the 9/11/08 experiment...........................................................................28

Figure 18. Helium volume percent is plotted as a function of time at the eight sensor heights

Indicated for the 9/12/08 experiment......................................................................................29

Figure 19. Time variations of wind velocity and the differential pressure between the garage interior

and outside ambience are plotted over the time period from 6300 s to 14400 s for the 

9/12/08 experiment. ................................................................................................................30

Figure 20. Wind speeds and differential pressure measurements are plotted over the twelve hours 

following the start of the experiments on 9/11/08 and 9/12/08. .............................................32

Figure 21. Wind speed and direction are plotted as a function of time for the 9/11/08 and 9/12/08 

experiments.............................................................................................................................33

Figure 22. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 9/12/08 in the family room, 

garage, and outside. ................................................................................................................34

Figure 23. The helium volume percent measured in the family room of the house is shown as a

function of time for the 9/12/08 experiment...........................................................................34

Figure 24. Helium volume percent is plotted as a function time at the nine sensor heights indicated

for the 7/29/10 experiment......................................................................................................35

Figure 25. Experimental helium volume percents measured along the vertical array at the end of

the 7/29/10 release are shown along with extrapolated values at the floor and ceiling.

The solid lines are approximations of the data that are used to estimate the average 

helium concentration along the vertical direction...................................................................36

Figure 26. Helium volume percent is plotted as a function of time at the indicated heights for four

sensors above the helium release location (center) and a single sensor at (x,y) = 

(4.343 m, 1.816 m) (front-right) for the 7/29/10 experiment..................................................37

Figure 27. Helium volume percent is plotted as a function of time for the five sensor heights

indicated for the initial 300 s of the 7/29/10 experiment........................................................38

Figure 28. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 7/29/10-7/30/10 in the family 

room, garage, and outside.......................................................................................................39
vi

Figure 29. The helium volume percent measured in the family room of the house manually (symbol)

and electronically (line) is shown as a function of time for the 7/29/10 experiment..............40

Figure 30. Helium volume percent is plotted as a function of time at the nine sensor heights

indicated for the 8/2/10 experiment........................................................................................40

Figure 31. Helium volume percent is plotted as a function time at the indicated heights for four

sensors above the helium release location (center) and a single sensor at (x,y) = 

(4.343 m, 1.816 m) (front-right) for the 8/2/10 experiment....................................................41

Figure 32. Helium volume percent is plotted as a function time for the five sensor heights indicated

for the initial 300 s of the 8/2/10 experiment..........................................................................42

Figure 33. The time variation of the differential pressure between the garage interior and outside is 

plotted over the time period from 0 s to 43 200 s for the 8/2/10 experiment. ........................42

Figure 34. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/2/10 in the family room,

garage, and outside. ................................................................................................................43

Figure 35. The helium volume percents measured in the family room of the house manually

(symbols) and electronically (line) are shown as a function of time for the 8/2/10

experiment. .............................................................................................................................43

Figure 36. Helium volume percent in the garage is plotted as a function time at the eight sensor

heights indicated along the vertical array for the 8/6/10 experiment......................................44

Figure 37. Experimental helium volume percents measured along the vertical array at the end of

the 7/29/10 release are shown along with extrapolated values at the floor and ceiling.

The solid lines are approximations of the data used to estimate the average helium 

concentration along the vertical direction...............................................................................45

Figure 38. The time variation of the differential pressure between the garage interior and outside is 

plotted over the time period from 0 s to 43 200 s for the 8/6/10 experiment. ........................47

Figure 39. Temperatures measured using thermocouples (symbols) and thermistors (lines) are 

plotted as a function of time for measurements recorded on 8/6/10 in the family room,

garage, and outside. ................................................................................................................48

Figure 40. The helium volume percents measured in the family room of the house manually

(symbol) and electronically (line) are shown as a function of time for the 8/6/10

experiment. .............................................................................................................................48

Figure 50. Experimental helium volume percents (solid symbols) measured along the vertical array

at the end of the 8/9/10 release are shown along with extrapolated values (open symbols)

at the floor and ceiling. The solid lines are approximations of the data used to estimate

the average helium concentration along the vertical direction. ..............................................57

Figure 51. Helium volume percent recorded by Sensor #4 is plotted as a function of time for the

8/9/10 experiment. The red line is the result of fitting the data to an exponential decay

using a nonlinear least squares curve fit. ................................................................................58
vii

Figure 52. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/9/10 experiment with

forced ventilation....................................................................................................................59

Figure 53. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/9/10 in the family room,

garage, and outside. ................................................................................................................60

Figure 54. The helium volume percent in the family room of the house measured electronically is

shown as a function of time for the 8/9/10 experiment. .........................................................60

Figure 55. Helium volume percent is plotted as a function of time for the eight sensor heights

indicated for the 8/12/10 experiment with forced ventilation.................................................61

Figure 56. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/12/10 experiment with

forced ventilation....................................................................................................................63

Figure 57. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/12/10 in the family room, 

garage, and outside. ................................................................................................................64

Figure 58. The helium volume percent measured in the family room of the house electronically is

shown as a function of time for the 8/9/10 experiment. .........................................................64

Figure 59. Helium volume percent is plotted as a function of time for the eight sensor heights

indicated for the 8/13/10 experiment with forced ventilation.................................................65

Figure 60. Experimental helium volume percents (solid symbols) measured along the vertical array

at the end of the 8/13/10 release are shown along with extrapolated values (open

symbols) at the floor and ceiling. The solid lines are approximations of the data used

to estimate the average helium concentration along the vertical direction.............................66

Figure 61. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/13/10 in the family room, 

garage, and outside. ................................................................................................................66

Figure 62. Helium volume percent is plotted as a function time at the indicated locations in the 

automobile centered over the helium release location for the 8/13/10 experiment with

forced ventilation....................................................................................................................67

Figure 63. Helium volume percent is plotted as a function time for the eight sensor heights indicated

for the 8/13/10a experiment with forced ventilation. .............................................................68

Figure 64. Experimental helium volume percents (solid symbols) measured along the vertical array

at the end of the 8/13/10a release are shown along with extrapolated values (open

symbols) at the floor and ceiling. The solid lines are approximations of the data used to 

estimate the average helium concentration along the vertical direction.................................69
viii

Figure 65. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/13/10a in the family room, 

garage, and outside. ................................................................................................................69

Figure 66. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/13/10a experiment with

forced ventilation....................................................................................................................70

Figure 67. Helium volume percent is plotted as a function of time for the eight sensor heights

indicated for the 8/19/10 experiment with forced ventilation.................................................71

Figure 68. Experimental helium volume percents (solid symbols) measured along the vertical array

at the end of the 8/19/10 release are shown along with extrapolated values (open

symbols) at the floor and ceiling. The solid lines are approximations of the data used to 

estimate the average helium concentration along the vertical direction.................................73

Figure 69. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/19/10 in the family room, 

garage, and outside. ................................................................................................................73

Figure 70. Helium volume percent is plotted as a function of time at the indicated locations in the

Passat centered over the helium release location for the 8/19/10 experiment with forced 

ventilation. ..............................................................................................................................74

Figure 71. Helium volume percent is plotted as a function time for the eight sensor heights indicated

for the 8/19/10a experiment with forced ventilation. .............................................................75

Figure 72. Experimental helium volume percents (solid symbols) measured along the vertical array

at the end of the 8/19/10a release are shown along with extrapolated values (open

symbols) at the floor and ceiling. The solid lines are approximations of the data used to 

estimate the average helium concentration along the vertical direction.................................75

Figure 73. Temperatures measured using thermocouples (symbols) and thermistors (lines) are

plotted as a function of time for measurements recorded on 8/19/10a in the family room, 

garage, and outside. ................................................................................................................76

Figure 74. Helium volume percent is plotted as a function of time at the indicated locations in the

Passat centered over the helium release location for the 8/19/10a experiment with forced 

ventilation. ..............................................................................................................................77

Figure 75. Helium volume percent is plotted as a function of time for sensors located 2.134 m above

the floor immediately above the helium release location and in the front-right quadrant

of the garage for the 7/29/10 experiment................................................................................78

Figure 76. The falloff of helium volume percent in the family room of the house following the end

of the helium release inside the garage on 7/29/10 is fit to an exponential curve using a

least squares curve fitting procedure. .....................................................................................83

Figure 77. The falloff of helium volume percent in the passenger compartment of the vehicle long

after the end of the helium release under the vehicle on 8/6/10 is fit to an exponential

curve using a least squares curve fitting procedure. ...............................................................88
1

1. Introduction

1.1 Overview

Concerns about the potential effects of the buildup of high carbon dioxide concentrations in the 

earth’s atmosphere due to burning of hydrocarbon fuels have led to efforts to develop technologies that 

utilize hydrogen as an energy carrier. One active area has been the commercial development of 

hydrogen-fueled fuel cell vehicles, with initial deployment currently predicted in less than five years. 

Due to substantial differences in physical properties and burning behavior between hydrogen and 

hydrocarbon fuels, a great deal of effort has been placed on the fire safety implications of the wide-spread 

use of hydrogen. One major focus has been the potential for hydrogen to be accidentally released into 

partially enclosed spaces such as fuel cell enclosures, buildings, and commercial and residential garages.

Both cryogenic liquid and compressed hydrogen stored in appropriate tanks have been utilized in 

prototype hydrogen-fueled vehicles. In the short term, it appears that high-pressure gaseous storage will 

be dominant. Thus the potential for accidental releases of hydrogen gas should be considered. Under the 

assumption that a large percentage of a fleet of hydrogen-fueled vehicles will be housed in the existing 

stock of residential garages, the National Institute of Standards and Technology (NIST) has undertaken an 

effort to characterize the hydrogen distribution and potential combustion behavior during and following 

the accidental release of hydrogen into a typical residential garage.

1.2 Summary of Related NIST Studies

An initial experimental effort utilizing helium as a surrogate for hydrogen investigated the effects of 

helium release rate, release location, and vent size and location on the distribution of and leakag of helium 

from an idealized 1⁄4-scale two-car garage. A NIST Technical Note and a manuscript presented during the 

Third International Conference on Hydrogen Safety describe this work. [1,2] Detailed time records of the 

measurements have been posted on the World Wide Web to provide well-characterized data sets for 

validating and improving models of the mixing process. [3] The total helium volume released was scaled 

to be equivalent to the release of 5 kg of hydrogen into the full-scale garage. Vent sizes were scaled to 

provide the equivalent of three garage air changes per hour, (ACHgar)4Pa, for a pressure difference of 4 Pa 

between the interior and surroundings of the garage. One and four hour releases at the lower center, 

lower rear, and upper center of the garage were studied. These experiments were used to characterize the 

ability of a computational code to predict such flows. [4]

A follow-on study investigated the effect of replacing the poly(methyl methacrylate) front wall of the 

1⁄4-scale garage with gypsum drywall. It was shown that helium readily diffuses through the dry wall, and 

the helium concentration drops rapidly inside following the end of the helium release. [5] The diffusion of 

helium through the dry wall is so much faster than the reverse diffusion of air, that the pressure inside the 

garage dropped rapidly by hundreds of Pascals when the garage was sealed. Experiments in which 

forming gas containing low levels of hydrogen (≈ 4 %) in nitrogen were released into the garage showed 

that the diffusion rate for hydrogen through drywall was similar to that for helium.

NIST sponsored a study carried out by Southwest Research Institute (SWRI) in which hydrogen was 

released into a structure having the dimensions of a two-car garage. Doorway fan tests were used to 

characterize the global leak characteristics of the structure before each test. Hydrogen was released at

low speed near the center of the floor. The constant hydrogen volume flow rate was sufficient to release 5 

kg in one hour. Experiments were performed with an empty garage and with a variety of vehicles 

centered inside. A range of instrumentation, including thermal conductivity sensors, thermocouples, 

piezoelectric pressure transducers, and different types of video cameras, was used to characterize the 

hydrogen volume fraction distribution and time behavior as well as flame spread rates and overpressures 

when the hydrogen was ignited. Ignition was initiated when the hydrogen level reached one of a number 

of pre-determined values. Post-analysis of the raw experimental data was performed by both SWRI and 
2

NIST researchers. Details concerning the experiment are available in a NIST Government Contractor 

Report [6], and a manuscript describing the experimental findings is available [7].

Two related studies were carried out in conjunction with work described in this report. The first was 

a modeling study of the effectiveness of forced ventilation in preventing the buildup of dangerous 

concentrations of hydrogen within a full-scale garage attached to a house, both with and without a vehicle 

present. [8] The second investigated the response of hydrogen detectors placed in the garage to hydrogen 

released as forming gas. [9]

A number of computational fluid mechanic and analytical studies by NIST have considered hydrogen 

release in a garage subject to a range of conditions. [10,11,12]

1.3 Related Literature on Hydrogen Dispersion in Residential Garages and 

Vehicles

The prospect of the deployment of a fleet of hydrogen fuel cell powered automobiles has raised 

concerns about the potential hazards associated with releases of hydrogen in partially enclosed spaces 

such as commercial and residential garages. As a result, a large number of experimental and modeling 

studies describing the distribution of hydrogen (or helium serving as a surrogate) have been reported. We 

have recently summarized such studies in a report describing the mixing of helium released within an 

idealized 1⁄4-scale two-car garage, which can be consulted for a more complete review. [1] Here we focus 

on studies directly related to the current study on mixing in residential garages and within vehicles.

Cariteau et al. considered the distribution of helium generated by various types of helium releases at 

different locations under a vehicle and in the empty garage. [13] The enclosure was scaled as a single-car 

garage, 5.76 m long × 2.96 m wide × 2.42 m high, and was very well sealed with a measured air change 

per hour rate, ACHgar of 0.01 h-1

. During each experiment, a total helium volume of 1.09 m3 was released 

at a constant rate for either 105 s or 316 s. Upward helium releases from a 7 cm diameter source in the 

empty garage showed that the higher flow rate resulted in higher concentrations near the ceiling at the end 

of a release. Vertical concentration profiles showed that the helium distribution was partially stratified,

with the higher flow rate resulting in a more stratified upper layer. Releases at two different horizontal 

locations in the garage had little effect on the helium distribution.

Helium was also released at several locations under a small commercial van either as a diffuse 

upward source located under the engine compartment hood or beneath the engine or as a downward jet 

located beneath the engine or passenger compartment. [13] The varying release conditions resulted in 

different helium concentrations near the ceiling as well as changing the vertical concentration distribution. 

The helium distributions for the releases under the hood were very similar to those observed when the 

vehicle was not present. When the diffuse source was moved below the engine, the helium concentration 

near the ceiling fell slightly, and the vertical distribution was not as stratified. The downward releases 

gave very different results. The helium concentrations near the ceiling were significantly decreased and 

the vertical helium concentration profile was nearly uniform. Cariteau et al. attributed these observations 

to changes in the mixing with the effective areas of the release. The downward jets effectively created 

large-area releases that increased mixing and limited stratification, while smaller area releases encouraged 

stratification. [13]

Maeda et al. performed a study of hydrogen buildup inside the front engine compartment of a 

conventional rear-drive passenger sedan. [14] Hydrogen was released at one of two locations near the 

ground—centered between the front wheels and centered under the passenger compartment. Various 

combinations of hydrogen volume flow rates (minimum of 9 × 10-5 m3

/s to a maximum of 

2.31 × 10-3 m3

/s, assuming a release temperature of 20 oC) and release periods (varying from 30 s to

600 s) were used. Hydrogen concentrations were measured in the engine compartment near the center 

just underneath the hood, at the front at the top of the radiator, and at the front near the bottom of the 

radiator. Quasi-steady concentrations of hydrogen developed quickly during the releases, with levels 

depending on release point, measurement location, and flow conditions. For a given flow condition, 

concentrations were higher for release under the passenger compartment as compared to under the engine.
3

Measured quasi-steady-state concentrations were generally higher at the measurement location under 

the hood, with slightly lower concentrations measured at the top of the radiator. Hydrogen concentrations 

at the bottom of the radiator were typically close to zero. The highest measured hydrogen volume 

percents of approximately 24 % were observed for the higher hydrogen volume flow rates and longer 

release periods. Once the hydrogen flow was halted, concentrations within the engine compartment 

dissipated quickly, requiring less than 200 s to fall below the lower flammability limit concentration for 

hydrogen.

Flammable mixtures of hydrogen and air (> 4 % hydrogen) were observed during these experiments. 

Spark ignition of these mixtures was used, and the resulting flames were characterized by temperature, 

thermal radiation, and sound intensity measurements. These authors concluded that the conditions 

generated outside of the engine compartment were not severe enough to cause major injuries or fatalities 

to people near the vehicle. [14]

Maeda et al. extended the findings of their original study [14] of mixing in the engine compartment of 

a vehicle to higher hydrogen volume flow rates, while adding an additional release location and several

hydrogen sensor locations. [15] Hydrogen volume flow rates for 600 s releases were varied over a range 

from 3.52 × 10-3 m3

/s to 0.0176 m3/s. Hydrogen sensors were added at the ventilator louver between the 

hood and windshield and at two vertical locations along a line between the bottom side of the hood and 

the top of the engine. The additional release location was centered under the rear axle of the rear-drive 

vehicle. Hydrogen was released either upwards or downwards from a 4 mm diameter tube.

For upward releases, hydrogen concentrations with volume flow rates above 3.53 × 10-3 m3

/s were 

nearly uniform throughout the upper volume of the engine compartment. In the lower volume of the 

engine compartment (bottom of the front bumper), no hydrogen was measured. These observations 

suggest that sharp concentration gradients were present at intermediate heights in the engine 

compartment. For lower volume flow rates, some stratification in the upper part of the engine 

compartment was evident. The quasi-steady-state concentrations increased relatively slowly with 

increasing volume flow rate. [15]

Downward releases under the vehicle resulted in changes to the distribution of hydrogen within 

interior volumes. For a given hydrogen volume flow rate, the quasi-steady-state concentration was lower 

than for the corresponding upward release, and there was a stronger dependence on the volume flow rate. 

Stratification of the hydrogen concentration in the upper volume of the engine compartment was observed

over the entire volume flow rate range. Unlike for upward releases, measurable hydrogen concentrations 

were present at the lower measurement location at the front of the engine compartment.

By covering the drive shaft tunnel under the vehicle with a flat metal sheet, the authors demonstrated 

that the geometry present under the vehicle had a large effect on the hydrogen concentrations trapped in 

the engine compartment. A similar dependence on release location was identified, with the highest 

concentration observed when the release point was centered under the passenger compartment.

Since hydrogen concentrations were higher in the second round of experiments, more vigorous 

combustion was observed when the hydrogen/air mixtures inside the engine compartment were ignited at 

the end of the release period. For the highest levels some deformation of the hood of the vehicle was 

reported. Even so, the authors reached a similar conclusion to their earlier work that such an ignition 

would be unlikely to cause major injury to a person located nearby, but outside the vehicle. [15]

Merilo et al. investigated the mixing and combustion behavior of hydrogen when released into a 

hardened building equipped with a plastic-sheet front wall and scaled to represent a single-car garage 

(3.64 m (w) × 6.10 m (l) × 2.72 m (h)). [16] Hydrogen was released from a 7.75 mm (ID) diameter tube 

at volume flow rates (assuming a 20 oC release temperature) between 2.9 × 10-3 m3

/s and 0.030 m3/s. The 

garage was equipped with fan ventilation, and the majority of the experiments were cases where active 

ventilation and hydrogen releases rates were varied for hydrogen flows into the empty garage in order to 

assess the effectiveness of the ventilation for limiting the buildup of hydrogen and the potential for 

burning.

Three experiments were run using natural ventilation with vents designed to meet the 

recommendations of the 2002 ICC Final Action Agenda, which included recommendations for residential 
4

garages. [17] These recommendations referred to cases where hydrogen generation or refueling 

operations were present and specifically excluded parked hydrogen-fueled vehicles in garages. The

requirements were based on experiments and modeling in a real-scale single car garage containing a 

vehicle mock up carried out at the University of Miami. [17] Hydrogen scenarios considered were 20 min 

releases at 5.1 × 10-4 m3

/s, 2.0 × 10-3 m3/s, and 6.1 × 10-4 m3/s. The vents were required to be located near 

the floor and ceiling and to have minimum areas of 0.0465 m2 per 28.3 m3 of garage volume.

Two of the experiments with natural ventilation were run for hydrogen volume flow rates near

0.030 m3

/s (assuming a temperature of 20 oC). The first was done with an empty garage and the second 

with a vehicle present. For the latter, the hydrogen release location was outside of the vehicle near the 

expected location of a refueling port. The third experiment was run at a lower volume flow rate of 

2.9 × 10-3 m3

/s with an empty garage. Hydrogen concentrations were monitored every 5 min to 10 min at 

three heights within the upper 1/3 of the garage. The measurements showed that the upper layer of the 

garage was well mixed for experiments where higher hydrogen volume flow rates were used. This was 

attributed to the mixing induced by overturning when the momentum-dominated flow reached the ceiling. 

For the lower flow rate case, some stratification of the upper layer was evident, which was judged to be 

consistent with a buoyancy-dominated flow. Maximum hydrogen volume percents at the ends of the 

releases near the ceiling were around 22 % for the higher volume flow rates and between 2.9 % and 7.1 % 

for the lower volume flow rate.

Ignition of the higher concentration levels resulted in observed maximum flame spread rates as high 

as 60 m/s. Ignition of the mixture at the lower release rate was not described. Overpressure 

measurements were reported for the higher concentration cases with and without a vehicle. Measurable 

overpressures were present for both experiments, but the magnitudes and fluctuations were much larger 

when the vehicle was present. The higher pressure was attributed to an internal explosion within the 

vehicle. The observed damage of the vehicle was consistent with this conclusion and showed that 

burning occurred in the passenger compartment as well as the engine compartment. [16]

Ekoto et al. investigated the interior hydrogen concentration distributions generated by releasing 

hydrogen or helium directly into the passenger compartment or trunk of a conventional automobile. [18] 

Releases were done in such a way that they were either momentum dominated or buoyancy dominated. 

Three volume flow rates of released gas, 8.3 × 10-4 m3

/s, 4.2 × 10-4 m3/s, and 1.97 × 10-3 m3/s, were used. 

Major findings were that 1) flammable concentrations of hydrogen developed in the release compartment 

over short periods of time (order of seconds), 2) very high levels of hydrogen were observed at longer 

times for the higher flow rates, 3) quasi-steady states developed over periods of tens of minutes which 

depended on release rates and locations, 4) momentum-dominated releases resulted in more homogeneous 

mixing, 5) releases in the passenger compartment led to high concentrations in the trunk, but the reverse 

was not the case, and 6) for the same volume flow rates helium concentrations were measurably lower 

than recorded with hydrogen.

Liu and Schreiber employed a computational fluid dynamics (CFD) model to study the similar 

problem of the release of hydrogen into a passenger compartment of a conventional automobile with 

active ventilation. [19] The hydrogen volume flow rate was assumed to be 3.1 × 10-3 m3

/s, and the 

compartment was actively ventilated with air at an unspecified rate. With the base configuration, the 

average hydrogen helium volume percent when a pseudo-steady state developed was 4.6 %, with over 

60 % of the compartment above the 4.1 % hydrogen flammability limit. The authors showed that moving 

the exhaust vents for the passenger compartment to the rear ceiling significantly reduced the passenger 

compartment hydrogen concentrations, with only 3.1 % of the volume having values greater than the 

lower flammability limit.

