RI 9694

REPORT OF INVESTIGATIONS/2014

Investigation of 

Purging and Airlock Contamination 

of Mobile Refuge Alternatives
Report of Investigations 9694 

Investigation of Purging and Airlock Contamination of Mobile 

Refuge Alternatives 

Eric R. Bauer, Timothy J. Matty, and Edward D. Thimons 

DEPARTMENT OF HEALTH AND HUMAN SERVICES 

Centers for Disease Control and Prevention 

National Institute for Occupational Safety and Health 

Office of Mine Safety and Health Research 

Pittsburgh, PA • Spokane, WA

March 2014
This document is in the public domain and may be freely 

copied or reprinted.

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Suggested Citation

NIOSH [2014]. Investigation of Purging and Airlock Contamination of Mobile Refuge 

Alternatives. By Bauer ER, Matty TJ, Thimons ED. Pittsburgh, PA: U.S. Department of Health 

and Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication 2014–116, RI 9694. 

DHHS (NIOSH) Publication No. 2014–116 

March 2014 

Cover photo by NIOSH.
Contents 

Executive Summary ................................................................................................................... 1 

Background and Methods................................................................................................... 1 

Summary of Findings ......................................................................................................... 2 

Summary of Discussion and Recommendations ................................................................ 2 

Introduction ................................................................................................................................ 4 

Background on Carbon Monoxide Toxicity ................................................................................. 5 

Mini Purge Box Experiments ...................................................................................................... 6 

Description of Mini Purge Box ............................................................................................ 6 

Test Set-up for Mini Purge Box Experiments ...