1.4 Overview of Gas Exchange Rates in Partially Enclosed Space

Leak rates for a partially enclosed space with a volume, Venc, are typically characterized in terms of 

the number of air changes per hour (ACHenc), which corresponds to a volume flow exchange rate across 

the enclosure boundary, Qenc, given by Qenc = Venc×ACHenc/3600 s/h. ACHenc can vary substantially with 
5

time and depends not only on the areas and characteristics of openings connecting across the enclosure 

boundary, but also on such factors as weather conditions and forced ventilation. Values of Qenc can be 

related to an effective leak area, ELAenc by use of the Bernoulli equation,

2

1

2

( ) ( )







 







ρ

∆

=

P

Q ELA enc H

enc H (1)

where the subscript H indicates evaluation at a specific pressure difference, ΔP, between the interior and 

exterior and gas density, ρ. A value of H = 4 Pa is often taken to be representative of pressure differences 

across a garage boundary that develop due to wind and temperature differences between the interior and 

ambience. [20] A table (their Table 1) included in a report by Chan et al. suggests that 4 Pa is on the high 

side, and actual values typically range from 0.2 Pa to 2 Pa for “mild” to “severe” weather conditions. [21] 

Note that (ELAenc)H does not normally correspond to the actual open area in a garage boundary since 

experimental flow rates typically obey the following relation [20] 

,

n Qenc = C×∆P (2)

where C is the flow parameter and n is an experimental parameter that varies between 0.5 and 1. [20]

Studies indicate that values of (ACHgar)4Pa and (ELAgar)4Pa vary widely for residential garages in the 

United States, e.g., see [20,22]. It is reasonable to consider values recommended as the minimum 

acceptable required levels. A recommended minimum value of Qgar = 2.73 m3

/min (100 ft3/min) per 

stored automobile was included in a version of an American Society of Heating, Refrigeration and Air 

Conditioning Engineers (ASHRAE) standard. [23] Swain et al. referred to this value in an earlier study of 

hydrogen release in garages. [24] Note that this recommendation is no longer included in current versions 

of the ASHRAE standard, but is incorporated in the 2009 International Mechanical Code published by the 

International Code Council (ICC). [25] A simple calculation reveals that this value corresponds exactly 

to an ACHgar = 3 h-1 for a single car garage sized 3.048 m (w) × 6.096 m (l) × 3.048 m (h). On this basis, 

an (ACHgar)4 Pa = 3 h-1 was adopted as the recommended lower value for our reduced-scale study. [1,2]

Values of (ACHenc)4 Pa are usually determined with a device known as a blower door that measures the 

ΔP across a boundary for a given volume flow rate of air, Qenc, into or from the space. Note that 

measurements are frequently taken at several higher pressures and extrapolated to 4 Pa using Eq. (2). 

Results described in [26] for 67 attached garages in four Canadian cities are reported in terms of 

(ACHgar)50 Pa. Averages can be roughly converted to H = 4 Pa values using Eq. (2) and an assumed value 

of n = 0.65. The results are (ACHgar)4 Pa = 7.2 h-1

, 3.5 h-1, 3.3 h-1, and 9.1 h-1 for Vancouver, Winnipeg, 

Saskatoon, and Ottawa, respectively. Emmerich et al. reported findings that can be converted to 

(ACHgar)4 Pa for five residential garages in the Washington, DC area. Values ranged from (ACHgar)4 Pa = 

1.4 h-1 to 25.3 h-1

. [22]

An approach known as the tracer gas method allows measurement of ACHenc values by either 

releasing short bursts or a continuous stream of a tracer gas into the enclosure and tracking its 

concentration as a function of time. Note that such values are expected and are observed to vary with 

time as conditions change. Several studies have reported such values for residential garages. Emmerich 

et al. summarized some early studies. [22] Studies of garages in the United States and Canada yielded 

ACHgar values covering a range from 0.3 h-1 to 2.7 h-1

. The most common values were between 0.7 h-1

and 1.0 h-1

. More recently, Batterman et al. reported ACHgar measurements of fifteen residential garages 

in Michigan. [27] ACHgar values ranged from 0.16 h-1 to 1.80 h-1

, with an average of 0.77 h-1 and standard 

deviation of 0.51 h-1

. Waterland et al. measured time-varying values of ACHgar for three garages in Texas 

and California. [28] Average values of ACHgar were approximately 0.5 h-1

, 0.5 h-1, and 0.2 h-1. Together, 

these findings suggest that typical residential garages in North America have ACHgar values for ordinary 

weather conditions between 0.5 h-1 and 1 h-1

, with a most probable value near 0.7 h-1. Note that most 
6

measured ACHgar values are more than a factor of three lower than the minimum level recommended by 

ASHRAE and the ICC. [23,25] Some of the studies above were reviewed by Adams et al. who provided a 

probability plot for ACHgar values that is consistent with, but skewed to somewhat lower values, than 

suggested above. [29] As a worst case value, they recommended a minimum value of ACHgar = 0.03 h-1

, 

i.e., 100 time less than the recommended minimum value, which appears to be very conservative.

1.5 Introduction to the Current Study

As discussed above and in a NIST technical note [1], there has been a great deal of recent activity on 

understanding the mixing behavior of hydrogen (or helium acting as a surrogate) when released into 

partially enclosed spaces. Much of this interest is based on concern about potential accidental releases 

from hydrogen fueled automobiles parked in residential garages. Concentration distributions have been 

measured in enclosures representing residential garages as well as within conventional automobiles.

Most of the experiments have been performed in specially constructed structures designed to 

represent residential garages. Potential leaks to the outside have generally been minimized or controlled 

by creating idealized vents. Most experiments have been designed to minimize temperature and wind 

variations, even though such effects are known to dominate air exchanges between structures and their 

surroundings. To our knowledge, no experiments have been performed with a garage attached to a 

residence, even though a large fraction of residential garages in the United States are attached.

In the study reported here, twelve experiments were performed in which helium, acting as a surrogate 

for hydrogen, was released into a garage attached to a house. Six of these tests involved natural 

ventilation of the garage, and six used a fan to create forced ventilation. For the natural ventilation cases,

helium was released as either momentum- or buoyancy-dominated flow. Experiments were performed 

with and without a conventional vehicle parked inside the garage. A vehicle was present during all six of 

the forced-ventilation experiments. The distribution of helium within the garage was monitored by timeresolved measurements at multiple locations and for a single location inside the attached house. When a 

vehicle was parked inside, helium concentrations were measured below the undercarriage, in a wheel 

well, at two locations inside the engine compartment, in the passenger compartment, and in the trunk. 

Supplemental measurements included wind speed and direction, differential pressures, and temperatures 

at various locations.

2 Experimental Set-Up

2.1 Overview and Accident Scenario

The experiments were designed to study the mixing behavior and concentration buildup when 

hydrogen is accidentally released from a hydrogen-fueled automobile parked in a residential garage 

attached to a single family home. Due to safety concerns, helium was used as a surrogate gas for 

hydrogen. As discussed above and in reference [1], numerous studies have demonstrated that the 

behaviors of hydrogen and helium are similar in experiments designed to study mixing and dispersion in 

partially enclosed spaces.

The highly idealized accident scenario considered was the release of 5 kg of pressurized hydrogen gas 

(a typical amount for full tanks on existing models of hydrogen-fueled automobiles) at a constant rate 

over a period of 4 h. Assuming an ideal gas at a temperature of 20 oC, this corresponds to a volume of 

59.7 m3

. The flow rate required to release this volume over 4 h is 4.15 L/s. This volume flow rate was 

chosen as the target value for the helium experiments.

Helium was released near the floor at the center of the garage. Cases were studied in which the 

garage was empty as well as when an automobile was parked directly over the release location. The 

experiments were designed to characterize the mixing and distribution of helium within the garage, the 

loss of the helium from the garage, and the buildup and decay of helium inside the house both during the 
7

release and in the post-release period. When the automobile was present, helium concentrations were 
8

Figure 1. Photograph showing the Indoor Environment and Ventilation Test House and attached garage at 

NIST.

Figure 2. This schematic shows a floor plan for the Indoor Environment and Ventilation Test House, 

Building 423.

monitored at multiple interior locations in order to characterize the potential buildup of hydrogen 

concentrations in such spaces.

Two sets of experiments were performed during September of 2008 and late July and early August of 

2010.

2.2 Test House and Garage

The single-car garage employed for this study is attached to Building 423 at NIST, which is known as 

the “Indoor Environment and Ventilation Test House.” An exterior photograph of the house and garage 

from the front is shown in Figure 1. Detailed descriptions of the house are available in two NIST Reports 

[30,31] which describe the characteristics of the house as originally built and following an airtightening 

refit. The single-story manufactured home has an outside width of 8.2 m and length of 17.1 m. It is 

oriented with its front facing towards the north. The house has three-bedrooms and two-baths with a large 

open area consisting of a living room, dining room, kitchen, morning room and family room. Figure 2

shows the floor plan. The house is equipped with a pitched roof above a low attic space separating the 
9

roof and an interior cathedral ceiling that runs along the full length of the house. The ceiling is 2.7 m 

above the floor at the center and slopes down to a height of 2.1 m at the front and rear walls. The internal 

floor area and volume are 140 m2 and 340 m3

, respectively.

The house is heavily instrumented with sensors at multiple locations to measure such properties as 

interior and exterior temperatures, differential pressure between the interior and exterior, atmospheric

pressure, relative humidity, fan operation, duct flow rates, and energy usage. Tracer gas measurements 

using sulfur hexafluoride were used to characterize air changes per hour for the house, ACHhou. [30,31] 

Ceramic-coated thermistors were used to measure temperatures at multiple locations within the house as 

well as the outdoor temperature on a weather tower. A sonic anemometer placed at the top of a 10 m high 

tower was used to record wind speed and direction at a location 5 m to the south of the house. The 

measurements are recorded continuously using a dedicated data acquisition system at a rate of 1 min-1

. 

Additional details concerning the sensor locations and uncertainties are available in the reports. [30,31]

Measurements showed that ACHhou values decreased marginally following the airtightness refit. As 

expected, ACHhou values were found to increase with increasing wind speed and temperature difference 

between the interior and outside ambience. Observed values varied over a range from roughly 0.1 h-1 to 

0.4 h-1 following the refit. [31] The range was somewhat wider, with values up 0.8 h-1

, for the as-built 

house. [30,31]

The weather measurement system described above was functional for the 2008 experiments. During 

the second series of experiments the anemometer malfunctioned, and local wind speed data is unavailable.

The attached garage was constructed on the west end of the structure after the house had already been 

updated for airtightness. A schematic floor plan for the garage is shown in Figure 3. It has a concrete 

foundation and poured concrete floor. The garage walls were constructed using standard “2×4” wood 

stud construction on 0.41 m centers. Outside walls were covered with plywood sheathing and exterior 

siding. The original siding between the house and the garage was removed and replaced with dry wall. 

The roof has a central height of 3.66 m and the same slope as the roof of the house.

The interior dimensions of the garage are 5.33 m (w) × 6.71 m (l) × 2.43 m (h) with a 36.5 m2 floor 

area and 86.9 m3 internal volume. The walls and ceiling are covered with finished, primed, and painted 

gypsum drywall with painted wooden molding around the lower edge. The drawings for the garage 

specify the use of 1.27 cm thick drywall and a one hour fire rating for the wall assembly. This implies the 

use of Type X fire-rated drywall, and inspection indicated that Type X had been used.

The north wall has a four-section 2.74 m (w) × 2.13 m (h) roll-up garage door equipped with outside 

sealing gaskets along the four edges. The side and top gaskets are sealed against 0.64 cm thick molding 

attached to the door frame, while the bottom gasket is press sealed against the concrete base. The 

effective open area of the doorway is 2.64 m (w) × 2.08 m (h). It appeared to be well sealed, and there 

were no obvious leaks visible when closed. The garage door is shown from the interior in Figure 4.

There are two additional doors into the garage. One is an exterior entrance with dimensions of 

0.914 m (w) × 2.032 m (h) located on the left side of the rear wall. This door is sealed against the exterior 

door frame molding on the sides and top and at the bottom by a gasket. This door can be seen in Figure 5. 

Inspection revealed ≈ 0.3 cm gaps around the door through which light was visible coming from the 

outside with the door closed. The second door connects to the house and is located where an original side 

entrance entered the manufactured home (see Figure 2). It has the same dimensions as the rear door. 

This door was well sealed at the bottom by a gasket pressed against floor molding. Elsewhere, there were 

gaps of ≈ 0.3 cm between the door and frame, including between the strike legs on the sides and the 

header at the top. Light was visible passing between the garage and interior at these locations. The 

doorway passed into a small utility room (see Figure 2), which was, in turn, connected to the family room 

by a second door. There was a 10 cm gap between the bottom of this second door and the floor.

The garage was constructed with two 0.902 m (w) × 1.219 m (h) windows located on the right rear 

wall and on the back side of the exterior side wall. The side-wall window had been removed, and the 

opening was sealed with a plywood sheet holding a centered 0.65 m × 0.65 m louvered aluminum box 

containing an exhaust fan. For these experiments, the fan was not operated, and the louvers were sealed.
10
11

Figure 3. This schematic shows a floor plan for the garage attached to the Indoor Environment and 

Ventilation Test House, Building 423.

Measurements of ACH for the garage were made using both tracer gas methods and doorway fan 

pressurization tests during related experiments carried out in conjunction with the measurements

discussed in this report. Prasad et al. have described the results of a series of doorway fan tests carried 

out in the garage. [8] For measurements with the fan located in the rear door frame, values of (ACHgar)4 Pa

= 2.5 h-1 and 3.6 h-1 were measured for pressurization and depressurization tests, respectively. The 

corresponding pressure exponents, n (see Eq. (2)), were 0.50 and 0.61. The n values suggest air exchange 

was occurring through openings of sufficient size that turbulent flow could develop. [20] By using low 

levels of released hydrogen as a tracer gas, Cleary and Johnsson made several measurements of ACHgar. 

[9] Their values ranged from 0.31 h-1 to 0.43 h-1

. The difference between (ACHgar)4 Pa and ACHgar

reinforces the conclusion that effective differential pressures between the interior and surroundings for the 

garage were normally considerably smaller than 4 Pa. These values of ACH suggest that the NIST test 

garage is better sealed by roughly a factor of two than a “typical” garage reported in the literature, but the 

measurements are well within the range of values observed in earlier studies.
12

Figure 4. An interior view of the garage looking toward the front in which the floor and garage door can 

be seen.

2.3 Helium Supply and Release Systems

The helium control flow system was assembled in the family room (see Figure 2) of the house. 

Helium was supplied from a Scott Specialty Gases1 Model 8403 Complete System Changeover Regulator 

System which combines a gas regulator with a two-way valve that allowed seamless switching between 

two different feed lines. Each of the two inlets, equipped with valves and bleed ports, was connected by 

tubing to short pressure manifolds equipped with two valves and gas lines running to two gas cylinders. 

A gas storage rack was used to safely hold the four gas storage bottles feeding the regulator system.

A typical experiment required twelve gas cylinders to provide the 10 kg of helium released over a 4 h

period. The procedure employed was to supply helium from one of the manifolds fed by two highpressure Type 1A cylinders until the pressure to the regulator dropped to roughly 350 kPa. At this point 

the flow was switched to the second set of gas cylinders, and the two empty cylinders were replaced by 

charged cylinders. This process was repeated until the end of the experiment.

The output of the regulator was connected through tubing to a valve leading to an American Meter 

Company Model AT-1400 diaphragm gas meter. The valve was used to control the flow rate. A pressure

gauge was included to determine the gauge pressure at the meter. The gas flow rate was determined by 

timing the period required for 0.142 m3 of helium to pass through the meter with a digital stopwatch. The 

gas pressure was measured at the same time. Volume flow rate measurements were recorded periodically 

(typically every 10 min) while the helium was flowing. Since the gas was released at atmospheric 

pressure, the measured volume flow rate was corrected to atmospheric pressure using the measured gauge 

pressure at the meter. The helium flowed into the garage through polyethylene tubing that passed through 

a duct placed in the wall between the family room and the garage.

 1 Certain commercial equipment, instruments, or materials are identified in this paper to foster understanding. Such identification 

does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the 

materials or equipment identified are necessarily the best available for the purpose.
13

Figure 5. An interior view of the garage looking toward the rear is shown. The exterior doorway on the 

rear wall can be seen. The helium release flow conditioner used for the 2010 experiments is 

visible on the floor in the foreground.

The diaphragm test meter is a volume displacement measurement. The uncertainty in displaced 

volume was estimated to be 0.2 %. Uncertainties for the recorded times were estimated as 0.3 s and 700 

Pa for the gauge pressure. A propagation of error calculation for the volume flow rate uncertainty gave 

an expanded uncertainty, i.e., coverage factor of 2, of ± 1.2 × 10-4 m3

/s.

The helium was released into the garage in two different ways. In the 2008 experiments, the helium 

was released through polyethylene tubing (I. D. = 6.35 mm, O. D. = 9.53 mm). The tube was attached 

along a vertical mount at the center of the garage (x,y) = (2.69 m, 3.40 m). The helium flow exited the 

tube vertically upwards at a height of 41.5 cm ± 0.1 cm above the floor. For a nominal volume flow rate 

of 4.14 L/s, the corresponding exit flow velocity was 131 m/s. Such flows are typically characterized by 

the Reynolds number, 

,

He

HeUoDo Re

μ

ρ = (3)

where ρHe is the density of the gas at the exit, Uo is the average velocity, Do is the diameter of the flow

exit, and μHe is the dynamic viscosity for helium, and the Froude number, 

 , ( ( ))1⁄2

1⁄2

o air He

He o

gD

U Fr

ρ − ρ

ρ = (4)

where g is the gravitational constant and ρair is the air density. The values for these flow conditions 

assuming a temperature of 20 oC were Re = 7070 and Fr = 210. The Re and Fr values indicate the initial

flow was turbulent and dominated by inertial forces.
14

An important characteristic of flows of buoyant gases in enclosures is the downstream distance, z,

where the transition from momentum- to buoyancy-dominated flow takes place. According to Chen and 

Rodi, the nondimensional parameter K, given by 

o

air

He

D

z

Fr

K 1/ 4

1

















ρ

ρ

= , (5)

is < 0.5 for momentum-dominated flows and > 5 for buoyancy-dominated flows. [32] For the tube flow 

used in 2008, the flow began to transition from momentum- to buoyancy-dominated at z ≈ 1.9 m and 

would have become buoyancy dominated at z ≈ 10.9 m. This indicates that by the time the flow reached 

the ceiling (height of 2.43 m) it was primarily momentum dominated.

For the series of experiments done in 2010, a system was constructed to release helium uniformly 

over a much larger area. A box with an open top having dimensions of 30.5 cm × 30.5 cm × 6.4 cm was 

constructed from sealed thin sheet metal. The helium entered through one side of the box near the base

and passed into a 3.8 cm high plenum. The top of the plenum was formed by a perforated metal sheet 

(diffuser) topped with a 10 cm layer of 12 mm crushed stone. The diffuser and stones were designed to 

smooth out the velocity profile before the flow exited upwards into the garage though the 30.5 cm × 

30.5 cm square opening located 6.4 cm above the floor. This helium release system is visible in the 

foreground of Figure 5. For these experiments the nominal helium flow velocity into the garage was 

0.045 m/s. The Reynolds and Froude numbers for this flow, calculated assuming a circular opening with 

the same release area, were Re = 131 and Fr = 0.075. These values indicate that the flow at the release 

location was laminar and buoyancy dominated. The distance required for the flow to become fully 

buoyancy dominated was calculated to be 0.21 m, which implies that the flow was a fully buoyancydriven plume when it reached the ceiling.

2.4 Instrumentation

2.4.1 Helium sensors

Local helium volume percent was measured at multiple locations using Xensor Integration TGC-3880 

thermal conductivity sensors. Approaches for calibrating these sensors and calculating volume percents

were developed during the study of helium mixing and dispersion when released into a 1⁄4-scale two-car 

garage. [1] Each sensor was covered with a standard 5 mm high × 8 mm diameter circular TO-5 cap with 

a 0.28 cm diameter hole drilled in the center that reduced its sensitivity to flow, while providing time 

responses on the order of a second. The sensor calibration and analysis procedures were identical to those 

employed in the earlier study. It was shown previously that for the range of helium volume percent in the 

current investigation, variations in measured helium concentrations between different sensors were less 

than 0.5 %, and estimated absolute uncertainties were less than 1 %.

Similar to the earlier reduced-scale study [1,2], eight sensors were used during the 2008 testing. The 

helium sensors were placed on mounts attached to a vertical steel pole located at (x,y) = (1.37 m, 5.09 m), 

which was close to the center of the left rear quadrant of the garage. The sensors and support pole are 

visible on the left side of Figure 5. The eight sensors were placed at heights of z = 0.152 m, 0.457 m, 

0.762 m, 1.067 m, 1.372 m, 1.676 m, 1.981 m, and 2.286 m with an uncertainty of ± 0.002 m (coverage 

factor = 1). Table 1 lists the coordinates of the sensors for these tests.

For the 2010 tests, an additional six helium sensors were added for a total of fourteen. As discussed 

further below, experiments were performed with and without an automobile present in the garage. A 

vertical array of eight sensors was located at the same horizontal position as for the 2008 measurements. 

The sensor heights were z = 0.305 m, 0.610 m, 0.914 m, 1.219 m, 1.524 m, 1.829 m, 2.134 m, and 
15

Table 1. Helium Sensor Locations for 2008 Tests

Sensor x (m) y (m) z (m)

#1 1.37 5.09 0.152

#2 1.37 5.09 0.457

#3 1.37 5.09 0.762

#4 1.37 5.09 1.067

#5 1.37 5.09 1.372

#6 1.37 5.09 1.676

#7 1.37 5.09 1.981

#8 1.37 5.09 2.286

Table 2. Helium Sensor Locations for 2010 Tests without Vehicle

Sensor x (m) y (m) z (m)

#1 1.37 5.09 0.305 

#2 1.37 5.09 0.610

#3 1.37 5.09 0.914

#4 1.37 5.09 1.219

#5 1.37 5.09 152.4

#6 1.37 5.09 1.829

#7 1.37 5.09 2.134

#8 1.37 5.09 2.388

#9 1.37 5.09 0.051

#10 2.69 3.34 0.673 

#11 2.69 3.34 1.219

#12 2.69 3.34 1.626

#13 2.69 3.34 2.134

#14 4.34 1.82 2.134

2.388 m. When an automobile was not present, a ninth sensor was added to the vertical array at z = 

0.051 m. Four of the remaining sensors were centered over the helium release location, (x,y) = (2.69 m, 

3.40 m), with heights of z = 0.673 m, 1.219 m, 1.626 m, and 2.134 m. The remaining sensor was placed 

2.134 m above the floor in the front right quadrant of the garage at (x,y) = (4.34 m,1.82 m). Table 2 lists 

the coordinates of the fourteen sensors for the 2010 experiments without a vehicle present.

As discussed in Section 2.5 below, either a Dodge Stratus or Volkswagen Passat was parked in the 

garage for some tests. When an automobile was present, Sensors #9 to #14 were moved to various 

locations underneath and inside the vehicle. Sensor #9 was placed in the driver’s side front wheel well 

directly above the tire. Sensor #10 was attached to the dome light or rear view mirror in the center of the 

passenger compartment 6.4 cm from the ceiling. Sensor #11 was centered inside the trunk at mid height. 

Sensor #12 was placed inside the engine compartment in front of the engine near the base. Sensor #13 

was placed at the top of the engine compartment. Sensor #14 was placed in the undercarriage of the 

vehicle. For the Stratus, a location centered crosswise in the area underneath the vehicle 61.0 cm in front 

(relative to the automobile) of the helium release system and 29.2 cm above the floor was used. Much of 

the underside of the Passat was protected by plastic splash shields which extended down to the level of 

the outer edge of the vehicle, and it was not possible to place the helium sensor in the same location as for 

the Stratus. As an alternative, Sensor #14 was placed on top of the left front wheel control arm. Table 3

lists the locations of Sensors #9 to #14 when the vehicles were present in the garage.

The voltages generated by the helium sensors were digitized using a SCXI 32-channel National 

Instruments 1102 signal conditioning board coupled with a SCXI-1300 interface board. The conditioned 

signal was fed to a PXI-6221 digitizer board. The data acquisition system was located in the garage near 

the sensors and connected by Ethernet to a personal computer located in the family room of the house.
16

Table 3. Locations of Sensors #9 to #14 inside Vehicle (when present) for 2010 Tests

Sensor Location in Vehicle

#9 Front driver’s side wheel well, 0.76 m above floor (Stratus), 0.74 m above floor (Passat)

#10 Attached to passenger compartment dome light at center, 6.4 cm from ceiling (Stratus), 

attached to rear view mirror at front 6.4 cm from ceiling (Passat)

#11 Centered in trunk, 24.1 cm above floor (Stratus), 31.8 above floor (Passat)

#12 Inside engine compartment, centered in front of engine 0.406 m above floor (Stratus), 0.46 

m above floor (Passat)

#13

Inside upper engine compartment, centered left to right, 8.9 cm from hood and 0.305 m 

from rear gasket (Stratus), above engine on driver’s side, 2.5 cm from hood, 25 cm from 

firewall and centerline (Passat)

#14

Centered crosswise in undercarriage, 0.61 m from front edge of helium release, 29.2 cm 

above floor (Stratus), attached above lower control arm on front driver’s side , 18.4 cm 

above floor (Passat)

National Instruments LabVIEW software was used to control the data acquisition. Voltages were 

digitized at a 2 kHz rate and then averaged for either 1 s or 10 s before being saved in a comma-delimited 

file along with the relative time of the sample. It was possible to change the data acquisition rate during 

an experiment.

2.4.2 Pressure measurement

A MKS Baratron 1333 Pa range transducer was used to record differential pressures between the 

interior of the garage and the outside for some experiments. The transducer was calibrated in the same 

way described in [1]. The expanded uncertainty based on a linear least squares fit to the calibration data 

was ± 0.3 Pa. The pressure transducer was placed inside the garage. The voltage output was connected to 

the same digitization system used for the helium sensors. One of the pressure ports for the transducer was 

open, while the second was attached to a section of flexible tubing. The end of the tubing was passed 

underneath the rear door and exposed to the outside conditions near the base of the door. The pressure 

measurements tended to be very noisy due to wind turbulence. The measurements provided a qualitative 

indication of pressure fluctuations associated with the wind on the south side of the garage.

2.4.3 Helium measurements inside the test house

Helium volume percent levels inside the family room of the house were monitored at a single point 

using a Siemens Calomat 6 helium gas analyzer. The gas was sampled at a point in the center of the room 

at a height of 1.52 m by using a small pump to withdraw gas from the room into a tube and pass it 

continuously through the analyzer. A valve was used to set the flow rate through the instrument to the 

manufacturer’s recommended value. The analyzer was calibrated by sequentially passing pure air and 

helium through the analyzer and adjusting the set points accordingly. The analyzer provided digital read

out of helium volume fraction as well as an analog output. During the 2008 experiments, helium volume 

fraction was periodically read manually (roughly every 10 min) and recorded. Similar measurements 

were made for the second series of experiments, but, in addition, the analog output was connected to the 

data acquisition system and recorded automatically along with the outputs of the Xensor Integration

sensors and the pressure transducer. The manual and recorded values were in good agreement. The 

manufacturer specifies the instrument to have an accuracy of 1 % helium or better over its full range.

2.4.4 Additional Temperature Measurements

Three Type K bead-type thermocouples were placed outside of the rear door for the garage, inside the 

garage near the side door, and inside the family room of the house in order to record exterior, interior 
17

garage, and interior house temperatures. The temperatures were recorded manually using portable 
18

Figure 6. Photograph showing the test vehicle parked inside the garage.

thermocouple readers equipped with digital readouts. Temperatures were read at the same times that 

volume flow rate measurements and family room helium volume percents were recorded.

2.5 Experiments with a Vehicle Parked in the Garage

Experiments were performed with either a 2005 Dodge Stratus or a 2003 Volkswagen Passat parked 

in the garage in order to study the effect of a vehicle on helium mixing within the garage and to 

characterize the buildup of helium inside conventional vehicles. The nominal dimensions of these

automobile were 1.79 m (w) × 4.85 m (l) × 1.37 m (h) for the Stratus and 1.74 m (w) × 4.70 m (l) ×

1.46 m (h) for the Passat. The automobiles were centered over the 0.305 m × 0.305 m helium release 

system, i.e., centered in the garage. Figure 6 includes a photograph shot from outside through the garage 

door which shows the Stratus parked inside the garage. Experiments were run with all vehicle doors, 

windows, hood, and trunk closed.

2.6 Experiments with Forced Ventilation

The effectiveness of forced ventilation for limiting the buildup of helium in the garage was tested by 

performing experiments during the 2010 test series in which a vent was connected to an exhaust fan 

placed in the rear exit doorway of the garage. A 1.27 cm thick oriented strand board (OSB) wood panel 

was cut to fit inside the inner doorframe of the rear door and sealed into the frame using duct tape. A 10.8 

cm diameter hole was cut in the panel centered 10.2 cm from the top of the doorway. A 20.3 cm × 

20.3 cm square plate with a 10.8 cm diameter flange was held in place by screws and sealed over the hole 

on the outside using duct tape The flange was connected to an Infiltec Model DL1-DM4-110 duct 

leakage tester by a section of 10.8 cm diameter flexible plastic duct. This leakage tester is designed for 

use with residential heating and air conditioning systems. It provides a measured air volume flow rate 

while recording the pressure difference between the interior of the space and the volume on the 

downstream side of the fan. The device can provide calibrated volume flow rates ranging from 
19

Figure 7. Photograph showing the OSB board with attached flange in place in the rear doorway of the 

garage. The flange was connected to the Infiltec Model DL1-DM4-110 duct leakage tester 

(visible at the base of the doorway) by flexible duct.

0.014 m3

/s to 0.146 m3/s. The manufacturer reports the uncertainty in volume flow rate as 5 %. The fan 

was operated by drawing air from the garage, i.e., in depressurization mode. Figure 7 shows a photograph 

of the arrangement from the rear of the garage.

Six measurements employing forced ventilation were performed. All were run with automobiles 

parked centered over the helium release flow conditioner. Four of the experiments were done with the 

2005 Dodge Stratus and two with the 2003 Volkswagen Passat.

The experiments were designed to mimic an automatically started fan responding to the detection of 

hydrogen by a sensor located near the ceiling. The fan on the Infiltec was started manually when the 

voltage for Sensor #8 (2.388 m above the floor) on the vertical array dropped to a voltage corresponding 

to a helium volume fraction of 1 %, which is 1⁄4 of the lower flammability limit for hydrogen in air.

3 Experimental Results

3.1 Experiments with Natural Ventilation of the Garage

3.1.1 Helium release from a vertical tube without vehicle

Six experiments were performed with natural ventilation of the garage during the two series of 

experiments. Table 4 list the major experimental parameters including the date of the test, the type of 

helium flow, the average helium volume flow rate, the total helium volume flow, and whether or not a

vehicle was present.

Figure 8 shows a plot of measured helium volume flow rate as a function of time for the 9/11/08 

experiment. The helium was released upwards from the 0.635 cm diameter vertical tube. The helium 

flow was initiated at 120 s and halted at 13 710 s as the helium in the twelve cylinders was fully depleted. 

The total flow time was about 800 s shorter than the 4 h target time.
20

Table 4. Summary of Natural-Ventilation Garage Experimental Flow Parameters

Date Helium Flow Average Flow 

Rate (m3/s)

Flow Period 

(s)

Total Flow 

Volume (m3)

Vehicle?

9/11/08 tube 4.44 × 10-3 13 590 60.3 No

9/12/08 tube 4.43 × 10-3 12 840 56.9 No

7/29/10 box 4.30 × 10-3 14 352 62.1 No

8/2/10 box 4.36 × 10-3 14 400 62.7 No

8/6/10 box 4.47 × 10-3 14 400 64.3 Yes

8/23/10 box 4.32 × 10-3 13 636 59.1 Yes

Figure 8. Measured helium volume flow rate is plotted as a function of time for the 9/11/08 garage 

experiment. Measurements were recorded roughly every 600 s.

A small repetitive fluctuation of the volume flow rate is evident in Figure 8. These fluctuations were 

roughly correlated with the times when the switchovers between pairs of helium cylinders were made, 

suggesting a weak sensitivity of the regulated flow to cylinder pressure or other property of the cylinders.

The average helium volume flow rate for the data shown in Figure 8 was 4.44 × 10-3 m3

/s with a 

variance of 6 × 10-5 m3

/s. This flow rate is 7 % higher than the target value of 4.15 × 10-3 m3/s. The total 

volume of helium released was estimated by performing a piecewise time integration over the

measurements. The result was 60.3 m3

, which was 1 % higher than the target value of 59.7 m3, even 

though the release time was less than 4 h.

Figure 9 shows measured helium volume percent for the eight sensor locations along the vertical 

array as a function of time for the 9/11/08 experiment. A period covering 12 h is shown. Several distinct 

behaviors are evident. The helium concentrations at all heights began to increase shortly after the flow

was started at 120 s. This is evident in Figure 10, which shows the concentration profiles during the 

initial 1000 s of the experiment. The initial detection of helium at Sensor #8 occurred at 136 s or 16 s 

after the flow started. The 16 s represents the time required for helium to flow to the release point, rise to

the ceiling, and spread out to reach the sensor location. The helium then moved downward from the 
21

Figure 9. Helium volume percent is plotted as a function of time for the eight sensor heights indicated for 

the 9/11/08 experiment.

Figure 10. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the initial 1000 s of the 9/11/08 experiment.
22

Figure 11. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

over the period from 6300 s to just after the end of the helium release for the 9/11/08 

experiment. The color codes for the sensor locations are the same as in Figure 10.

ceiling, reaching the lower sensors sequentially from top to bottom. Only about 300 s from the start of 

the flow was required for helium to reach the lowest sensor.

In Figure 9, it is evident that a well mixed layer rapidly developed in the upper portion of the garage 

at heights of 1.1 m and higher and remained well mixed as the helium concentration increased. At the 

lower measurement points there was a rapid falloff in helium concentration with height that was evident 

over the entire release period. After roughly two hours the helium concentrations in the garage 

approached constant values for all of the measurement heights, indicating a quasi-steady state was 

developing. The well mixed upper layer and the concentration stratification at lower levels were present 

when the helium flow was halted.

Close inspection shows that the concentration profiles at the lower sensor positions had distinct rapid

fluctuations (recall data were recorded at 1 Hz), while those in the well mixed upper layer were 

considerably smoother. This “unmixedness” resulted from significant concentration variations over 

relatively small distances, an indication of significant ongoing mixing, in the lower part of the garage

volume. Note that the helium volume percent measurements at z = 1.067 m showed some fluctuations

even though the concentration was very close to that of the well mixed upper layer. This provides 

evidence that the height of the interface between the well mixed upper layer and the stratified lower layer 

varied with time.

The helium volume percents recorded during the quasi-steady-state are shown on an expanded time 

scale in Figure 11. Additional correlated, lower frequency helium concentration variations are evident at

all sensor heights. This suggests that an experimental parameter capable of affecting the quasi-steady 

state concentration levels was varying slowly over this period. One possibility is that variations in wind 

speed and direction altered the gas exchange rate between the garage and its surroundings. Figure 12

shows time plots of wind speed recorded by the sonic anemometer located 10 m above the ground and the 

differential pressure measured between locations outside the garage rear door and garage interior between 

6 300 s and 14 400 s. Wind direction measurements showed that the wind was primarily from the south 

during this time. Even though these measurements were recorded at different locations using different
23

Figure 12. Time variations of wind velocity and the differential pressure between the garage interior and 

outside ambience are plotted over the time period from 6300 s to 14400 s for the 9/11/08 

experiment.

data rates, visual comparison reveals that there was a strong correlation between wind velocity and the 

differential pressure, with periods of higher wind velocity corresponding to periods of higher differential 

pressure. Comparison of the data in Figure 12 with the helium concentration data in Figure 11 confirms 

that small dips in the quasi-steady-state concentrations were correlated with periods when wind velocity 

and differential pressure were higher. Apparently, the exterior weather condition had a small, but 

measurable, effect on the quasi-steady-state helium volume percents inside the garage.

The development of a quasi-steady state allowed a number of parameters characterizing the helium 

release phase to be defined. The first is the upper-layer helium volume percent at the end of the release 

period, which was determined to be 23.1 %. It was also possible to estimate the average concentration in 

the garage by assuming the absence of horizontal concentration gradients and extrapolating the 

measurements along the vertical array to the floor and ceiling. Justification for this assumption is 

provided below. Figure 13 shows an example of the approach for the 9/11/08 experiment. The 

experimental quasi-steady-state helium volume percents at the various sensor heights were determined by 

visual inspection of the plots shown in Figure 9 and are plotted in Figure 13. The measurements reveal 

that the vertical concentration profile can be fit well by two straight lines describing the constant 
24

Figure 13. Experimental helium volume percents measured along the vertical array at the end of the 

9/11/08 release are shown along with extrapolated values at the floor and ceiling. The solid 

lines are approximations of the data that are used to estimate the average helium concentration 

along the vertical direction.

Table 5. Helium Concentration Parameters for Garage at the End of Helium Release

Date Upper Layer He 

Vol. % at End of 

Release

Average He Vol. 

% in Garage at 

End of Release

Volume of Released 

He in Garage at End 

of Release (m3)

Fraction of Released He in 

Garage at End of Release

9/11/08 23.1 % ± 0.3 % 18.8 16.3 0.27

9/12/08 23.1 % ± 0.3 % 19.0 16.5 0.29

7/29/10 22.3 % ± 0.3 % 19.5 16.9 0.31

8/2/10 22.7 % ± 0.3 % 20.2 17.6 0.28

8/6/10 22.3 % ± 0.3 % 20.2 17.6 0.27

8/23/10 22.1 % ± 0.3 % 20.1 17.5 0.30

concentration in the upper layer and the falloff with height in the lower layer. The lines allow 

extrapolation of values to the ceiling and floor as indicated by the open symbols in the plot. The upperlayer helium volume percent, the floor and ceiling extrapolated values, and the height where the lines 

intersected were varied qualitatively until the observed agreement between the idealized lines and the 

experimental data was achieved. The two lines were used to integrate the concentration along the height 

to obtain the average value for the helium volume percent. The result was 18.8 %.

By using the estimated average helium volume percent and the known volume of the garage 

(86.9 m3

), the volume of helium present in the garage at the end of a release could be calculated. For the 

9/11/08 experiment, the result was 16.3 m3

. The fraction of released helium remaining in the garage 

could now be determined by dividing by the total released volume from Table 4 to give 0.27. This value 

along with the upper-layer helium volume percent and the average helium volume percent at the end of 

the release period is included in Table 5, which summarizes these values for the six experiments.
25

Figure 14. Helium volume percent is plotted as a function of time at the eight sensor heights indicated for 

the period from 43 200 s to 90 000 s in the 9/11/08 experiment.

In Figure 9 and Figure 11 it can be seen that the helium concentrations at the heights of Sensors #1 to 

#4 began to fall within seconds of the end of the helium release at 13 710 s. The most rapid initial drop 

was for Sensor #4, located at a height of 1.067 m, which was shown above to have been located close to 

the interface of the well mixed upper layer and the lower volume of the garage where a strong vertical 

concentration gradient developed. As time passed, the falloff in concentration moved upward through the 

upper layer, reaching the upper sensor location 2.286 m above the floor roughly 1500 s after the helium 

flow was stopped. Note that fluctuations in the quasi-steady state concentrations introduce some 

uncertainty, on the order of ± 200 s, in this estimate,

The helium concentration at each measurement location fell with a different rate in such a way that all 

of the profiles collapsed sequentially to a common curve from bottom to top. It is straightforward to 

estimate the times when concentration readings for a given sensor collapsed onto the common curve. The 

period required for the uniform concentration of helium to develop within the garage can be approximated 

as the time from the end of the release until the helium volume percent at Sensor #8 collapsed onto the 

common curve. For the data shown in Figure 9, roughly 18 000 s was required. At this time the helium 

volume percent was nearly uniform throughout the garage at ≈ 2.5 %. Interestingly, during the period 

when the collapse to a uniform helium concentration was occurring, the value for the lowest sensor at a 

height of 0.152 m remained nearly constant as the helium volume percents for the higher sensors 

approached this value sequentially from Sensor #2 to Sensor #8.

When the helium concentrations in the garage at the eight vertical sensors collapsed to a common 

decay curve, the falloff became relatively slow. Figure 14 shows the helium concentration decays for the 

period from 43 200 s to 90 000 s. The helium concentrations dropped from around 1 % to 0.2 % over this 

period. By assuming the helium was acting as a tracer gas, it was possible to estimate a value for ACHgar

by curve fitting an exponential curve to the falloff. The data for Sensor #4 was fit using a nonlinear least 

squares curve fit transform in SigmaPlot. Figure 15 shows the experimental data and the result for the fit. 

The calculated time constant was 7.47 × 10-5 s

-1

, which corresponds to ACHgar = 0.27 h-1. The R2 value 

for the fit was 0.973, indicating that an exponential curve was a good representation for the concentration 

decay.
26

Figure 15. Helium volume percent recorded by Sensor #4 is plotted as a function of time for the 9/11/08 

experiment. The red line is the result of fitting the data to an exponential decay using a 

nonlinear least squares curve fit.

Table 6. Garage Parameters for Post-Release Period

Date Highest 

Sensor 

(m)

Time for Sensor 

#8 Initial Roll 

Off (s)

Time for 

Sensor #8 

Collapse (s)

He Vol % at 

Sensor #8 

Collapse

Concentration 

Decay Constant 

(s-1)

ACHgar (h-1)

9/11/08 2.286 1 500 ± 200 18 000 2.5 7.47 × 10-5 0.27

9/12/08 2.286 1 200 ± 200 15 200 3.6 3.14 × 10-5 0.11

7/29/10 2.388 800 ± 200 12 400 3.0 9.08 × 10-5 0.33

8/2/10 2.388 1100 ± 200 14 200 2.2 6.04 × 10-5 0.22

8/6/10 2.388 900 ± 200 14 100 2.4 5.53 × 10-5 0.20

8/23/10 2.388 900 ± 200 14 100 2.2 4.46 × 10-5 0.17

The behavior of the helium concentration in the garage following the end of the helium release can be 

characterized in terms of the parameters defined above. Values for the periods between the end of the 

release and the initial falloff of concentration and for the collapse to the uniform concentration for the 

highest sensor, Sensor #8, at 2.286 m, the helium volume percent at collapse, and the decay rate at long 

times with associated value of ACHgar are summarized in Table 6 for the 9/11/08 data. Values for later 

experiments discussed below are also included.

During the experiment, temperatures were measured in the family room of the house and outside 

using the dedicated thermistors and added thermocouples. Figure 16 compares the two sets of 

measurements. The measurements in the family room agreed very well when the different rates used to 

record data are taken into consideration. The distinct fluctuations in temperature resulted from the 

cycling of the air conditioning system used to cool the house. The temperature measurements outside do 

not agree as well, with the thermocouple measurements generally being somewhat higher. There are two 

possible reasons for this. The temperature may have been slightly higher just above the concrete outside 

the rear door of the garage where the thermocouple was places as opposed to the open air higher up where 
27

Figure 16. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 9/11/08 in the family room, garage, and 

outside.

the thermistor was located. It is also possible that the thermocouple was not adequately shielded from 

radiation from the sun and sky and was radiatively heated. Figure 16 also includes temperature 

measurements made inside the garage using a thermocouple. Since this thermocouple was placed at an 

interior location, it would not have been subject to radiative heating. The temperature inside the garage 

rose during the helium release in a way that roughly tracked the outside temperature, even though the rate 

was a bit higher. As a result, the temperature difference between the interiors of the house and garage 

increased from about 1 oC to 5 oC during the helium release.

Figure 17 shows a plot of measured helium volume percent in the family room of the house as a 

function of time during and immediately following the helium release inside the garage. The helium 

concentration increased continuously, reaching a value of about 2.5 % shortly after 12 000 s, when a 

sudden drop occurred. This concentration drop happened when a window in the family room was opened 

around 12 300 s. The window remained open until 13 620 s. When the window was shut, the helium

concentration quickly started to rise again, reaching values nearly as high as before the window was 

opened. The helium concentration remained relatively constant after the helium flow into the garage was 

halted at 13 710 s, only starting to decrease at longer times.

During the experiment, the fan for the air conditioning was on continuously as confirmed by the time 

record of the fan status recorded by the monitoring system for the house. Nabinger and Persily found that

with the fan operating, a tracer gas was fully mixed throughout the house within ten minutes. [30] While 

helium at the levels observed in the family room may not act purely as a tracer gas, it is likely that it was 

mixed throughout the house volume within a similar time frame. This suggests that the helium volume 

percent measured in the family room was representative of a well mixed helium/air mixture throughout 

the interior volume of the house.

By assuming that the helium inside the house was fully mixed, it was possible to estimate the total 

volume of helium present inside the house at the end of the helium release in the garage by multiplying 

the measured volume percent, 2.3 %, at this time by the interior volume of the house. The result for the 
28

Figure 17. The helium volume percent measured in the family room of the house is shown as a function 

of time for the 9/11/08 experiment.

Table 7. Helium Parameters in the House at the End of Helium Release in the Garage

Date He Vol. % in 

House 

Volume of He in 

House (m3)

Fraction of Released He

9/11/08 2.3 7.8 0.13

9/12/08 4.8 16.3 0.29

7/29/10 3.7 12.6 0.20

8/2/10 4.1 13.6 0.22

8/6/10 3.2 10.9 0.17

8/23/10 2.5 8.5 0.14

9/11/08 data was 7.8 m3

, which corresponds to 13 % of the total helium volume released into the garage. 

The helium volume percent in the house, corresponding total helium volume in the house, and the fraction 

of the total volume of helium released present in the house at the end of helium flow into the garage are 

summarized in Table 7.

The experiment with helium release from the straight tube was repeated on 9/12/08. The measured 

helium volume flow rate had a similar variation with time to that shown in Figure 8 for the 9/11/08 

experiment. The average helium volume flow rate, flow period, and the total volume released for the 

9/12/08 experiment are included in Table 4. The average helium volume flow rates were similar for the 

two experiments. However, since the release period for the second experiment was shorter, the total 

volume of helium released was reduced by 5.6 %.

Figure 18 shows a plot of measured helium volume percent as a function of time at the eight sensor 

locations for the 9/12/08 experiment. The general time dependence of the concentration profiles are 

similar to those for the 9/11/08 case shown in Figure 9. The uniform upper layer helium volume percent 

at the end of the release period was 23.1 % (see Table 5), which is the same as observed on 9/11/08. 

Close inspection reveals that the helium concentrations for each of the sensors in the lower part of the

garage were slightly higher on 9/12/08. As a result, when the experimental data was fit and integrated 
29

Figure 18. Helium volume percent is plotted as a function of time at the eight sensor heights indicated for 

the 9/12/08 experiment.

using the same approach as above to determine a quasi-steady-state average helium volume percent at the 

end of the release, the value was slightly higher for the second set of data. While concentration 

fluctuations are evident in the quasi-steady-state portion of the helium release for the second experiment

in Figure 18, they were not as intense as for the original data.

The wind velocity and differential pressure between the garage and outside ambience are shown in 

Figure 19 during the quasi-steady-state release portion of the 9/12/08 data. During this time the wind was 

primarily from the south. As observed for the 9/11/08 experiment, there was a clear correlation between 

the strength and fluctuations of the wind velocity and the differential pressure fluctuations. Comparison 

with Figure 18 shows that dips in the measured helium concentrations and the strength of the fluctuations 

were anti-correlated with the wind velocity and differential pressure magnitudes as was the case for the 

earlier experiment. Figure 12 and Figure 19 show that both the wind speed and differential pressure 

fluctuations were higher on 9/11/08. Assuming gas exchange rates between the garage and its 

surroundings increase with higher wind speeds and differential pressures, this observation provides a 

plausible explanation for the slightly higher average helium volume percent at the end of the release 

observed on 9/12/08.

Following the helium release, the concentrations fell off in similar ways for both experiments, but, as 

evident from the results listed in Table 6, the time required for the concentration profiles to collapse was 

shorter, and the helium volume percent when all of the curves collapsed was higher for the second 

experiment. Comparison of the concentration profiles for the two experiments reveals that their decay 

behaviors were similar. The principal reason for the faster collapse of the curves on 9/12/08 was the 

higher helium volume percent when the single decay curve developed. A fit of the falloff data at long

times when the helium volume percent was on the order of 1 % and lower yielded an estimate of ACHgar

= 0.11 h -1

, compared to ACHgar = 0.27 h-1 for the earlier data. The experimental observations for the two 

experiments during the release and post-release periods are consistent, with slower air incursion into the 

lower level of the garage for the 9/12/08 experiment.
30

Figure 19. Time variations of wind velocity and the differential pressure between the garage interior and 

outside ambience are plotted over the time period from 6300 s to 14400 s for the 9/12/08 

experiment.

Figure 20 shows comparisons of the wind speed and differential pressure between the garage interior 

and outside during the twelve hours following the start of the experiments on 9/11/08 and 9/12/08. The

corresponding plots of helium volume percent over the same time periods are shown in Figure 9 and 

Figure 18. During both experiments the wind shifted from the south to primarily from the north shortly 

after the helium flow was halted. Over most of the release and immediate post-release periods the wind 

and differential pressure magnitudes and fluctuations were noticeably higher for the earlier experiment. If 

a higher wind is partially responsible for increased gas exchange with the garage, as expected, this 

provides an explanation for the higher rate of helium losses from the garage over these times.
31
32

Figure 20. Wind speeds and differential pressure measurements are plotted over the twelve hours 

following the start of the experiments on 9/11/08 and 9/12/08.

A steep increase in wind velocity starting around 36 000 s for the 9/12/08 experiments was not 

reflected in the differential pressure measurement. The wind direction during this period was from the 

north, as opposed to measurements during the quasi-steady-state helium release period when the wind 

was from the south for both experiments. This suggests that the differential pressure measurements are 

wind-direction sensitive, with the largest response observed for winds blowing in the direction of the 

measurement location, i.e., from the south.

The importance of wind direction becomes clearer when considering differences in measured ACHgar

for the two experiments. Wind velocities and directions are compared in Figure 21 over longer times, 

which include the periods when ACHgar values were determined. A value of 0o is defined to be north. At 

roughly 12 h (43 200 s) after the start of the experiment on 9/12/08, the wind began to blow strongly from 

the north, while on 9/11/08 the wind was from the south over the same period. Even though the wind 

magnitude at long times was higher, the value of ACHgar for the 9/12/08 experiment was 2.5 times smaller 

than measured on 9/11/08. This is strong evidence that gas exchange between the garage and its 

surroundings was strongly dependent on wind direction as well as magnitude.

The weather during the 9/12/08 helium release was cool and cloudy with rain. These conditions were 

reflected in the temperature measurements shown in Figure 22 for locations in the family room, garage, 
33

Figure 21. Wind speed and direction are plotted as a function of time for the 9/11/08 and 9/12/08 

experiments.

and outside. There were relatively small differences between the locations. Unlike the similar 

measurements shown in Figure 16 for measurements on 9/11/08, outside temperatures recorded by the 

thermocouples were slightly lower than measured with the thermistor. This may suggest that interactions 

with a nearby surface played a role in the thermocouple measurements. The temperature fluctuations 

associated with the cycling of the air conditioning were evident in the family room. The fan was operated 

in continuous mode.

The helium volume percent recorded in the family room during the 9/12/08 helium release is shown 

in Figure 23. The concentration rose continuously over the period, approaching 4 % helium by the end of 

the release. As discussed above, since the fan was running, it is likely that the entire volume of the house 

reached a level around 4 %. This value along with derived values for the helium volume in the house at 

the end of the release and the fraction of the total released helium it represented are included in Table 7. 

All of the values were much larger than those measured on 9/11/08. Comparison of Figure 23 with the 

corresponding plot for the 9/11/08 data in Figure 17 shows that the helium concentrations roughly tracked 

each other until the window in the family room was opened during the earlier experiment.
34

Figure 22. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 9/12/08 in the family room, garage, and 

outside.

Figure 23. The helium volume percent measured in the family room of the house is shown as a function 

of time for the 9/12/08 experiment.
35

Figure 24. Helium volume percent is plotted as a function of time at the nine sensor heights indicated for 

the 7/29/10 experiment.

3.1.2 Helium release over large area without vehicle

Two helium release experiments for the empty garage similar to those performed in 2008 were done 

in 2010. The major change was that helium was released over a 0.0929 m2 area as opposed to the 

3.17 × 10-5 m2 tube area of the earlier experiments. Helium volume flow rates were measured using the 

same approach. The time behaviors for the volume flow rates were similar to that shown in Figure 8. 

The average volume flow rates and total flow volumes are summarized in Table 4 for the experiments 

performed on 7/29/10 and 8/2/10.

Figure 24 shows plots of helium volume percent as a function of time for the nine sensor locations 

along the vertical array in the left rear quadrant. The overall time behaviors and magnitudes were similar 

to those observed in the 2008 experiments (see Figure 9 and Figure 18). Note that care should be 

employed when comparing plots since the vertical sensor array for the later experiment utilized different 

sensor separations and heights (see Table 1 and Table 2).

As observed in the earlier experiments, a uniform upper layer developed during the release. The 

quasi-steady-state helium volume fraction of 22.3 % listed in Table 5 was slightly less than observed in

the 2008 experiments. Based on the variations of helium volume percent with height in Figure 24, it is 

clear that the interface separating the well-mixed upper layer from the lower volume where vertical 

concentration gradients developed was located just above 0.914 m. This is consistent with the 2008 

results, which indicated that the interface location was between 0.76 m and 1.07 m above the floor.

The average helium volume percent in the garage at the end of the release was estimated using the 

approach described earlier. Figure 25 shows a plot of the quasi-steady-state concentrations versus height 

along with extrapolated values at the floor and ceiling and line approximations for the variations with 

height. Integration using the lines yielded an average value of 19.5 %. This value is higher than 

determined for the 2008 experiments even though the upper-layer quasi-steady-state concentration was 

lower. Comparison of Figure 25 with Figure 13 shows that for given heights in the lower layer the 

helium concentrations were higher for the 2010 experiment.
36

Figure 25. Experimental helium volume percents measured along the vertical array at the end of the 

7/29/10 release are shown along with extrapolated values at the floor and ceiling. The solid 

lines are approximations of the data that are used to estimate the average helium concentration 

along the vertical direction.

The concentration profiles in the lower portion of the garage in Figure 24 show that the concentration 

gradient increased with decreasing height. The rapid and slow concentration fluctuations observed in the 

lower layer for the 2008 experiments were also present in the latest data. It is interesting that the quasisteady-state helium volume percent was similar (≈ 6 %) for measurements at 0.0152 m in 2008 and 

0.051 m in 2010. This appears to have been the result of the different helium concentration distributions 

in the lower layer.

The helium concentration profiles following the end of the release also had similar appearances to the 

falloffs observed for the earlier results. One difference was that the helium volume percent at the lowest 

measurement height, which was nearer the floor than for the 2008 experiments, rose slightly as it 

approached the uniform helium concentration around 26 700 s. Values characterizing the falloff behavior 

of helium concentration during the post-release period are included in Table 6. The initial roll off of 

concentration at Sensor #8 was earlier than observed during the 2008 experiments. A shorter period was 

also required for the concentration at this height to collapse to the common decay curve. Even though 

these times were shorter, the helium volume percent for the collapsed curve at the time the falloff curve 

for Sensor #8 joined it was 3.0 %, which was intermediate between the two values from 2008. An 

exponential least squares fit to the common decay curve at long times yielded a time constant of 

9.08 × 10-5 s

-1 and ACHgar = 0.33 h-1

. This value is somewhat higher than the values, 0.27 h-1 and 0.11 h-1, 

observed for the experiments in 2008. The decay behaviors suggest that the gas exchange rate for the 

garage varied with time, with the 7/29/10 experiment being faster during the initial part of the post-release 

phase, but slower at longer times.

Additional helium sensors were added for the 2010 experiments. For the 7/29/10 experiment, four of 

these sensors were placed above the release location, and a fifth was added in the front-right quadrant at a 

height of 2.314 m. Figure 26 shows the helium volume percents recorded by these sensors. The large

fluctuations observed for the sensors located over the helium release location show that the local
37

Figure 26. Helium volume percent is plotted as a function of time at the indicated heights for four sensors 

above the helium release location (center) and a single sensor at (x,y) = (4.343 m, 1.816 m) 

(front-right) for the 7/29/10 experiment.

concentrations fluctuated with time due to mixing inside the buoyant plume. The caveat concerning the 

effect of time-averaging on reducing the magnitudes of fluctuations noted earlier applies here as well. 

The concentrations at the various heights above the release location were higher, but not by large 

amounts, than those observed in the upper layer in the right front quadrant of the garage (Sensor #14).

The effects of mixing along the plume were particularly clear at early times, as can be seen in Figure

27, where the first three hundred seconds of the experiment are plotted. In less than a second after the 

flow started at 60 s, helium was detected at the lowest sensor 0.673 m above the release location. Helium 

was then sequentially observed at one second intervals at the three higher sensors above the release point. 

Somewhat surprisingly, the initial concentration was higher at the 1.219 m height than at the lower 

sensor. Normally, it would be expected that the helium concentration would decrease with distance from 

the release point. This observation may be tied to the mixing in the lower layer discussed above. An 

initial spike in helium concentration for the sensor located in the upper layer in the front-right quadrant 

was observed 15 s after the start of the release, and the concentration began to rise steadily 15 s later.

Over much of the release period shown in Figure 26, the helium volume percent at the lowest sensor 

position above the release location was the highest concentration, as expected. However, during much of 

the quasi-steady-state period, it was lower than observed for the higher sensors and even in the upper 

layer outside the plume. This is similar to the behavior seen at the start of the flow. All of the 

concentrations measured above the release locations were similar to the upper-layer value. Since the 

helium concentration leaving the distribution box should have been nearly 100 % helium (it is possible 

that some outside air mixed with the helium by back flow before it exited the box), this implies that a 

great deal of mixing with surrounding air took place between the release height and the lowest sensor 

0.673 m above the floor.

As soon as the helium flow stopped, the rapid fluctuations dissipated, and helium concentrations for 

the three highest sensors (1.219 m and higher) above the release location and in the front-right upper layer 

collapsed to a common helium volume percent of 22.5 %. This is very close to the upper-layer value of 
38

Figure 27. Helium volume percent is plotted as a function of time for the five sensor heights indicated for 

the initial 300 s of the 7/29/10 experiment.

22.3 % given in Table 5 based on measurements along the vertical sensor array in the left-rear of the 

garage. The corresponding helium concentration for the lowest sensor at 0.673 m was estimated to be 

19.8 %, which falls midway between the corresponding concentrations of 17.8 % and 21.3 % recorded by 

the vertical-array sensors at heights of 0.610 m and 0.914 m, respectively.

The initial falloff behaviors following the end of the release for the data in Figure 26 were similar to 

those described for the sensors along the vertical array, with the drops in concentration starting 

immediately for the lowest sensor and progressing with time upwards through the uniform upper layer. 

The concentration falloffs collapsed to a common helium volume percent around 26 700 s or 12 300 s

after the end of the release. This compares favorably with the value of 12 400 s included in Table 6 based 

on the measurements along the vertical array. The helium volume percents when the concentration 

collapses occurred were roughly 3.1 % and 3.2 % for the sensors in the rear-left quadrant and above the 

release location, respectively. During the post-release phase, it is difficult to identify the data for the 

sensor located at a height of 2.134 m above the release location in Figure 26 because the curve for the 

sensor located at the same height in the right-front of the garage overlays it. Comparison with the 

response for Sensor #7 shown in Figure 24, which was located at the same height in the left-rear quadrant, 

shows that its falloff was indistinguishable from those at the center and right front. This provides strong 

evidence that there were no horizontal concentration gradients at this height, and the upper layer was fully 

mixed.

Neither wind speed nor differential pressure measurements were recorded for the 7/29/10 experiment.

Figure 28 compares temperatures measured in the family room of the house, inside the garage, and 

outside. The outside and family room temperatures were taken from the measurement system for the 

house, and the garage temperatures were recorded manually using a thermocouple reader. It was a hot 

summer day, and temperatures in the garage and outside were 8 oC to 11 oC higher than the airconditioned interior. Temperature fluctuations in the family room reflect the cycling of the air 

conditioning. The fan for the air conditioning was operated continuously.
39

Figure 28. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 7/29/10-7/30/10 in the family room, garage, 

and outside.

Figure 29 shows a plot of the helium volume percent measured in the family room over a period of 

twenty-four hours, including the 4 h release period. Measurements recorded manually and electronically 

agree well. Once the helium release began at 60 s, the helium volume percent in the house rose 

continuously, reaching a maximum value of 3.7 % approximately 400 s after the end of the release. This 

value is slightly less than that observed in the 9/12/08 experiment (see Figure 23.). Corresponding values 

of helium volume and fraction of released helium in the house at the end of the release are listed in Table 

7. Very shortly after the end of the release, the helium concentration began to fall slowly, approaching 

zero after 20 h. Some fluctuations are evident during the decay phase, which appear to be correlated with 

the air conditioning cycling. The source of the fluctuations is unclear.

The helium release into the empty garage was repeated on 8/2/10. The average helium volume flow 

rate and total flow volume are included in Table 4. Figure 30 is a plot of the helium volume percent for 

the nine locations along the vertical sensor array in the left-rear quadrant of the garage. Both the general 

appearance and quantitative behavior are similar to the results for 7/29/10 shown in Figure 24. The most 

obvious differences between the two experiments occurred during the post-release period. The helium 

volume percent recorded by the lowest sensor in the second experiment did not show the minimum that 

was evident on 7/29/10 as the concentrations collapsed to a common curve. Careful comparison showed 

that the collapsed curve for the later experiment fell slightly below that for the earlier. These observations 

suggest a slightly faster helium loss rate for the repeated experiment.

Quantitative characterizations of the data in Figure 30 are included in Table 5 and Table 6 for the 

release and post-release phases. The quasi-steady-state upper layer and average concentrations at the end 

of the release were both slightly higher than observed on 7/29/10, as was the volume of helium remaining 

in the garage at this time. The characteristics for the post-release period in Table 6 are similar to those 

determined from earlier measurements. The largest difference was for the helium volume percent at the 

time when the decay curves collapsed to a single curve, which at 2.2 % was the lowest of the experiments 

discussed thus far. This was the case even though the time required to decay to this level was 
40

Figure 29. The helium volume percents measured in the family room of the house manually (symbols) 

and electronically (line) are shown as a function of time for the 7/29/10 experiment.

Figure 30. Helium volume percent is plotted as a function of time at the nine sensor heights indicated for 

the 8/2/10 experiment.

intermediate to the other experiments. The ACHgar = 0.22 h-1 determined at long times was also near the 

median for the earlier experiments.
41

Figure 31. Helium volume percent is plotted as a function of time at the indicated heights for four sensors 

above the helium release location (center) and a single sensor at (x,y) = (4.343 m, 1.816 m) 

(front-right) for the 8/2/10 experiment.

The concentration time profiles recorded by the remaining five sensors for the 8/2/10 experiment are 

shown in Figure 31. The corresponding results for 7/29/10 were plotted in Figure 26. Comparison shows 

that concentrations along the centerline above the release location were slightly higher (≈ 5 %) during the 

second experiment. This difference was several times larger than the relative difference in helium volume 

flow rates listed in Table 4. There were also differences in concentration for Sensor #10, centered 

0.673 m above the helium release location, near the end of the release. For the 7/29/10 data, the helium 

concentrations were strongly fluctuating and were apparently lower than recorded by sensors located 

above the release point. Such behavior was not as evident for the second run. A similar difference is also 

evident in the plot of the data over the initial 300 s of the experiment shown in Figure 32. The 

concentrations show the expected falloff from top to bottom for sensor locations above the release 

location. Unlike for the 7/29/10 results, the helium volume percent for Sensor #10 abruptly dropped to 

19.6 %, very close to the value of 19.8 % estimated from the 7/29/10 experiment, when the helium flow 

was stopped and then began to fall at a slower rate. These observations suggest that conditions in the 

lower layer had a reduced influence on the helium plume during the second experiment.

When the helium flow was halted, the fluctuations at all four locations above the plume rapidly 

disappeared, and the smooth falloff curves were similar to those observed on 7/29/10. As before, the two 

curves for a height of 2.134 m (above the release point and in the front right) were superimposed. Similar 

to the results in the left rear, the concentration falloffs eventually collapsed to a single curve.

The differential pressure between the interior of the garage and outside was recorded on 8/2/10. The 

results are plotted in Figure 33. Note that the averaging time for the measurements was changed from 1 s 

to 10 s at 28 800 s, which is the reason for the abrupt decrease in fluctuation magnitude. The differential 

pressure behavior was similar to those observed on 9/11/08 (Figure 12) and 9/12/08 (Figure 19) and 

suggests that wind levels were not dramatically differently for these three experiments.

Figure 34 shows plots of temperature for the 12 h time period following the start of the experiment on 

8/2/10 for locations in the family room, garage, and outside. The day was cloudy and breezy, and the 

temperatures in the garage and outside were somewhat lower than measured on 7/29/10. The fluctuations 
42

Figure 32. Helium volume percent is plotted as a function of time for the five sensor heights indicated for 

the initial 300 s of the 8/2/10 experiment.

Figure 33. The time variation of the differential pressure between the garage interior and outside is

plotted over the time period from 0 s to 43 200 s for the 8/2/10 experiment.

due to the cycling of the air conditioning were evident in the family room. The blower for the air 

conditioning was operated continuously.
43

Figure 34. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/2/10 in the family room, garage, and outside.

Figure 35. The helium volume percents measured in the family room of the house manually (symbols) 

and electronically (line) are shown as a function of time for the 8/2/10 experiment. 

The helium volume percent measured in the family room for the 8/2/10 experiment is shown in 

Figure 35. The growth and decay of the helium concentration were similar to those for the 7/29/10 

experiment shown in Figure 29, but the maximum concentration for the later experiment is 4.1 % versus
44

Figure 36. Helium volume percent in the garage is plotted as a function of time at the eight sensor heights 

indicated along the vertical array for the 8/6/10 experiment.

3.7 % for the earlier. This difference is larger than the difference in volume flow rates and suggests that a 

higher fraction of helium was transported into the house from the garage during this experiment. This is 

confirmed by comparing the values of fraction of released helium in the house listed in Table 7. The 

maximum value was recorded 450 s after the end of the helium release inside the garage. Fluctuations 

due to the cycling of the air conditioning are apparent. The source of these fluctuations was not clear.

3.1.3 Helium release over large area with vehicle

Two 4 h releases of helium were made with an automobile parked at the center of the garage. Helium 

from the release box flowed upward and struck the undercarriage of the automobile before spreading out 

under the automobile, accumulating at various locations, and eventually escaping to the surrounding 

garage.

The first experiment was run on 8/6/10. The measured volume flow parameters are included in Table 

4. The helium volume flow rate and total volume released were slightly higher than for the earlier 

experiments. The helium flow was initiated at 60 s and lasted 14 400 s.

The locations of the sensors were changed from those used in the experiments without an automobile. 

Sensor #9 was moved from its position near the floor on the vertical sensor array to the front driver’s side 

wheel well and Sensors #10 to #14 were placed at other locations within the vehicle.

The concentration time behaviors recorded by the vertical array of sensors are shown in Figure 36. 

Comparison with the corresponding plots (Figure 9, Figure 18, Figure 24, and Figure 30) recorded 

without a vehicle shows that the growth of the concentration profiles during the helium release phase and 

the decay during the post-release phase had the same general appearances. The helium volume percents

grew rapidly during the initial release phase before leveling off, while the concentrations dropped 

following the end of the release, with the concentrations at the lowest sensor falling the earliest and fastest 

and concentrations at higher positions decaying later. The helium concentrations for all sensors 

ultimately collapsed to a single nonzero value, which then decayed slowly with time.
45

Figure 37. Experimental helium volume percents measured along the vertical array at the end of the 

8/6/10 release are shown along with extrapolated values at the floor and ceiling. The solid 

lines are approximations of the data used to estimate the average helium concentration along 

the vertical direction.

Table 5 lists values of the parameters used to characterize the experiment at the end of the release 

period. These are nearly identical to those determined for the 8/2/10 data, but the two sets of results differ 

somewhat when compared to the earlier experiments without a vehicle. There is evidence that mixing 

inside the garage was improved slightly by the presence of an automobile. As observed in the 

experiments without a vehicle (compare Figure 36 with Figure 24 and Figure 30), there was a well-mixed 

region in the upper part of the garage. However, the location of the interface with a vehicle present was 

slightly lower with Sensor #3 at 0.914 m reaching a quasi-steady-state concentration very close to that 

characteristic of the upper layer, even though sharp concentration spikes were evident at this height. For

the cases without a vehicle on 7/29/10 and 8/2/10, the quasi-steady-state helium volume percents at this 

height were clearly lower than in the well-mixed upper layer. Additional evidence for increased mixing 

in the presence of the vehicle can be seen in Figure 37, which shows the quasi-steady-state helium volume 

percents as a function of height along the sensor array. Quasi-steady-state helium concentrations in the 

lower layer at 0.305 m (Sensor #1) and 0.610 m (Sensor #2) were considerably higher with the 

automobile present. As a result, comparison with Figure 13 and Figure 25 shows that the average 

concentration gradient in the lower layer was reduced compared to experiments without a vehicle present.

The decay of the helium volume fraction in the garage following the end of the release for the 8/6/10 

experiment also appeared to be very similar to the corresponding results for 8/2/10 in Figure 30. This is 

confirmed by the agreement between quantitative characteristics for the two experiments included in 

Table 6. The close agreement extends to parameters that characterize the concentration falloff 

immediately following the end of the helium release, as well as the decay constants and ACHgar derived 

from measurements at longer times when helium levels in the garage were 1 % and lower.

Figure 38 shows differential pressure measurements between the garage and the outside. During the 

early part of the helium release, there were negative pressure fluctuations that were generally more 

intense and larger than were observed in earlier experiments (see Figure 12, Figure 19, and Figure 33). 
46
47

Figure 38. The time variation of the differential pressure between the garage interior and outside is 

plotted over the time period from 0 s to 43 200 s for the 8/6/10 experiment.

Even though the fluctuations were quite large, the effects on the loss rates of helium from the garage were 

relatively small. Note that the data averaging time was changed from 1 s to 10 s at 28 800 s.

The temperatures recorded in the family room, garage, and outside during the 8/6/10 experiment are 

shown in Figure 39. It was a hot summer day, and during the helium release the outside and garage 

temperatures were roughly 6 oC higher than in the air conditioned house. Shortly after the end of the 

helium release, the compressor on the air conditioning system failed, and the interior temperature began 

to increase. At some point shortly afterwards, windows on the front and rear of the house were opened.

A plot of helium volume percent recorded in the family room during the 8/6/10 experiment is shown 

in Figure 40. Table 7 summarizes parameters related to the buildup of helium in the house. The growth 

curve of the helium concentration was similar to that observed in earlier experiments, but the maximum 

level reached, 3.2 %, was somewhat lower. The reduced concentration was likely due to a higher 

exchange rate with the outside air, which is consistent with the wind observations, or to a reduced gas 

exchange rate between the garage and house, which might have been due to the relatively large 

temperature difference between the two locations. During the post-release phase, the falloff of the family 

room helium concentration was much faster than observed when the vehicle was not present (compare 

Figure 40 with Figure 29 and Figure 35). The most likely reason was the opening of the house windows,

even though the exact time when this occurred was unknown.

During the 8/6/10 experiment, helium concentrations were measured in the undercarriage of the 

vehicle near the helium release location, in the passenger-side front wheel well, at two locations within 

the engine compartment, and inside the passenger compartment and trunk. Figure 41 shows the time 

behaviors for the helium volume percent at these locations during the release and post-release periods. 

Note that the thermal conductivity sensor (Sensor #10) inside the passenger compartment was originally 

attached to the compartment light near the center of the roof, but that it fell at some unknown time and 

was found on the floor behind the passenger seat at the end of the experiment.

The data shown in Figure 41 are replotted in Figure 42 on an expanded time scale to emphasize the 

concentration behavior during the initial part of the helium release. It is evident that the helium 

concentration in the volume underneath the vehicle rose sharply very shortly (within 4 s) after the helium 
48

Figure 39. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/6/10 in the family room, garage, and outside.

Figure 40. The helium volume percents measured in the family room of the house manually (symbols) 

and electronically (line) are shown as a function of time for the 8/6/10 experiment.

flow was started at 60 s and rapidly attained a helium concentration of roughly 28 %. This concentration 

was close to the stoichiometric concentration for hydrogen burning in air [33], which indicates that the 
49

Figure 41. Helium volume percent is plotted as a function of time at the indicated locations inside the 

automobile centered over the helium release location for the 8/6/10 experiment.

Figure 42. The data shown in Figure 41 are replotted on an expanded time scale emphasizing the helium 

increases when the flow was initially released.

experimental configuration used here would have rapidly generated a dangerous flammable mixture at 

this location if hydrogen had been released instead of helium. It can be seen in Figure 41 that after 
50

Figure 43. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the initial 190 s of the 8/6/10 experiment.

reaching this level the concentration slowly increased further until it reached a quasi-steady-state value 

around 37 % prior to the end of the release.

The corresponding plot for the initial part of the release period for the eight sensors along the vertical 

array is shown in Figure 43, which was obtained by expanding the time scale for the data in Figure 36. It 

can be seen that 43 s was required for the initial detection of helium at the 2.39 m height of Sensor #8, 

which is nearly 40 s after helium was first detected under the automobile. This delay can be compared to 

a value of 15 s measured for the corresponding data shown in Figure 30 for a case without a vehicle 

present. This indicates that when releasing the helium under the vehicle, an additional period of 25 s was 

required for helium to reach a sensor location close to the ceiling.

The high concentration of helium that built up within the undercarriage of the Stratus apparently 

flowed to other accessible areas in the vehicle. The next location where helium was detected following 

the start of the release was at the top of the engine compartment about 4 s later. The helium volume 

percent increased rapidly to about 17 %. The nearly factor of two drop in concentration between the 

undercarriage measurement location and the top of the engine compartment shows that additional air was 

mixing into the flow either by entrainment from below or through openings in the engine compartment. 

The next sensor reached by the helium was located in the wheel well 0.76 m above the floor. The 

concentration increased in a manner similar to that at the top of the engine compartment and reached a 

level that was only slightly lower. Since a pathway existed between the engine compartment and the 

wheel well, this suggests that a relatively uniform concentration of helium developed in the upper part of 

the engine compartment.

The area of relatively high concentration did not extend uniformly over the full height of the engine 

compartment. While the helium concentration at the front of the engine 0.41 m above the floor began to 

increase shortly after the other sensors detected helium, the rate of increase was more gradual, and the 

quasi-steady-state levels attained were only slightly more than half of those in the wheel well and upper 

portion of the engine compartment. This indicates that there was a significant falloff in helium 

concentration in the lower part of the engine compartment. In fact, the quasi-steady-state helium volume

percent of around 17.5 % is very close to the extrapolated concentration at the same height measured 
51

along the vertical array (see Figure 36). This suggests that very little of the higher concentration 

helium/air mixture trapped underneath the vehicle reached this location.

The helium concentrations trapped in the undercarriage and engine compartment of the automobile 

during the release phase indicate that hydrogen released at similar flow rates would create significant 

volumes of flammable hydrogen/air mixtures a few seconds following the start of a release. Due to the 

higher concentrations trapped within the vehicle as compared to the garage interior, the combustion 

process inside these enclosed spaces would be expected to be more vigorous since the hydrogen/air flame 

speed increases until the concentration exceeds the stoichiometric ratio. [33]

During the helium release, concentrations underneath the vehicle and within the engine compartment, 

after jumping abruptly at the start of the release, grew slowly with time before attaining quasi-steady-state

levels. The general time behaviors were similar to those observed for the sensors located along the 

vertical array inside the garage. This shows that the helium concentrations within the vehicle grew due to 

mixing of the released helium with surrounding gases in the garage for which the helium concentration 

was increasing. The quasi-steady-state inside the vehicle only developed when it developed in the garage.

The growing helium concentrations underneath the automobile and in the engine compartment as well 

as in the surrounding garage penetrated the passenger compartment and trunk of the car as is evident in 

Figure 41. However, the exchange rate was far from instantaneous. The roughly comparable helium 

concentrations at these two locations increased at considerably slower rates than elsewhere under the 

vehicle and outside and were still increasing at the end of the 4 h release period, despite the development 

of quasi-steady-state concentrations elsewhere. This shows that the passenger compartment and trunk 

behaved as partially enclosed spaces with relatively lower exchange rates with their surroundings than the 

garage, which attained a quasi-steady-state helium distribution in about two hours. Assuming hydrogen 

penetration would behave in a similar way, the results indicate that the development of flammable 

mixtures inside the passenger compartment and trunk would require longer periods than in the other 

partially enclosed spaces of the vehicle or inside the garage.

When the helium flow was shut off, the high helium concentrations underneath the vehicle and in the 

engine compartment rapidly dissipated to levels comparable to those of the lower layer inside the garage 

(compare Figure 41 and Figure 36). The most rapid drop was for the location under the vehicle (Sensor 

#14). By expanding the curve in Figure 41, it was found that the helium volume percent at this location 

dropped by half of the difference between the level present at the end of the release and the value for 

Sensor #1 in roughly 7 s. The falloff rate decreased somewhat after this, but the concentration still 

approached the external value in 30 s. The falloffs at the tops of the engine compartment and wheel well 

were somewhat slower. They initially fell to levels that were intermediate between those recorded by 

Sensors #2 and #3, and then tracked the falloff of these concentrations. Roughly 50 s was required for the 

concentrations to decrease by one half of the difference between the engine compartment and the garage 

interior at comparable heights. The sensor located in the lower front of the engine compartment measured 

helium concentrations that were a couple of percent higher than recorded by the lowest sensor in the 

garage (Sensor #1, 0.305 m above floor). This remained the case after the helium flow was stopped. This 

provides further evidence that very little of the helium trapped under the vehicle reached this location.

The helium concentration trapped inside the passenger compartment and trunk decayed much more

slowly after the helium flow was stopped than observed for other locations in the vehicle or the garage. 

This is evident in Figure 41 where the helium volume percent levels inside the passenger compartment 

and trunk rapidly became higher than those trapped under the car or in the engine compartment once the 

helium flow was stopped. The falloff rates of the helium concentration inside the passenger compartment 

and trunk were clearly slower than observed elsewhere in the vehicle. Helium concentrations were 

monitored at the locations in the garage and vehicle until 240 000 s following the start of the experiment. 

Inspection revealed that concentrations outside of the passenger compartment and trunk tracked each 

other, eventually falling to zero around 85 000 s after the end of the release. Concentrations in the 

passenger compartment and trunk, which tracked each other closely, required over 200 000 s to reach this 

level. These observations show that there was rapid gas exchange between the passenger compartment 
52

Figure 44. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/23/10 in the family room, garage, and 

outside.

and trunk, but the exchange between these volumes and their surroundings was much slower than 

between the garage interior and its surroundings.

The repeat experiment with a vehicle in the garage was run on 8/23/10. The weather was relatively 

cool and sunny. The central air conditioning for the house was still not functioning at the start of the 

experiment. A small window unit air conditioner located in the family room was used to maintain a 

nearly constant temperature of 25 oC in the family room area where the instrumentation was located. At 

approximately 13 200 s after the start of the experiment, repairs to the central air conditioning were 

completed, and the system began to cool the entire interior of the house.

Figure 44 shows temperatures recorded in the family room, garage, and outside during the 

experiment. The temperature in the family room actually increased slightly when the central air 

conditioning was initially turned on. This was likely due to recirculation of higher temperature air from 

elsewhere inside the house. Shortly afterwards, the temperature began to fall from 26 C to 21 oC over a 

period of 8 500 s. After this time the air conditioning began to cycle in the normal manner. Outside 

temperatures were similar to those inside the house, while temperatures in the garage were a couple of 

degrees higher.

The volume flow rate for helium had the same type of oscillations evident in Figure 8. The measured 

helium volume flow rate, flow time, and total volume release are included in Table 4. The helium supply 

was exhausted before the 4 h release period was complete, and, as a result, the total volume of helium 

released was slightly less than for the 8/6/10 experiment with a vehicle (59.1 m3 versus 64.3 m3

).

Figure 45 and Figure 46 show time behaviors of helium volume percent measured for the various 

heights along the vertical array in the garage and for the six sensors located inside the vehicle, 

respectively. Comparison of Figure 45 and Figure 36 reveals that the vertical profiles for the two 

experiments with vehicles were similar when the difference in helium release periods is accounted for. 

This was the case for the initial growth and the approach to a quasi-steady state during the release phase,

as well as the concentration decays during the post-release phase. The close agreement in Table 5 and 
53

Figure 45. Helium volume percent in the garage is plotted as a function of time at the eight sensor heights 

indicated along the vertical array for the 8/23/10 experiment.

Figure 46 Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/23/10 experiment.

Table 6 between the parameters used to characterize these two phases confirms the agreement. This 

suggests that helium losses from the garage occurred at similar rates for the two experiments. Fits of the
54

Figure 47.The time variation of the differential pressure between the garage interior and outside 

ambience is plotted over the time period from 0 s to 43 200 s for the 8/23/10 experiment.

helium concentrations at long times when helium volume percents had fallen to the order of 1 % gave 

decay constants and ACHgar values that were similar, but the values for 8/23/10 were slightly smaller, 

indicating slightly reduced gas exchange between the garage and surroundings.

Figure 47 shows a plot of the time variation of the differential pressure between the garage and the 

outside. There appears to have been a particularly windy period just after 7200 s. There is a leveling off 

of the helium volume percent curves in Figure 45 and a drop in concentration in Figure 46 that are

correlated with the wind increase. The pressure fluctuations and magnitudes on 8/23/10 overall were 

smaller than observed during the 8/6/10 experiment (see Figure 38). As discussed earlier, these

observations suggest that winds of the magnitude present during these experiments have measurable, but 

generally small, effects on the loss rate of helium from the garage during the release and early postrelease periods. The reduced decay rate on 8/23/10 at longer times was likely associated with the lower 

wind speeds.

The appearances of the concentration profiles for the measurement locations inside the vehicle shown 

in Figure 46 and Figure 41 were also very similar for the repeated experiments. Detailed comparisons 

showed that the quasi-steady-state helium volume percent under the vehicle (Sensor #14) reached a 

slightly higher value on 8/23/10, while those under the hood and in the wheel well were slightly lower. 

The concentration profiles measured inside the passenger compartment and trunk were nearly identical. 

Recall that the passenger compartment sensor may have been on the floor instead of at the ceiling for the 

8/6/10 experiments. If this were the case, the sensor location had little effect on the measurements.

The decay behaviors of the volume percents following the end of the release were very similar for all 

measurement locations inside the garage and vehicle. As discussed above, this suggests that helium 

losses from the garage were comparable in both experiments.

The helium volume percent measured in the family room as a function of time is shown in Figure 48. 

As in earlier experiments, the concentration began to increase shortly after the helium flow into the 

garage was started and continued until shortly after the flow stopped. As compared to the earlier

experiments (see Figure 17, Figure 23, Figure 29, Figure 35, and Figure 40), the buildup of helium 
55

Figure 48. The helium volume percent measured in the family room of the house manually (symbols) and 

electronically (line) is shown as a function of time for the 8/23/10 experiment.

Table 8. Summary of Forced Ventilation Garage Experimental Flow Parameters

Date Average He Flow 

Rate (m3/s)

Flow 

Period (s)

Total Flow 

Volume (m3)

Ventilation 

Fan On (s)

Fan Volume 

Flow Rate (m3/s) Vehicle

8/9/10 4.44 × 10-3 1378 6.1 80 0.0901 Stratus

8/12/10 4.21 × 10-3 3605 15.2 80 0.0934 Stratus

8/13/10 4.24 × 10-3 3600 15.3 100 0.0922 Stratus

8/13/10a 4.28 × 10-3 3600 15.4 101 0.1071 Stratus

8/19/10 4.43 × 10-3 4130 18.3 100 0.0910 Passat

8/19/10a 4.27 × 10-3 3600 15.4 138 0.1066 Passat

concentration was considerably reduced, except for the 9/11/08 experiment shown in Figure 17. As 

discussed earlier, a window was opened during this test, which reduced the interior helium concentration. 

During the later experiment doors were opened briefly as people entered and left the house, but these 

were not excessive compared to earlier experiments. The major differences appear to have been that the 

central air conditioning and fan were not functioning during the 8/23/10 experiment, and the temperature 

inside the house was somewhat higher. It is interesting that the helium volume percent showed a strong 

spike when the air conditioning fan was initially turned on when the repairs were completed, increasing 

by nearly a full percentage point before dropping back down. This is an indication that mixing inside the 

house was not uniform. This observation does not explain the lower interior concentration measured in 

the family room. It is considered likely that this was due to a lower exchange rate between the house and 

garage due to the house fan not operating or the garage-house temperature difference.

3.2 Experiments with Forced Ventilation of the Garage

Six experiments were run during August, 2010 in which the garage was vented with a fan in order to 

limit the buildup of helium concentration during a release. All experiments were run with a conventional 

automobile parked above the helium release location at the center of the floor. Table 8 lists the dates of 
56

Figure 49. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/9/10 experiment with forced ventilation.

the experiments, average helium volume flow rates, helium flow periods, total helium flow volumes, the 

time following the start of the helium flow when the fan was started (roughly corresponds to when the 

helium volume fraction at Sensor #8 reached 1 %), the fan volume exhaust rates, and the model of the 

vehicle parked in the garage.

All of these experiments were run during a period when the central air conditioning for the test house 

was not functioning. Since outside temperatures were generally high during the testing period, interior 

temperatures inside the house rose significantly. For the last four experiments, a window-unit air 

conditioner was used to lower temperatures in the family room where electronic equipment was housed, 

but temperatures elsewhere in the house remained 1 oC to 2 oC higher.

The initial test on 8/9/10 involved a helium release that lasted 1378 s. The exhaust fan was turned on 

80 s after the helium flow began. Figure 49 shows plots of helium volume percent versus time for the 

eight sensors located along the vertical array. Comparison of the concentration profiles with those shown 

in Figure 36 and Figure 45 for similar helium releases with the Stratus present, but without an exhaust

flow, reveals significant modifications of the helium concentration field in the garage due to forced 

ventilation. Upper-layer helium concentrations for the earlier measurements had increased to around 

14 % after 1380 s compared to the roughly 4 % value evident in Figure 49. Helium concentrations were 

still rapidly increasing without forced ventilation, while they were leveling off with the exhaust fan 

operating.

The relative vertical concentration profiles were also substantially modified. For the natural 

ventilation cases, there was a well mixed upper layer above a lower layer where the helium concentration 

decreased rapidly with height. The transition between the layers was close to Sensor #3 located 91.4 cm 

above the floor. With forced ventilation, the helium concentration in the lower layer was much more 

uniform, and the transition height between the layers was slightly higher since the concentration recorded 

by Sensor #4 at 1.219 m was just barely lower than recorded by the four higher sensors.

When the flow was halted, the helium distribution with forced ventilation responded in the same 

general way as the tests without a fan operating. Helium concentrations in the lower layer began to fall
57

Figure 50. Experimental helium volume percents (solid symbols) measured along the vertical array at the 

end of the 8/9/10 release are shown along with extrapolated values (open symbols) at the floor 

and ceiling. The solid lines are approximations of the data used to estimate the average 

helium concentration along the vertical direction.

immediately, while those in the upper layer first leveled off before starting to decay. Vertical 

concentration gradients developed, and a substantial period was required for the decaying curves to 

collapse to a single curve, with concentrations at higher sensors requiring longer periods. A major 

difference between the behaviors was the substantially faster concentration falloffs with forced 

ventilation. Roughly 1500 s were required for helium concentrations to approach zero with the exhaust 

fan operating compared to roughly 60 000 s without forced ventilation.

A number of parameters have been used to characterize the forced-ventilation experiments. These 

include the upper-layer helium volume fraction at the end of the release period, the average helium 

concentration along the vertical array at the end of the release period, the period following the end of 

release required for the helium concentration to fall to 0.2 %, and the helium concentration falloff rate 

near the end of the experiment when the helium volume fractions dropped from ≈ 0.5 % to ≈ 0.1 %.

The upper-layer concentration was estimated by inspection. The average concentration was 

determined by an approximate integration performed as follows. Upper and lower concentrations 

(assumed constant) were estimated visually from the data. The heights were then identified where the 

constant upper-layer concentration began to fall off and where the constant lower layer concentration 

began to increase. A linear concentration increase rate was assumed between the two heights. The 

resulting piecewise linear curve was then easily integrated to give the average concentration. Figure 50

shows an example for the 8/9/10 data. The falloff rate was estimated by least squares fitting an 

exponential to the experimental data for Sensor #4 over a range where the helium volume percent dropped 

from 0.5 % to 0.1 %, as shown in Figure 51. The values of these parameters determined for the 8/9/10 

test are listed in Table 9.

It was possible to obtain an alternative estimate for the helium volume percent falloff rate by 

assuming that only air entered the garage to replace gas withdrawn from the interior by the fan, any air 

that entered was instantly mixed with the interior gas, temperatures were constant, and the exhaust fan 
58

Figure 51. Helium volume percent recorded by Sensor #4 is plotted as a function of time for the 8/9/10

experiment. The red line is the result of fitting the data to an exponential decay using a 

nonlinear least squares curve fit.

Table 9. Helium Concentration Parameters for Forced-Ventilation Garage Experiments

Date Upper Layer He

Vol. % at End of 

Release

Average He Vol. 

% in Garage at 

End of Release

Time Required for He 

Vol. % to Drop to 0.2 

% at End of Release (s)

Helium Falloff Rate for 

Sensor #4 at End of 

Decay (s-1)

8/9/10 4.0 3.1 2060 1.49 × 10-3

8/12/10 4.2 3.3 -- --

8/13/10 4.3 3.4 2030 1.37 × 10-3

8/13/10a 3.7 2.7 1625 1.81 × 10-3

8/19/10 4.3 3.7 2350 1.29 × 10-3

8/19/10a 3.8 3.0 1560 1.86 × 10-3

efficiency was constant throughout the experiment. For these conditions, the garage helium concentration 

would have fallen by a factor of two when the amount of exhausted gas was equal to the interior volume, 

86.9 m3

, of the garage. Using the exhaust rate provided in Table 8, this would have required 964 s. The 

corresponding falloff rate is obtained by multiplying the inverse of this time by e-1⁄2 = 0.693. The result is 

7.2 × 10-4 s

-1

. Comparison with the measured falloff rate in Table 9 shows that the measured value was 

2.1 times larger. The most likely source of the difference between the two results is the vertical 

concentration gradient that developed during the post-release period. Since the fan was exhausting gas 

from the upper part of the garage, the exhausted gas had a higher helium concentration than the average, 

with the result that helium was extracted faster than predicted assuming instantaneous mixing.

The concentration time profiles measured at five locations inside the car during the 8/9/10 experiment 

are shown in Figure 52. Corresponding measurements for cases without forced ventilation were provided 

in Figure 41 (8/6/10) and Figure 46 (8/23/10). The initial concentration increases with forced ventilation 

were very similar to those observed with natural ventilation. The first detection of helium at the sensor 

located in the undercarriage of the vehicle (Sensor #14) was 4 s after the flow was initiated, while helium 

was first detected near the ceiling (Sensor #9) 35 s later. The initial helium volume percent jumps for 
59

Figure 52. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/9/10 experiment with forced 

ventilation.

sensors under the automobile and in the engine compartment are indistinguishable in magnitude from the 

corresponding experiments with natural ventilation. These findings are reasonable since these 

measurements were recorded prior to the fan being turned on. Note that the helium concentrations 

trapped under the vehicle correspond to hydrogen concentrations that would be highly flammable if 

ignited.

The concentrations for Sensors #9, #13 and #14 increased very slowly with time following the abrupt 

large increases when the helium flow was started. Even though the initial concentration jump for Sensor 

#12 in the lower engine compartment was much smaller, the slope of the concentration increase with time 

was similar. The slopes for the concentration increases were noticeably smaller than observed for the 

natural ventilation cases. This is consistent with the earlier conclusion based on natural-ventilation tests 

that concentrations at these locations increased as the concentrations exterior to the vehicle increased. 

The helium concentration at the sensor located near the bottom of the engine compartment (Sensor #12) at 

the end of the release was nearly a factor of 2 smaller than observed for the cases with natural ventilation 

at comparable times. When the helium flow was halted, concentrations under the vehicle and in the 

engine compartment dropped very rapidly to levels equal to the exterior helium volume percent.

As observed for the natural-ventilation experiments, concentrations inside the passenger compartment 

and in the trunk increased much more slowly during the release period and then fell off much more slowly 

in the post-release period. Helium volume fraction magnitudes at these locations were slightly reduced at 

the end of the release period as compared to the natural ventilation cases. This likely reflects the lower 

helium concentrations trapped under the vehicle and in the engine compartment as well as outside of the 

vehicle. Once the helium flow was halted, these concentrations fell off very slowly.

Temperatures measured in the family room, inside the garage, and outside the structure during the 

8/9/10 experiment are shown in Figure 53. The relatively high temperatures inside the house due to the 

absence of air conditioning are apparent. Helium volume fractions inside the family room were also 
60

Figure 53. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/9/10 in the family room, garage, and outside.

Figure 54. The helium volume percent in the family room of the house measured electronically is shown 

as a function of time for the 8/9/10 experiment.

recorded and are plotted in Figure 54. A very small increase in helium volume percent was observed 

during the release period with a maximum concentration of 0.2 % at the end of the release.
61

Figure 55. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/12/10 experiment with forced ventilation.

Figure 55 shows the concentration profiles for locations along the vertical sensor array for the 

experiment performed on 8/12/10. Once initiated, the helium flow continued until the data acquisition 

system malfunctioned 2902 s after the flow started. The helium volume flow and garage exhaust rates 

provided in Table 8 were similar but differed slightly from those used for the 8/9/10 experiment. 

Comparison of Figure 55 with the concentration profiles for the earlier experiment in Figure 49 shows 

that the time profiles during the release phase had similar appearances, with two distinct layers present. A 

detailed comparison of the helium volume percents at the time when the helium flow was halted on 8/9/10 

revealed that the helium concentrations were reduced roughly 0.2 % in the lower layer and 0.3 % in the 

upper layer for the 8/12/10 experiment. The lower values are consistent with both the lower helium 

volume flow rate and the higher fan exhaust flow rate used during the 8/12/10 experiment (see Table 8).

It is evident in Figure 55 that the helium volume percents continued to rise slowly beyond the time 

(release period of 1378 s, plotted time of 1438 s) when the helium release was terminated for the 8/9/10

experiment. The helium concentrations did reach asymptotic values after a helium flow period of roughly 

2100 s. A detailed comparison showed that the additional concentration increases between 1400 s and 

2100 s were 0.4 % at each of the sensor heights. These observations indicate that the upper layer 

concentration included in Table 8 for the 8/9/10 experiment was likely not the true asymptotic value for 

this experiment. An estimate for the asymptotic value can be obtained by adding 0.4 % to the listed value 

to give an upper-layer asymptotic concentration of 4.4 %.

Values of the parameters used to characterize the garage concentration behavior for the 8/12/10 

experiment are included in Table 9. Note that no values are included for the post-release phase since 

measurements were not recorded for this period.

The measured helium volume fractions observed inside and under the vehicle are shown in Figure 56

for the 8/12/10 test. The shapes of the curves at early times as well as the concentration levels are nearly 

identical to those shown in Figure 52 for the 8/9/10 experiment. Since the release on 8/12/10 lasted for a 

longer time, it is clear that the helium volume percents at the measurement locations in the engine 

compartment and immediately under the vehicle were nearly constant following the initial rapid increases 
62
63

Figure 56. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/12/10 experiment with forced 

ventilation.

when the helium release was started. The helium concentration in the lower engine compartment at the 

time when the 8/12/10 flow was stopped was 2.9 %. This value can be compared with the asymptotic 

value for the lower layer in the surrounding garage of roughly 2.3 %. This observation suggests that a 

small fraction of the helium in the lower region of the engine compartment came directly from the release. 

Helium concentrations inside the passenger compartment and trunk rose slowly over the entire release 

period, reaching levels around 1.6 % at the end of the release. Even after 48 min, these values were well 

below those trapped in cavities of the vehicle and in the surroundings.

The outside temperature on 8/12/10 was quite hot, and the interior temperature was nearly as high 

inside the family room as evident in Figure 57. Interestingly, temperatures inside the garage were a 

couple of degrees cooler. The time profile for the family-room helium concentration is shown in Figure 

58. The values appear to have reached a maximum of 0.2 % around 1800 s. This is very close to the 

value observed on 8/9/10 (see Figure 54). At later times the helium volume percent appeared to drop. It 

is difficult to know if this drop was real or due to a slow drift of the instrument. The zero shift apparent in 

the 8/9/10 data suggests that the baseline may have drifted.

Figure 59 shows the helium volume percent time profiles for the vertical sensor array recorded during 

the first of the two helium releases on 8/13/10. These experiments differed from the earlier two with 

forced ventilation in that a window air conditioner was used to cool the family room of the attached 

house, and the weather was considerably cooler. Comparisons with earlier results for 8/9/10 (Figure 49) 

and 8/12/10 (Figure 55) show that there were marked differences between the earlier concentration 

profiles and those in Figure 59. Concentrations in the upper layer were roughly 10 % higher for the later 

experiment, and there were noticeably larger concentration gradients over the upper Sensors #5 to #8. 

The concentrations at the 1.219 m height (Sensor #4) were much lower relative to the upper-level values 

than observed for the earlier measurements, indicating a higher interface between the layers, and the 

concentration fluctuations at this height were increased many fold.
64

Figure 57. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/12/10 in the family room, garage, and 

outside.

Figure 58. The helium volume percent measured in the family room of the house electronically is shown 

as a function of time for the 8/12/10 experiment.
65

Figure 59. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/13/10 experiment with forced ventilation.

The concentrations in the lower layer, which were nearly uniform on 8/9/10 and 8/12/10, developed 

gradients on 8/13/10 with concentrations at the lowest sensor (0.305 cm) lying well below those measured 

at 0.610 m and 0.914 m. Though close, the helium concentrations measured at 0.61 m (Sensor #2) were 

slightly higher than recorded at 0.914 m, i.e., there was a negative concentration gradient. This implies 

that the density at the lower sensor was somewhat less than for the sensor located immediately above. 

Such a density distribution is said to be “inverted” and creates a condition referred to as “unstable” since 

higher fluid tends to sink downward. Such a density inversion apparently existed only over a narrow 

range of heights above the floor. One effect of a density inversion is to increase mixing and concentration 

fluctuations. It is unclear if the high fluctuation intensities observed at 1.219 m on 8/13/10 were 

associated with the unstable density distribution.

For the 8/13/10 experiment the helium concentrations rose steeply at all levels during the initial 

1500 s of the release and then more slowly. However, unlike the two earlier experiments, the 

concentrations continued to increase slowly throughout the 1 hr release and did not asymptote as observed 

earlier. The average concentration over the three lowest sensors seemed to be roughly equal to the 

asymptoted lower-level uniform values of the earlier experiments.

The substantial changes in vertical concentration distribution between the 8/13/10 data and the earlier 

experiment on 8/9/10 become clearer when the vertical helium volume percent profile at the end of the 

release for the later data shown in Figure 60 is compared with Figure 50. 

Values used to characterize the post-release phase for the 8/13/10 experiment are included in Table 9. 

The period required for the helium volume percent to decease to 0.2 % and the concentration decay rate 

agree well with those for the 8/9/10 data.

There is no apparent correlation between the helium volume or exhaust flow rates listed in Table 8 for 

the earlier two experiments and the 8/13/10 experiment that provides an explanation for the shift in 

vertical concentration distribution or higher average concentration. This suggests that these changes must 

have resulted from a change in one or more external parameters. Figure 61 shows the temperatures 

measured in the family room, garage, and outside during the 8/13/10 experiment. As already mentioned, 

the outside temperature was relatively low, and interior and outside temperatures were close to each other. 
66

Figure 60. Experimental helium volume percents (solid symbols) measured along the vertical array at the 

end of the 8/13/10 release are shown along with extrapolated values (open symbols) at the 

floor and ceiling. The solid lines are approximations of the data used to estimate the average 

helium concentration along the vertical direction.

Figure 61. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/13/10 in the family room, garage, and 

outside.
67

Figure 62. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/13/10 experiment with forced 

ventilation.

As can be seen in Figure 53 and Figure 57, temperatures were several degrees higher, and the temperature 

differences between the garage and house were somewhat larger on 8/9/10 and 8/12/10. It seems unlikely 

that these relatively small differences are sufficient to provide an explanation for such substantial changes 

in vertical concentration distributions. The helium concentration inside the family room was not recorded 

on 8/13/10 or 8/19/10.

Unfortunately, wind measurements in the immediate vicinity of the test house were not available for 

the 2010 experiments. A review of nearby amateur historical weather records for stations located within a 

few km of NIST did provide some insight. These measurements indicated that 8/9/10 and 8/12/10 were 

relatively breezy during the experimental periods with measured winds between 2.3 m/s and 10 m/s, 

while the recorded winds on 8/13/10 were either calm or under 2.3 m/s. On the earlier days, the wind 

direction was primarily from the southwest, while on 8/13/10 the wind was coming from the northeast. 

Recall that earlier results without ventilation indicated that wind speed and direction affected helium 

mixing inside and loss rates from the garage, with higher winds and winds from the south favoring 

mixing and loss of helium. The observed differences between the 8/13/10 experiment and the two earlier 

tests are consistent with such wind effects, thus providing a possible explanation for the differences.

Figure 62 shows the helium volume percents recorded by the various sensors placed under and inside 

the Stratus for the 8/13/10 experiment. The profiles are similar to those observed in the earlier 

experiments with forced ventilation (see Figure 52 and Figure 56). There are some variations in 

maximum concentrations achieved and whether or not the values reached asymptotic values.

A second test was run on 8/13/10 that is referred to as 8/13/10a. As can be seen from values included 

in Table 8, the helium release rates were very close for the two tests, while the fan exhaust rate was 

increased by about 16 % for the second test. The concentration time profiles along the vertical sensor 

array are shown in Figure 63 for the 8/13/10a experiment. The relative vertical ordering of the time 

profiles was similar to those observed earlier in the day (compare with Figure 59), but the profile shapes 

differed somewhat. For the first experiment the helium concentrations continued to increase slowly until 
68

Figure 63. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/13/10a experiment with forced ventilation.

the end of the helium release following the initial rapid rises, while they reached asymptotic values for the 

later experiment. More significantly, helium volume percents at the end of the release period for the 

second experiment were reduced by 0.6 % to 0.7 % at all heights. These reductions in concentration 

between the 8/13/10 and 8/13/10a experiments are evident by comparing the vertical profiles of the 

helium volume percents at the end of the release on 8/13/10a shown in Figure 64 with the similar profile 

in Figure 60 for the 8/13/10 test. The average concentrations along the vertical direction listed in Table 9

for the two experiments confirm this difference, dropping from 3.4 % to 2.7 %. The 25 % decrease in 

average concentration is larger than the 16 % increase in the fan exhaust rate.

The results indicate that there were higher volume exchange rates between the garage interior and its 

surroundings for the second experiment on 8/13/10. The increased volume exchange rate was also 

reflected in the shorter period required for the helium volume percent to drop to 0.2 % during the postrelease period for the 8/13/10a experiment, 2030 s versus 1625 s. It should be kept in mind that the 

average concentrations were different at the ends of the releases. Perhaps more definitive is the observed 

increase in the helium falloff rate at Sensor #4 as the concentration fell from 0.5 % to 0.1 %, increasing 

from 1.37 × 10-3 s

-1 to 1.81 × 10-3 s-1 for the 8/13/10 and 8/13/10a experiments, respectively (see Table 9).

Figure 65 is a plot of the temperatures inside the house and garage and outside during the period of 

the 8/13/10a experiment. The outside temperature had risen slightly from the time of the earlier 

experiment (see Figure 61), while the temperature inside the house had fallen. The garage temperatures 

remained roughly constant. Given the small changes, it seems unlikely that temperature variations 

provide an explanation for differences between the two 8/13/10 experiments. Wind measurements 

recorded by nearby amateur weather stations indicated that conditions remained relatively calm during 

both experiments on 8/13/10. These observations lead to a conclusion that differences in vertical helium 

concentration distributions at the end of release periods for the two 8/13/10 experiments were the result of 

the change in fan exhaust flow rate.

The helium concentrations measured at the various locations inside the Stratus during the 8/13/10a 

experiment are plotted in Figure 66 and can be compared with those for the 8/13/10 test in Figure 62. The 

two sets of time profiles have very similar time dependencies and magnitudes. Careful comparison
69

Figure 64. Experimental helium volume percents (solid symbols) measured along the vertical array at the 

end of the 8/13/10a release are shown along with extrapolated values (open symbols) at the 

floor and ceiling. The solid lines are approximations of the data used to estimate the average 

helium concentration along the vertical direction.

Figure 65. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/13/10a in the family room, garage, and 

outside.
70

Figure 66. Helium volume percent is plotted as a function of time at the indicated locations in the 

automobile centered over the helium release location for the 8/13/10a experiment with forced 

ventilation.

reveals that, similar to the profiles along the vertical sensor array in the garage, the profiles under the auto 

and in the engine compartment achieved asymptotic values for the later experiment, while the same 

profiles recorded earlier continued to increase slowly over the entire release phase. These small 

differences are likely due to the changes in helium volume percents immediately outside the vehicle. As 

discussed previously, the surrounding helium concentration distribution influences those inside the 

vehicle due to the strong mixing of released helium with outside gas. Since the concentrations exterior to 

the vehicle and their rate of increase were small, their influence was relatively weak.

Two additional experiments, 8/19/10 and 8/19/10a, were run during which forced ventilation was 

used. For these tests the Stratus was replaced with a 2003 Volkswagen Passat. The underside of the 

Passat has a different configuration than the Stratus, which is open underneath the vehicle. Much of the 

undercarriage of the Passat is protected by plastic splash shields which extend down to the level of the 

outer edge of the vehicle. The shields do not extend all of the way to the center of the undercarriage, and 

there is a narrow recessed channel containing the engine exhaust lines running along the center from the 

gasoline tank at the rear and connecting into the engine compartment at the front. The different underside 

configuration of the Passat is likely to have directed more helium directly to the sides of the vehicle. It 

was not possible to place a helium sensor under the Passat in the same location as for the Stratus. As an 

alternative, it was centered on top of the left front wheel strut 18.4 cm above the floor of the garage.

The concentration profiles for locations along the vertical sensor array are shown in Figure 67 for the 

8/19/10 test. Note that this release lasted 4130 s, 530 s longer than the 3600 s periods used for the two 

8/13/10 tests, and the helium volume flow rate was roughly 4 % higher. The concentration profiles had 

behaviors that were intermediate between those observed for the two forced-ventilation experiments on 

8/9/10 and 8/12/10 and the two experiments on 8/13/10. Similar to the first two experiments, the upper 

layer was well mixed, and the helium concentration for the 121.9 m height was close to the upper layer 

value, even thought the concentration fluctuations were greater than observed for these experiments. On 

the other hand, the lower layer developed concentration gradients, but these were somewhat reduced 
71

Figure 67. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/19/10 experiment with forced ventilation.

compared to those observed on 8/13/10. The weak density inversion was still evident between Sensor #2 

and Sensor #3. Figure 68 shows the experimental helium volume percent values at the end of the release 

plotted as a function of height. The lines are approximations for the concentration distribution that were 

used to estimate the average value of 3.7 % included in Table 9.

Temperatures recorded during the 8/19/10 experiment are shown in Figure 69. The temperature 

differences between the garage and the house interiors and the outside ambient were not very large. It is 

unlikely that these small variations were responsible for observed differences between experiments.

The average helium volume percent estimated for the 8/19/10 experiment was the largest of the five 

experiments discussed thus far that employed an exhaust fan. This was likely due to the higher helium 

volume flow rate and longer release period used for this experiment. With the exception of the 8/13/10a 

experiment, the exhaust volume flow rates were similar to those used in the earlier measurements.

The concentration profiles at locations inside the Passat for the 8/19/10 experiment are shown in 

Figure 70. The profiles appear very different than earlier experiments primarily because Sensor #14, 

which was located under the vehicle, recorded a maximum helium volume percent around 1.5 % instead 

of the roughly 30 % measured when the tests were run with the Stratus parked in the garage. This large 

difference was due to the change in the sensor location mentioned above. The placement of the sensor on 

the wheel strut apparently moved it out of the layer of helium trapped underneath the car. The helium 

concentration measured by this sensor overlaps that recorded by Sensor #12, which was located near the 

bottom-front of the engine compartment at the base of the radiator. Helium volume percents recorded in 

the upper part of the engine compartment and in the passenger side front wheel well had similar time 

behaviors and magnitudes to those observed for the Stratus, but the levels at the top of the engine 

compartment were somewhat higher for the Passat. Helium concentrations inside the passenger 

compartment and trunk of the Passat grew in a similar way as observed with the Stratus, increasing 

slowly during the helium release and then decaying much slower than observed inside the garage or under 

the vehicle. After 4130 s of helium release, the volume percent in the passenger compartment and trunk 

had reached levels of 2.1 % and 2.7 %, respectively. Comparable values were 1.7 % at both locations for 
72
73

Figure 68. Experimental helium volume percents (solid symbols) measured along the vertical array at the 

end of the 8/19/10 release are shown along with extrapolated values (open symbols) at the 

floor and ceiling. The solid lines are approximations of the data used to estimate the average 

helium concentration along the vertical direction.

Figure 69. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/19/10 in the family room, garage, and 

outside.
74

Figure 70. Helium volume percent is plotted as a function of time at the indicated locations in the Passat

centered over the helium release location for the 8/19/10 experiment with forced ventilation.

the two 8/13/10 experiments. Apparently, the transport of helium between the surroundings and the 

compartments was slightly higher for the Passat.

The helium concentration profiles along the vertical sensor array are shown in Figure 71 for the 

second test, 9/19/10a. This experiment employed a 17 % higher exhaust flow rate than the earlier test

(see Table 8). Comparison with Figure 67 reveals substantial differences between the two vertical

profiles. The later data yielded a vertical concentration distribution which appeared to be very similar to 

those recorded on 8/9/10 and 8/12/10. A fairly well mixed upper layer (a weak concentration gradient is 

evident) and well mixed lower layer were present in contrast to the nonuniform lower layer observed 

earlier in the day. Helium volume percents recorded at Sensor #4 (1.219 m) were closer to the upper 

layer concentration and had fluctuations that were substantially reduced.

The change in vertical concentration profiles can be seen more clearly by comparing the profile at the 

end of the release for the 8/19/10a data shown in Figure 72 with that for 8/19/10 in Figure 68. The solid 

lines in Figure 72 are an approximation to the concentration distribution which was used to estimate the 

average concentration at the end of the helium release of 3.0 % listed in Table 9. The average value is

23 % smaller than the value estimated for the 8/19/10 data. A similar reduction of 26 % was obtained 

when the exhaust volume flow rate was increased by a comparable value for the two experiments on 

8/13/10. The fall off rate for Sensor #4 over the 0.5 % to 0.1 % helium volume percent range for the 

8/19/10a test is included in Table 9. This value is in excellent agreement with that measured for the 

9/13/10a experiment, which was run using a nearly identical exhaust flow rate.

The first experiment on 8/19/10 was run in the morning. The second was started in early afternoon. 

As is evident by comparing Figure 73 with Figure 69, the outside temperature rose rapidly over the day. 

Even though the temperature was rising, there was only a 2.5 oC difference between the garage 

temperature, which was only slightly warmer than the family room in the house, and the outside. Pressure 

differences generated as the result of these temperature differences were likely negligible compared to 

those associated with the fan-induced flow.
75

Figure 71. Helium volume percent is plotted as a function of time for the eight sensor heights indicated 

for the 8/19/10a experiment with forced ventilation.

Figure 72. Experimental helium volume percents (solid symbols) measured along the vertical array at the 

end of the 8/19/10a release are shown along with extrapolated values (open symbols) at the 

floor and ceiling. The solid lines are approximations of the data used to estimate the average 

helium concentration along the vertical direction.
76

Figure 73. Temperatures measured using thermocouples (symbols) and thermistors (lines) are plotted as a 

function of time for measurements recorded on 8/19/10a in the family room, garage, and 

outside.

The helium volume percent time profiles measured at various locations under and within the Passat 

are plotted in Figure 74 for the 8/19/10a experiment. The profiles are similar to those measured for the 

same locations in the morning and shown in Figure 70, but there are some subtle and interesting 

differences. For both experiments, the helium volume percents recorded in the lower part of the engine 

compartment (Sensor #12) and under the vehicle (Sensor #14) overlap at the end of the release period. 

However, the approximate values at this time were reduced from 1.6% to 1.2 % for the later test. The 

measurements inside the passenger compartment and trunk also differed slightly. Measurements in the 

trunk were the same after 3600 s of helium release for the two releases, reaching values of 2.7 %. 

However, as can be seen in the figures, the concentrations in the passenger compartment were lower, 

2.1 %, for the 8/19/10 run and higher, 3.1 %, for 8/19/10a. Interestingly, for the latter case, the helium 

concentration in the passenger compartment dropped relatively quickly at the end of the release period 

and fell below the value in the trunk so that the long period of slow decay looked very similar to that for 

the 8/19/10 test. It is difficult to explain the reasons for these observations, but they were likely 

associated with the different vertical concentration distribution in the surrounding garage.

The change in vertical concentration distribution between the two experiments on 8/19/10 are similar 

to that identified between the 8/9/10 and 8/12/10 experiments and the two tests performed on 8/13/10, 

with the 8/19/10a results being most similar to the results on 8/9/10 and 8/12/10. Wind effects have been 

tentatively identified as having been at least partially responsible for the variations among the earlier 

measurements. Local weather conditions recorded by several nearby amateur weather stations for 8/19/10 

were reviewed. There were inconsistencies between stations, but these historical records generally 

indicated increasing winds between the two experiments on 8/19/10, with the wind shifting from the north 

to the west. These observations are consistent with a conclusion that higher winds from the south and 

west resulted in more uniform concentration distributions in the garage during experiments with active 

ventilation.
77

Figure 74. Helium volume percent is plotted as a function of time at the indicated locations in the Passat

centered over the helium release location for the 8/19/10a experiment with forced ventilation.

4 Discussion

4.1 Naturally Ventilated Garage Experiments

Six experiments with natural ventilation of the garage performed over a two year period have been 

described. Both momentum- and buoyancy-dominated helium flows were released into the garage. Two 

experiments were done with a vehicle parked over the buoyancy-dominated helium source. Helium 

volume percents were recorded as a function of time and height at a location centered in the left-rear 

quadrant of the garage. Figure 9, Figure 18, Figure 24, Figure 30, Figure 36, and Figure 45 show plots of 

helium volume percent for the six tests spanning times covering the helium release and post-release 

periods. Comparisons showed that the general appearances of the concentration time behaviors were 

similar for all of these tests.

At the start of a release there were relatively rapid increases in measured concentrations, but after

roughly two hours the rates fell dramatically, and the concentration profiles developed quasi-steady-state 

behaviors that persisted until the helium flow was halted. The development of common curves for the 

highest sensors showed that well-mixed upper layers developed during each of the experiments. The

quasi-steady-state helium volume percents for each experiment are listed in Table 5. The values are 

similar for each test, yielding an average helium volume percent of 22.6 % with a standard deviation of 

0.4 %. The standard deviation is only slightly larger than the estimated uncertainty of 0.3 % for an 

individual test. Comparisons of quasi-steady-state upper-layer helium volume percent profiles with time 

records of wind speed and direction and differential pressure between the garage interior and outside 

indicated a weak dependency on these parameters that was sufficient to account for variations of this 

magnitude. The 2008 experiments had slightly higher quasi-steady-state concentrations, but the 

differences were small and of the same order of magnitude as the variations due to changing wind

conditions. The data were checked to see whether there was an identifiable dependence on the helium 

volume flow rate or total volume of helium released. No such dependence was identified.
78

Figure 75. Helium volume percent is plotted as a function of time for sensors located 2.134 m above the 

floor immediately above the helium release location and in the front-right quadrant of the 

garage for the 7/29/10 experiment.

In our earlier reduced-scale study, it was shown that horizontal gradients were quickly smoothed out 

by flows that developed when vertical density profiles varied across the garage. [1] As a result, the 

measurements recorded along the vertical array should be representative of the vertical helium volume 

percent profiles elsewhere in the garage. This assumption was used to estimate quasi-steady-state average 

helium volume percents for the entire garage.

The hypothesis that vertical concentration profiles did not vary with lateral location was checked 

using measurements made along the flow release vertical centerline and in the front-right quadrant of the 

garage during the 7/29/10 and 8/2/10 experiments (see Figure 26 and Figure 31). In these figures it is 

evident that the concentrations measured at 1.219 m and higher above the floor along the plume centerline 

and at the right-front height of 2.134 m had similar values. The fluctuations and slightly higher helium 

concentrations present along the plume axis introduce some ambiguity during the releases, but, as evident 

in the figures, these fluctuations dissipated, and the local concentrations rapidly recovered immediately 

after the helium flow was halted. Inspection showed that at these times all of the helium volume fractions 

at the upper-layer locations above the release point had nearly identical values. Using the results for 

locations 2.134 m and higher, characteristic concentrations were obtained in the same way as for 

measurements along the vertical array. The results were 22.5 % ± 0.1 % for the 7/29/10 experiment and 

22.9 % ± 0.1 % for the 8/2/10 experiment. Note that the small variations observed indicate that the upperlayer helium volume fractions above the release point and in the front-right quadrant of the garage were 

identical. Comparison with the results in Table 5 shows that the estimated helium volume percents for 

the center and right-front upper layer were 0.2 % higher for both experiments. This difference is well 

within the variations observed and indicates that the assumption that horizontal concentration gradients 

were negligible is valid.

Additional confirmation of a horizontally uniform upper layer is obtained by directly comparing the 

7/29/10 helium concentrations recorded by the sensors located 2.134 m above the helium release location 

and at the same height in the right-front quadrant of the garage in Figure 75. The slightly higher 

concentration and fluctuations above the release location as compared to the right-front position were due 
79

to the presence of the plume during the helium release. As soon as the helium flow stopped, the curves 

for the two locations became completely overlapped and remained so as the helium concentration fell off.

Average quasi-steady-state helium volume percent values for the garage are listed in Table 5. The 

average for the six tests was 19.6 % with a standard deviation of 0.6 %. The variations between tests are 

relatively small, but larger than the variation in the uniform upper-layer values. While the two highest 

quasi-steady-state upper-layer helium volume percents were recorded for the 2008 experiments with 

helium release from a tube, the corresponding average values for the garage were the lowest. Comparison 

of the vertical concentration profiles indicates the differences were at least partly due to variations in 

concentration gradients in the lower part of the garage. Due to the spacing of the sensors, it was difficult 

to identify the exact location of the interface between the uniform upper layer and the lower region where 

the concentration fell off. Comparisons of the vertical profiles for the 2010 experiments showed that the 

interface did move lower in the garage for the experiments with a vehicle present as compared to those 

without. The average concentration in the garage also changed as the interface height varied.

The measurements indicate that uniform upper layers formed during the helium releases for all six 

experiments. Based on the Froude numbers, the helium flows for the experiments without a vehicle in 

2008 were momentum dominated when they reached the ceiling, while those in 2010 were buoyancy 

dominated. Both types of flow formed uniform upper layers with interfaces located near 0.9 m. Cariteau 

et al. [13], Merilo et al. [16] and Ekoto et al. [18] reported that momentum-dominated upward flows favor

the formation of well mixed upper layers, while buoyancy-dominated flows favor the formation of 

stratified upper layers. These findings differ from the current work where both types of helium sources 

generated well mixed upper layers.

The concentration measurements along the centerline of the release for the experiments without a 

vehicle provide insight into the reason for the different conclusion. These measurements (see Figure 26, 

Figure 31, and Figure 75) reveal that the plume rapidly entrained surrounding gas and was highly mixed 

by the time it reached the sensor located 0.67 m above the floor and that helium concentrations inside and 

outside the plume were very similar. In contrast, Cariteau et al. showed that sensors placed near the 

ceiling inside their buoyancy-dominated flows recorded significantly higher helium concentrations than 

observed for locations at the same height outside the plume. [13] The different plume mixing behaviors 

provide the likely explanation for why the upper layers displayed different stratification behaviors.

Cariteau et al. considered helium flows having Fr = 0.44 and 1.29. [13] These values are considerably 

higher than the Fr = 0.075 used for the releases from the 0.305 m × 0.305 m flow conditioner in the 

current experiments. Merilo et al. reported that the Froude number for their buoyancy-dominated flow 

was 62. [16] This value suggests that the flow was momentum-dominated at the release point. However, 

Merilo et al. argued that the flow became primarily buoyancy dominated by the time the flow reached the 

ceiling. The corresponding value for their momentum-dominated case was Fr = 664. Apparently, the 

development of upper-layer stratification within a garage depends strongly on Fr, with maximum 

stratification occurring for Fr around one and more complete mixing occurring for Fr much greater than 

or much less than one.

The results in Table 5 indicate that slightly more than 2/3 of the volume of helium released over a 

roughly 4 h period was lost from the garage during the releases. Since quasi-steady-state helium volume 

percents, HeVol%ss, were achieved, it is possible to estimate the effective air changes per hour rate, 

ACHeff,ss, for the garage at the end of the helium release period. The helium volume percent for the quasisteady state can be written as

 % 100,

, ,

, ×















+ =

He ss air ss

He ss

ss Q Q

Q HeVol (6)

where QHe,ss and Qair,ss are the steady-state volume flow rates of helium and air into the garage, 

respectively. Based on results listed in Table 5, a characteristic quasi-steady-state helium volume 

percent was 20 %. Substituting this value in Eq. (6) leads to Qair,ss = 4QHe,ss. A good estimate for QHe,ss

from Table 4 is 4.4 × 10-3 m3

/s, which leads to a value of 0.018 m3/s = 63 m3/h for Qair,ss. Dividing by the 
80

garage volume gives ACHeff,ss = 0.73 h-1. This value is roughly a factor of two greater than ACHgar values 

measured using tracer gases.

The presence of a helium/air mixture is expected to lead to a higher gas exchange rate than when only 

air is present. Since helium has a lower density than air, its presence inside the garage leads to increased 

hydrostatic pressure differences between the interior and exterior similar to those created by higher 

temperatures inside the garage. An average helium volume percent of 20 % creates a density difference 

equivalent to a temperature difference of 60 oC. This value is much greater than normal temperature

differences between the garage interior and either the house or outside. The net result should be a faster 

exchange rate than when only air is present inside the garage. Another factor is the potential for helium to 

diffuse rapidly through gypsum drywall. In work described elsewhere, it was demonstrated that such 

diffusion was a significant removal mechanism for helium from a 1⁄4-scale model of a two-car garage. [5] 

This was the case even when the drywall was primed and painted, as was the case for the garage in the 

current study. Due to uncertainties concerning the leak distribution for the garage, it is unclear what the 

relative roles of the two loss mechanisms were, but it seems likely that both contributed.

Helium volume percent measurements inside the house provide an indication of where a portion of 

the helium loss from the garage ended up. Since previous work had shown that with the air conditioning 

fan running, the volume of the entire house was well mixed in about ten minutes [30,31] and the helium 

releases lasted roughly 4 h, it was a good approximation to assume that any helium entering the house 

was uniformly mixed in the interior volume. Based on this, it was possible to estimate the helium volume 

in the house at any given time. Measured helium volume percents in the family room are listed in Table 7

for the six experiments along with estimates for the total helium volume in the house at the end of a 

release and the corresponding fraction of total helium released present in the house. Recall that results for 

9/11/08 and 8/23/10 were considered suspect since a window was opened during the earlier experiment,

and the air conditioning was not functioning during the experiment on 8/23/10.

Considerable scatter is evident for the volumes and fractions of released helium present in the house 

at the end of the release period, with values for the four most reliable tests ranging from 10.9 m3 to 

16.3 m3 and 0.17 to 0.29, respectively. The relative scatter is much greater than for comparable values 

included in Table 5 for the garage. The larger scatter suggests that transport of helium from the garage to 

the house interior and helium losses from the house were more sensitive to some experimental 

parameter(s) than was the total helium loss from the garage.

The helium volumes present in the house at the end of the releases were comparable (ratios ranging 

from 0.5 to 1 for the six tests and 0.6 to 1 for the four tests considered more reliable) to those inside the 

garage. This indicates that anywhere from 30 % to 40 % of the helium loss from the garage during a 

release in the garage ended up inside the house at the end of a release. This can be compared with the 

percentage of the wall surface area between the garage and house relative to the total interior surface area 

for the garage walls and ceiling of 17 %. Presumably, the remainder of the helium was loss to other 

interior spaces, e.g., attic spaces, and to the outside ambience with lower efficiency.

Next, we focus on helium concentration behavior within the garage during the post-release period. 

The general concentration behaviors observed following the end of a release were similar for all six 

experiments, as can be seen in Figure 9, Figure 18, Figure 24, Figure 30, Figure 36, and Figure 45. As 

evident in Figure 11 for the 9/11/08 experiment, the first effect of halting the helium flow was that helium 

concentrations at the three lowest sensor locations began to drop almost immediately, while 

concentrations in the well-mixed upper layer did not immediately respond. As additional time passed, the 

concentrations at the higher sensor locations began to roll off, with the falloff beginning later for higher 

sensors. Values of the period required for the helium concentration at the highest sensor to start to fall 

were summarized in Table 6. Substantial variations are evident. The initial falloff rates were also slower 

for higher locations.

The roll-off curves in the post-release period were very distinctive. As time passed, helium 

concentrations at the sensors decayed in such a way that all of the volume percents approached a common 

value, with measurements at successively higher locations requiring longer periods to reach the common 

curve. As evident in Figure 75, which shows the helium volume percent time behaviors for sensors 
81

located 2.134 m above the floor for the 7/29/10 experiment, there was a distinct change in the helium 

volume percent falloff rate when the concentration for a given sensor approached the common curve. 

This change in rate was apparent for upper-layer sensors in all of the experiments. The time required for 

all of the concentration decays to collapse and the helium levels at this time are listed in Table 6. As 

discussed earlier, comparison of the decay behaviors for the two 2008 tests (see Figure 9 and Figure 18) 

suggested that the concentration falloff rates were comparable and that differences in the collapse times 

resulted from the decays collapsing to different concentrations. This conclusion does not explain all of 

the variations present in Table 6. As an example, the helium volume percents when a single decay 

developed for the 8/2/10, 8/6/10, and 8/23/10 experiments were nearly identical to those for the 9/11/08 

test, but the period required for the volume percent collapse was much longer for the earliest test.

One of the more interesting aspects of the concentration fall off behaviors concerned the observed 

time dependence at the lowest sensors. For the 2008 experiments, the helium levels at the 0.152 m sensor 

dropped rapidly immediately following the end of the release from just above 5 % to a value between 3 % 

and 4 %. After reaching this level, the concentration remained essentially constant until all of the curves 

for the higher sensors collapsed to this value. Two slightly different behaviors were observed for the 

repeated releases without a vehicle present in the garage in 2010. These tests had sensors located at 0.051 

m and 0.305 m on the vertical array. For the 7/29/10 test the helium concentration at 0.305 m during the 

immediate post-release period behaved in a similar way to that observed for the lowest sensor at 0.152 m 

in the 2008 experiments, while the concentration near the floor actually increased as the higher 

concentration curves were collapsing. During the 8/3/10 run the helium concentration at the 0.051 m 

height dropped quickly to a nearly constant value close to the concentration present when all of the higher 

curves collapsed to a single curve, while the measurements at 0.305 m approached the collapsed curve 

more slowly. These observations suggest that the mixing behavior in the lower part of the garage 

depended on some external parameter that varied between the two experiments. Such a dependence is

also consistent with differences in the plume mixing behavior between the two experiments described 

earlier. These findings suggest that more intense mixing was taking place in the lower layer during the 

7/29/10 experiment.

Helium volume percent decay constants and corresponding ACHgar values calculated for Sensor #4 

when concentrations were less than 1 % are included in Table 6. ACHgar ranged from 0.11 h-1 to 0.33 h-1

. 

These values are 2.2 to 6.5 times smaller than the ACHeff estimated above for the quasi-steady-state 

periods at the end of the helium release phase. The higher ACHeff values require that gas exchange rates 

into and from the garage did not remain constant during the post-release phase. Even so, if the relative 

changes were the same for all of the experiments, it would have been reasonable to expect an inverse 

correlation between the times required for the concentration decay curves to collapse to a common curve 

and the ACHgar values since faster exchange rates should lead to more rapid mixing and a faster decay. 

As evident from Table 6, this assumption is consistent with the results for the 7/29/10 data, which had the 

fastest collapse to a common curve and the largest measured ACHgar value. However, this was not the 

case for the 9/11/08 and 9/12/08 experiments. The longest collapse period was measured for the 9/11/08 

data, but this experiment yielded an ACHgar more than twice as large as that for the 9/12/08 data. The 

extremely low ACHgar value determined from the 9/12/08 data was accompanied by a time of collapse to 

a single decay curve that was close to the median value for all of the experiments and was similar in 

magnitude to three of the other experiments. These observations suggest that gas exchange rates not only 

decreased during the post-release phase but that there were substantial variations in the relative changes 

between experiments.

As already discussed, a plausible explanation for these observations was suggested by considering the 

wind behaviors recorded during the 9/11/08 and 9/12/08 experiments. It was argued that faster exchange 

rates were favored by higher wind speeds and winds from the south as opposed to the north. The lack of 

wind data in the immediate vicinity of the house limited our ability to further test this conclusion using 

the 2010 results.

The period required for the helium concentration to become uniform throughout the garage during the 

post-release phase was on the order of 4 h. Experiments in the 1⁄4-scale garage showed that molecular 
82

diffusion was capable of smoothing out concentration gradients over the entire height on timescales of 

roughly one hour. [1] The similarity of the mixing times involved suggests that molecular diffusion was 

playing a role in the observed mixing inside the garage during the post-release period.

The distribution of leaks from the garage and their characteristics were unknown. To our knowledge 

there have been no previous investigations of release of large amounts of buoyant gas into an actual 

garage attached to a house in which the concentration measurements have been made. For this reason, it 

is not known if the observations summarized here are characteristic of all garages or will vary 

substantially from garage to garage.

Our recent NIST Technical Note is the only study of which we are aware that systematically 

investigated the effects of vent placement and size on helium losses from a volume. [1] These 

experiments were done with either one or two vents located in one wall of the 1⁄4-scale two-car garage. 

For a 4 h release with vents near the top and bottom of the wall, a quasi-steady-state helium distribution 

developed after roughly two hours, similar to the experiments shown here. In contrast, however, there 

was a uniform concentration layer only near the ceiling, and the quasi-steady-state helium volume percent 

decreased with height. When the helium release ended, concentrations at each sensor height began to fall 

immediately. These behaviors were very different than observed in the current experiments. At long 

times the concentrations of helium became quasi steady, similar to the observations here, but the collapse 

for different heights was concurrent, not sequential as in the current experiments. The configuration of 

the two vents in the reduced-scale garage created significant hydrostatic pressure differences between the 

interior and ambience that generated strong flows out of the enclosure at the top and into the enclosure at 

the bottom when high interior helium concentrations were present. The result was a strong recirculating

interior flow that generated the observed stratification until the helium levels fell sufficiently to reduce the 

hydrostatic pressure difference. This induced flow was primarily responsible for the observed 

concentration behaviors during the release and the immediate post-release periods. The observed 

differences between the reduced- and full-scale experiments provide strong evidence that a similar vent

distribution was not present for the garage experiments. Reduced-scale experiments were also performed 

with single vents in the center of the front wall. The resulting release and post-release concentration

distributions and time behavior were totally different than observed in the current experiments.

In the absence of knowledge of the actual leak distribution, it is only possible to speculate about the 

mixing flows in the garage based on the observed concentration distributions. The experiments show that 

during a release, a stable uniform layer of mixed helium and air formed in the upper roughly two thirds of 

the garage, while a lower concentration region with significant mixing between air from outside and the 

lower-layer gases existed in the approximately lower one third of the volume. The mixing and air 

incursion suggest that either a pass-through flow containing large amounts of air entered through at least 

one wall in the lower part of the garage and exited through one or more different walls or that a 

recirculating flow with air entering near the floor and lower-layer gas mixture exiting near the top of the 

lower layer had developed in a single wall. The relatively rapid development of a quasi-steady-state 

concentration distribution suggests that large amounts of air were entering the garage. This conclusion is 

supported by the relatively high values of ACHeff,ss estimated above for the gas volume exchange rate 

between the garage interior and its surroundings.

The experiments also indicated that a large fraction of the helium loss from the garage was 

preferentially transported into the house. While it is possible that gas from the house interior was also 

flowing back into the garage, it seems more plausible that the make-up flow consisted primarily of air 

entering through the garage walls exposed to the outside ambience. If this hypothesis is correct, it 

requires that the differential pressure between the garage and house was negative. Unfortunately, this 

measurement was not recorded during the experiments.

As mentioned earlier, rapid helium and hydrogen loss from the 1⁄4-scale garage was observed when 

the front wall was replaced with gypsum drywall. [5] Both bare and primed and painted walls were tested. 

Priming and painting slowed down, but did not eliminate the diffusion. A fan was used to mix the gases 

so that the helium volume fraction measurements could be more easily used to derive effective diffusion 

coefficients for the gases passing through the walls. Additional experiments not described in [5] were
83

Figure 76. The falloff of helium volume percent in the family room of the house following the end of the 

helium release inside the garage on 7/29/10 is fit to an exponential curve using a least squares 

curve fitting procedure.

done without the fan. For these experiments, strong vertical concentration gradients developed, with the 

largest variations near the ceiling. The absence of such gradients in the current experiments provides 

evidence that diffusion of helium through the walls did not play a large role in the experiments. It is not 

currently understood why diffusion should have been less important in the current experiments.

The experiments where helium concentration inside the house was monitored continuously were used 

to characterize the loss of helium from the house after the helium flow inside the garage was halted. 

Figure 29 shows data for 7/29/10. In Figure 76 the decay portion of the curve following the end of the

helium release has been fit by a least squares procedure assuming the decay was exponential. The 

resulting time constant was 5.50 × 10-5 s

-1

, which corresponds to an air change per hour rate for the house, 

ACHhou, of 0.20 h-1

. The R2 parameter of 0.9974 for the fit indicates the curve was well represented by an 

exponential. Inspection of Figure 76 revealed that the experimental data fell slightly faster than the 

calculated fit at long times. The measured ACHhou is well within the range of measurements reported for 

the house after it was refit. [31]

A similar analysis for the repeat experiment on 8/2/10 yielded a time constant of 7.11 × 10-5 s

-1

, 

giving ACHhou = 0.26 h-1

. This value was slightly higher than for the 7/29/10 experiment. The opposite 

order was found for the long-time ACHgar values for the garage (see Table 6). The corresponding results 

for the 8/6/10 data were substantially different, with a time constant of 2.62 × 10-4 s

-1 and ACHhou = 

0.94 h-1

. The much higher loss rate was due to windows being opened in the house shortly after the 

helium release was completed. A similar analysis was done for the 8/23/10 experiment even though 

conditions in the house were likely modified by the absence of air conditioning and fan recirculation until 

just before the helium flow into the garage was halted. The time constant for the helium volume percent 

falloff was 8.54 × 10-5 s

-1

, corresponding to ACHhou = 0.31 h-1. These values are similar to those for the 

7/29/10 and 8/2/10 experiments and agree with earlier measurements for the house. [31] This suggests 

that turning on the air conditioning and cooling the house (see Figure 44) had a minimal effect on the 

house air exchange with its surroundings.
84

The ACHhou values were very similar in magnitude to corresponding values listed in Table 6 for the 

garage. This finding reinforces the earlier conclusion that the air tightness of the garage studied here was 

unusually high, since previous investigations had shown it is not unusual for garages to have ACHs ten 

times higher than measured for an attached house.

4.2 Garage Experiments with Forced Ventilation

The six experiments employing forced ventilation were performed in 2010 during a period when the 

central air conditioning for the attached house had malfunctioned, and the whole house fan was not 

operating. The initial four experiments were done with the Stratus parked over the helium release 

location, while the Passat was used for the last two. The helium flow and fan exhaust rates for each 

experiment are listed in Table 8, while parameters used to characterize mixing during the release and 

post-release periods are included in Table 9.

Four of the experiments (8/12/10, 8/13/10, 8/13/10a, and 8/19/10a) had helium release rates that were 

between 4.2 × 10-3 m3

/s and 4.3 × 10-3 m3/s while two (8/9/10 and 8/19/10) had slightly higher rates near 

4.43 × 10-3 m3

/s. Four of the experiments were run with exhaust fan settings that generated flows of 

≈ 0.092 m3

/s. This value was chosen based on calculations with an analytical model [8] of mixing in the 

garage in order to limit buildup of helium in the enclosure to volume percents less than the lower 

flammability limit of hydrogen (4 %) [33]. The two remaining experiments were run with higher exhaust 

volume flow rates of 0.107 m3

/s, which was the highest flow rate possible with the fan used for the 

experiment.

The results listed in Table 9 show that the upper-layer concentrations attained quasi-steady-state 

values of just over 4 % with the lower exhaust flow rates, but that average values along the vertical 

direction were considerably less than 4 %. When the exhaust flow rate was increased to 0.107 m3

/s, the 

upper-layer helium volume percent dropped to less than 4 %, and the average vertical concentration fell 

lower.

The vertical helium volume percent profiles in Figure 49, Figure 55, Figure 59, Figure 63, Figure 67, 

and Figure 71 show that well-mixed upper layers with higher concentrations than observed near the floor 

developed in the garage as the helium releases proceeded and extended below the mid height of the 

garage. After roughly 1800 s of helium flow, the upper-layer concentrations attained a quasi-steady-state 

behavior in which concentrations grew slowly or leveled off with time. For three of the tests (8/9/10, 

8/12/10, 8/19/10a), concentrations for the three lowest sensors located at heights of 0.305 m to 0.914 m 

were nearly the same, indicating the lower layer was also well mixed. In contrast, the helium volume 

fraction varied with height over these sensors for the remaining three experiments. For the 8/13/10, 

8/13/10a, and 8/19/10 tests, concentration readings for Sensor #2 at 0.610 m were slightly greater than 

measured for the higher Sensor #3 at 0.914 m, which indicates that a weak density inversion was present 

in this region of the lower layer. Such inversions are unstable and should result in enhanced mixing. 

Concentration values recorded by Sensor #4 (1.219 m), which were intermediate between the lower- and 

upper-layer values, displayed intense fluctuations for the experiments with stratified lower layers, while 

the helium volume fractions at this sensor height were closer to upper-layer values and had much smaller 

fluctuations when the lower-layer was well mixed. Whether the intense fluctuations were associated with 

the density inversions is unknown.

The substantial changes in lower-layer mixing behavior are apparently not correlated with helium 

release or fan exhaust rates as shown by comparison of the parameters included in Table 8 and Table 9. 

The lack of weather measurements at the house introduces uncertainties, but comparison with nearby 

amateur weather stations suggested that the changes were due to weather conditions, with improved 

mixing in the lower layer associated with higher wind velocities from the south. Wind effects seem to 

have had little, if any, effects on upper-layer concentrations.

When the helium flow was halted, the volume percents measured at the various sensors began to 

decrease in a similar manner, but at much faster rates, than observed for the earlier experiments with 

natural ventilation. Concentrations in the lower layer began to fall almost immediately, while those in the 
85

upper layer leveled off for short periods before beginning to fall sequentially from the lower positions to 

the highest sensor. Eventually, the individual decays collapsed to a common curve. The collapses 

occurred earlier and for higher helium volume percents for the experiments which had concentration 

gradients in the lower layer. Two parameters, the required time needed for the helium volume percent to 

drop to 0.2 % and the decay constant for Sensor #4 at long times, were used to characterize the postrelease behavior. The values listed in Table 9 show that both the time period and the falloff rate fell into 

two groups, with shorter periods and higher falloff rates associated with the higher fan exhaust rate, as 

expected.

As discussed earlier, the measured decay rates for Sensor #4 were roughly a factor of two higher than 

the exchange rates that would have been expected based on the exhaust flow rate. It was hypothesized 

that this was due to the concentration gradients present that resulted in removal of upper-layer gas having 

helium volume percents greater than the average value.

Helium volume percent measurements were made in the family room of the attached house for the 

8/9/10 and 8/12/10 experiments. In both cases, small increases on the order of 0.2 % were observed. 

These values are not very different from values observed in the house at comparable times for the 8/6/10 

(0.07 %, 0.32 %) and 8/23/10 (0 %, 0.15 %) experiments with natural garage ventilation at flow times 

(1378 s, 2902 s) corresponding to the ends the 8/9/10 and 8/12/10 tests. It should be kept in mind that the 

house air conditioning fan was not operated on 8/9/10 and 8/12/10, and the helium may not have been

fully mixed throughout the house. Even so, the similarity of the helium concentrations is somewhat 

surprising since the forced ventilation removed upper layer helium from inside the garage and transported 

it outside. This should have limited helium entering the house in two ways. First, the helium 

concentrations available for transport into the house were lower, and, second, the fan would have lowered 

the pressure in the garage somewhat relative to the house, which should have slowed exchange between 

the garage and the house.

The use of an exhaust fan was effective in limiting the helium volume percents to levels 

corresponding to hydrogen/air mixtures that would have been too lean to burn. The simple approach 

described in reference [8] for estimating the required exhaust rate was effective. It should be kept in mind 

that the ability to limit the buildup of hydrogen depends on both the hydrogen release rate and the fan 

exhaust rate. The design of an appropriate ventilation system for residential garages will depend critically 

on the accident scenario chosen to estimate possible hydrogen release rates.

4.3 Buoyant Gas Trapping within Conventional Automobiles

The 8/6/10 and 8/23/10 experiments were done with a vehicle parked over the helium release location

and natural ventilation of the garage. Helium volume fraction measurements inside the vehicle provide 

insights as to how helium concentrations built up inside the automobile during the helium releases and 

decayed once the helium flow stopped. The relevant concentration measurements were shown in Figure

41 and Figure 46. A plot of the early helium concentration development was included in Figure 42.

The results showed that helium levels built up very rapidly under the vehicle and within the upper 

part of the engine compartment when helium started to flow. Similar rapid hydrogen concentration 

growth in a vehicle engine compartment was reported by Maeda et al. for releases under a vehicle located 

in the open, confirming that hydrogen and helium behave similarly in this type of experiment. [14,15] 

Measurements inside the upper regions of engine compartments of several different conventional vehicles 

during hydrogen releases inside a garage at Southwest Research Institute showed rapid increases to 

similar levels of hydrogen volume percent. [6,7] The current measurements near the bottom of the engine 

compartment showed much lower concentrations that were close to those measured outside of the vehicle, 

indicating that the helium concentrations inside the engine compartment were stratified. Maeda et al. 

reported similar observations for their experiments. [14,15]

The initial helium concentrations formed in the volume of the undercarriage and in the upper part of 

the engine compartment indicated significant mixing of helium with air as the flow traveled from the

release location. The observation that the concentrations continued to increase as helium built up in the 
86

surrounding garage provided additional evidence of the importance of this mixing. Additional support for 

the role of released helium mixing with surrounding gases in the garage is provided by the findings of 

Maeda et al. [14,15] who showed that, for experiments performed in the open where a buildup of 

hydrogen in the surroundings was not possible, trapped concentrations of hydrogen attained quasi-steadystate values after only a few seconds and the Southwest Research Institute study, which showed that

trapped hydrogen in vehicles inside a garage continued to slowly increase [6,7].

To our knowledge, the only earlier study that has reported the penetration of released gas into 

passenger compartments of vehicles is the Southwest Research Institute study. [6,7] A number of 

different conventional automobiles were used. The measurements showed that the growth of hydrogen 

concentration inside the passenger compartments was much slower than observed inside the engine 

compartments. There were significant variations in the rate at which hydrogen concentrations increased 

inside the passenger compartments, suggesting that there were significant vehicle-to-vehicle differences 

in the degree to which passenger compartments were isolated from the outside environment. It should be 

kept in mind that the Southwest Research Institute study used older model vehicles that had been 

scrapped. Such vehicles may have passenger compartments that are leakier than more recent models that 

are in road condition.

No previous studies were identified in which buoyant gas penetration into the trunk of an automobile 

was measured. The natural-ventilation experiments revealed that the helium concentration in the 

passenger compartment (see Figure 41 and Figure 46) rose slightly faster than in the trunk during the 

early portion of the release period, but helium levels were nearly identical by the end of the 4 h release 

period. Maximum helium volume percents were just over 16 % for the 8/6/10 and 8/23/10 data. Note 

that even though the helium concentrations increased relatively slowly over the 4 h release period, they 

attained values that would have been highly flammable if hydrogen had been released instead of helium. 

During the post-release phase the helium concentrations in the two compartments decayed relatively 

slowly and had values which were nearly indistinguishable.

The relatively slow buildup and decay of helium inside the passenger compartment of the Stratus

implies a small number of air changes per hour, here denoted ACHveh. In order to estimate ACHveh, an 

exponential curve was least squares curve fit to the passenger compartment concentration decay at long 

times. Figure 77 shows the result for the 8/6/10 experiment. The time constant from the fit was 

1.87 × 10-5 s

-1 corresponding to ACHveh = 0.067 h-1

. As expected, this value is nearly 4 times smaller than 

values determined for the house and garage. The experimental data for 8/23/10 were not collected for as 

long, and it was only possible to make a similar calculation as the helium volume percent fell from 2.1 % 

to 1.4 %. Nonetheless, the resulting value of ACHveh = 0.088 h-1 was only slightly higher than estimated 

for the 8/6/10 experiment.

Unlike the current investigation in which the helium penetrated the passenger compartment and trunk 

from the outside, Ekoto et al. considered the case where hydrogen or helium was injected directly into the 

passenger compartment or trunk of a conventional automobile. [18] These authors observed that when 

helium was released directly into the passenger compartment, there was a rapid exchange with the trunk,

and concentrations there were slightly lower but tracked those inside the passenger compartment. 

Interestingly, when the release was into the trunk, the concentration in the passenger compartment did not 

increase nearly as rapidly as in the trunk. The authors attributed this observation to the effects of 

stratification and the resulting locations of the interface between the upper and lower layers in the 

passenger compartment and garage. The rapid attainment of pseudo-steady state concentration 

distributions and the relatively high concentrations trapped in the compartments provide evidence that the 

volume exchange rate between the vehicle compartments and outside surroundings was also relatively 

slow for the vehicle used in their study.
87
88

Figure 77. The falloff of helium volume percent in the passenger compartment of the vehicle long after 

the end of the helium release under the vehicle on 8/6/10 is fit to an exponential curve using a 

least squares curve fitting procedure.

Six additional experiments with forced ventilation were run with vehicles present in the garage. Four 

of these were done with the Stratus parked over the release location. As can be seen by comparing the 

concentration time profiles for locations in the Stratus shown in Figure 41 and Figure 46 for the natural 

ventilation cases and in Figure 52, Figure 56, Figure 62, and Figure 66 for the experiments with forced 

ventilation, it is evident the helium volume percent time profiles were nearly identical at short times after 

the helium flow was initiated. Any minor changes in concentration levels between experiments were 

most likely associated with small variations in the helium volume flow rates. Recall that for the forcedventilation experiments, the fan was started within 2 min after the helium flow started. The similarity of 

the time profiles at short times and the lack of changes in the profiles when the fan flow started show that 

the garage exhaust flow had little or no direct effect on mixing under the vehicle.

Large differences in the time profiles between the natural- and forced-exhaust experiments appeared

as the helium releases continued. For the experiments without forced ventilation, helium volume percents 

at locations where helium was trapped under the automobile, i.e., in the upper engine compartment, 

driver’s side front wheel well, and undercarriage, continued to increase more slowly after the sharp 

increases associated with starting the helium flow. These increases were on the order of 25 % at the top 

of the engine compartment and 75 % in the wheel well and inside the undercarriage at the end of the 

releases. Helium concentrations in the passenger compartment, trunk, and in the lower part of the engine 

compartment grew slowly to levels around 15 %. In contrast, during the forced-ventilation experiments,

the increases in helium concentration in the trapped areas were much smaller, ≈ 7 % in the upper engine 

compartment and ≈ 15 % in the wheel well and undercarriage. Concentrations in the passenger 

compartment and trunk were ≈ 1.5 %, while a value of ≈ 2.5 % was present in the lower engine 

compartment.

The differences in helium volume percent time development for the two types of ventilation confirm 

and emphasize the importance of mixing of the surrounding gas with released helium under a vehicle on 

the levels of helium trapped under and inside a vehicle during a release. The ultimate concentration levels 

were reduced significantly by using forced ventilation to limit the buildup of helium in the lower layer of 

the garage. It should be recognized that the helium volume fractions measured under the car in the 

undercarriage, engine compartment and wheel well correspond to hydrogen concentrations that would be 

highly flammable. On the positive side, helium levels inside the passenger compartment and trunk were 

reduced below the flammability limit and would not be expected to ignite if hydrogen was present at the 

same concentrations.

The final two forced-ventilation experiments were run with the Stratus replaced by the Passat. 

Comparisons of Figure 62 and Figure 66 (8/13/10 and 8/13/10a) with Figure 70 and Figure 74 (8/19/10 

and 8/19/10a) reveal that the general appearances of the concentration time profiles inside the two 

vehicles were similar. As discussed earlier, concentrations recorded in the undercarriage were not 

directly comparable since the sensors were located in different areas due to the need to avoid the splash 

shields present on the Passat. Helium volume percents recorded at the end of a release were somewhat 

higher in the upper engine compartment and wheel well for the Passat. This may have been due to 

reduced mixing with surrounding gas as the helium was transported from the release location through the 

narrow channel present in the undercarriage between the splash guards. Helium levels inside the 

passenger compartment and trunk of the Passat were about a factor of two higher than observed for the 

Stratus.

Maeda et al. investigated the build up of hydrogen in the engine compartment of a front-engine, reardrive unnamed passenger sedan. [15] Concentrations in the upper part of the engine compartment were 

shown to be nearly uniform. For releases under the center of the vehicle and rates similar to those used in 

the current work, the quasi-steady-state helium volume percent reported by Maeda et al. was very close to 

16 %. This value is similar to those observed following the initial concentration increases during 
89

experiments reported here using the Stratus. When the Passat was used, the upper engine compartment 

helium concentrations were slightly higher. The agreement of experiments using three different vehicles 

suggests that such concentration levels may be characteristic of mid-size sedans.

Maeda et al. investigated the effect of a vehicle’s underside configuration by placing a flat sheet metal 

cover over the drive-train “tunnel” of their test automobile. [15] Quasi-steady-state concentrations in the 

engine compartment were reduced by roughly a factor two. A similar affect might have been expected for 

the Passat, since the undercarriage was partially covered with flat splash shields. However, as noted 

earlier, there was a deep channel in the undercarriage that passed over the release location between the 

areas covered with shields. As emphasized by Maeda et al., helium/air mixtures likely flowed along this 

channel and were conducted into the engine compartment. [15]

The hydrogen trapped under and inside vehicles during releases has been shown to play an important 

role if hydrogen is subsequently ignited. Studies at SRI International [16] and Southwest Research 

Institute [6,7] showed that when hydrogen trapped inside a garage with a vehicle parked inside was 

ignited, the resulting explosions were much more vigorous and damaging than for comparable 

experiments without a vehicle present. This is likely due to two augmenting effects associated with the 

presence of a vehicle. The first is the buildup of higher hydrogen concentrations in interior spaces of the 

vehicle as compared to the garage volume outside. Unless these mixtures become rich, i.e. reach a 

hydrogen concentration higher than the stoichiometric value, the higher concentration hydrogen trapped 

inside a vehicle will develop higher flame speeds when ignited. The second effect involves the role of the 

vehicle itself in accelerating flame speeds. The trapped hydrogen in vehicles is typically found in small 

partially enclosed spaces containing a great deal of clutter. These conditions are known to accelerate 

turbulent burning rates and result in stronger explosions than characteristic of comparable hydrogen/air 

mixtures in the open. [33] For the SRI experiments, it is likely that only the second effect played a role

since the hydrogen was released outside of the vehicle, and it is difficult to identify a mechanism whereby

higher hydrogen concentrations than present inside the garage would have been present inside the vehicle. 

In the case of the Southwest Research Institute study, both effects were present since the hydrogen was 

released under the vehicle and measured concentrations in the engine compartment were considerably 

higher than in the surrounding garage at the time of ignition. As observed in the current experiments, it is 

likely that high hydrogen concentrations were also trapped inside the undercarriages of the vehicles used 

in the Southwest Research Institute investigation.

5 Summary

Twelve experiments have been described in which helium, serving as a surrogate for hydrogen, was 

released into a residential garage attached to a house at rates between 4.21 × 10-3 m3

/s and

4.44 × 10-3 m3

/s. Helium was released as either momentum- or buoyancy-dominated flow. Cases with 

natural and forced ventilation of the garage were studied, and the garage was either empty or had a 

conventional vehicle parked over the release location. Helium concentrations were tracked at multiple 

locations inside the garage and vehicles (if present). The helium concentration at a single location inside 

the house was also monitored. Additional measurements included temperatures inside the garage and 

house and outside and wind velocity for some of the experiments.

Thirty major findings and observations are listed here:

1. The garage had ACH4Pa ≈ 3 h-1 based on doorway blower measurements, but direct 

measurements using tracer gas methods gave values of ACHgar on the order of 0.4 h-1

. The 

difference in the two values demonstrates that ΔP = 4 Pa, commonly taken as a characteristic 

pressure difference between building interiors and the outside, is a considerable overestimate. A 

value closer to ΔP = 1 Pa would be more representative.

2. The air tightness of the test garage was better than typical attached garages in the United States.

3. The leak sizes and their locations were not characterized for the garage.

4. Four hour helium releases with momentum- or buoyancy-dominated flow into the empty garage 

with natural ventilation yielded similar time profiles of concentration. Quasi-steady-state 
90

vertical helium concentration distributions developed after ≈ 2 h that had nearly uniform upper 

layers extending down to between 1.0 m and 1.2 m above the floor and lower layers with 

concentrations that fell with decreasing height.

5. Measurements indicated that lateral helium concentration gradients were minimal in the empty 

garage.

6. Literature findings suggesting more complete mixing with momentum-dominated releases and 

more concentration stratification with buoyancy-dominated releases were not confirmed by the 

results in Item 4. It is hypothesized that the reason for the discrepancy is that buoyancydominated flows with extremely low Fr mix rapidly and result in reduced stratification and 

more complete mixing than occurs for buoyancy-dominated flows with Fr closer to the 

transition value between momentum- and buoyancy-dominated flows.

7. Wind velocity and direction were found to have small, but measurable effects on the rate of 

exchange between the empty garage interior and its surroundings for natural-ventilation 

experiments. Higher wind speeds and winds from the south increased the exchange rate. Wind 

effects were generally small since they were overwhelmed by the larger pressure differences 

resulting from the large density difference between helium/air mixtures and air.

8. Temperature differences between the garage and house and the garage and outside were 

generally no larger than a few oC, and no clear correlations of mixing behavior with temperature 

were observed.

9. Roughly two-thirds of the helium released into the naturally ventilated garage was lost from the 

garage during the 4 h release period, and between 15 % and 30 % of the released helium was 

present in the attached house at the end of a release.

10. The maximum helium volume percent in the house for a 4 h experiment with natural ventilation 

reached just above 4 %, which is close to the lower flammability limit for hydrogen/air 

mixtures. This result is likely specific to this house and garage combination. In general, helium 

concentration buildup inside an attached house would be expected to vary with a range of 

parameters including house volume, interior mixing efficiency, presence of smaller volumes 

capable of trapping hydrogen, permeability of the house/wall envelop, and physical conditions 

in the house and garage.

11. Post-release helium concentrations for experiments in the empty, naturally ventilated garage 

decayed in such a way that the measurements at each height collapsed to a common decay curve

at different times, with the period required for collapse increasing with measurement height. 

Parameters used to characterize this behavior were the helium volume percent present when the 

curves collapsed and the period required for the highest sensor to reach this concentration.

12. Values of ACHgar estimated using the tails of the helium concentration decay curves for the 

naturally ventilated garage tests were between 0.11 h-1 and 0.33 h-1 and were consistent with 

earlier measurements using tracer gas approaches in the same garage.

13. Placing a vehicle over the helium release location at the center of the floor improved mixing in 

the garage with natural ventilation by a small, but measurable amount, as reflected by a lowered 

interface between the uniform upper layer and the lower layer, higher concentrations and 

smaller gradients in the lower layer, and higher averaged concentrations at the end of the 

release.

14. Following helium release initiation into a naturally ventilated garage with a Dodge Stratus 

parked over the release location, helium volume percents jumped abruptly to values around 

30 % in the undercarriage volume and around 15 % in the upper engine compartment and at the 

top of the driver-side wheel well. These values tracked higher as helium concentrations in the 

surrounding garage increased and then leveled off as the exterior concentrations reached quasisteady-state levels. Values measured in the lower engine compartment were close to those 

found in the surroundings immediately outside the vehicle.
91

15. For the conditions of Item 14, helium concentrations inside the passenger compartment and 

trunk began to increase slowly after a delay at the start of the release, ultimately reaching levels 

of ≈ 15 % by the end of a release.

16. The helium concentrations trapped at the various locations in the vehicle during a release 

attained levels that would be highly flammable if helium were replaced with hydrogen.

17. Helium concentrations in the undercarriage, upper engine compartment, and wheel well for the 

conditions of Item 14 dropped abruptly within a few 10s of seconds following cessation of the 

helium release.

18. Helium concentrations for the conditions of Item 14 in the passenger compartment and trunk 

decayed slowly after the helium flow stopped. Decay rates at long times for repeated

experiments were ACHveh = 0.067 h-1 and ACHveh = 0.088 h-1

. These values are considerably 

smaller than similar parameters for the house and garage.

19. Active ventilation of the garage containing a vehicle by withdrawing air with a fan significantly 

reduced helium volume percents inside the garage.

20. Quasi-steady-state concentration distributions developed in less than an hour for the release 

phase of experiments in the forced-ventilated garage containing a vehicle.

21. Well-mixed upper layers with an interface located above 1.22 m formed during the release 

phase for the forced-ventilated garage experiments with a vehicle.

22. Significant variations in the lower-layer helium concentration distributions during helium 

release were observed between different forced-ventilation experiments with vehicles. For 

some, the lower layer was well mixed and displayed low concentration fluctuations, while for 

others, vertical concentration gradients developed with indications of an unstable density 

distribution and large concentration fluctuations. A tentative conclusion, based on comparison 

with weather data from amateur weather stations near NIST, associated more completely mixed 

lower layers with higher winds from the south.

23. Observed average helium volume percents at the end of the release for fan exhaust rates around 

0.92 m3

/s were on the order of 3.4 %, which is slightly lower than predicted a priori using an 

analytical model described elsewhere. [8]

24. Increasing the fan exhaust rate by ≈ 15 % resulted in a ≈ 25 % drop in the average quasi-steadystate helium concentration.

25. Small differences in trapped helium concentrations inside and under the vehicle were observed 

for the two vehicles used in the forced-ventilation garage experiments. The helium volume 

percents for both vehicles were similar to those reported in an earlier published paper. [15] The 

similarities suggest the current results will be characteristic for a range of mid-sized 

automobiles.

26. Helium concentrations present in the engine compartment and trunk of a conventional 

automobile were reduced at the end of a helium release by forced ventilation since the exterior 

helium levels surrounding the compartments were reduced.

27. During the post-release period for the forced-ventilation experiments with a vehicle, the vertical 

concentration distributions developed a stratified structure with a similar appearance to those 

observed during natural-ventilation tests, but with a much faster falloff.

28. Helium falloff rates at low helium concentrations in the forced-ventilation experiments with a 

vehicle based on the sensor at 1.22 m were roughly a factor of two higher than predicted 

assuming instantaneous mixing of entering air with the garage contents. This was attributed to 

the finite mixing time that led to observed vertical concentration gradients.

29. For the forced-ventilation experiments with a vehicle in the garage, the long term helium falloff 

rates depended linearly on the fan exhaust rate.

30. During the forced-ventilation experiments with a vehicle in the garage, observed helium volume 

percents in the family room of the attached house were similar to those measured at comparable 

times for experiments without forced ventilation. Since the helium release periods for the 
92

forced-ventilation tests were shorter, it is not known if the higher levels observed in 4 h releases 

with natural ventilation would have been attained.

The findings and observations justify the following conclusions concerning the dispersion behavior 

and mixing of a highly buoyant gas when released into a residential garage attached to a single family 

home.

a. The degree of vertical concentration stratification within the garage depends on the Froude 

number of the release, with large and small Froude numbers favoring more complete mixing.

b. The development of upper and lower layers depends on the Froude number (favored by Froude 

numbers close to one) and release location (favored by release near the ceiling).

c. The rate of helium loss from the garage depends on the outside wind velocity and direction, but 

the dependence is relatively weak due to the dominance of hydrostatic pressure differences 

generated by the substantial density difference between the helium/air mixtures inside the 

garage and the air in the surroundings.

d. Helium released inside the garage was found to be transferred preferentially through walls into 

the attached house as opposed to the outside surroundings.

e. Helium released underneath a conventional vehicle builds up substantial concentrations in the 

undercarriage and engine compartment due to trapping in these partially enclosed spaces, but the 

spaces were sufficiently open that helium volume fractions dropped rapidly to levels similar to 

those in the surrounding garage when a release ended.

f. The passenger compartments and trunks of the conventional automobiles investigated were 

relatively well isolated from exterior volumes inside the vehicle, and, as a result, the 

surrounding garage, and helium concentration build ups and fall offs were substantially slower 

than observed at these exterior locations.

g. Moderate active air ventilation was shown to be effective in limiting helium concentrations 

inside the garage to levels corresponding to nonflammable hydrogen/air mixtures.

h. Moderate active air ventilation was shown to be ineffective in reducing high helium 

concentrations trapped in the undercarriage and engine compartments of conventional mid-sized 

passenger vehicles.
93

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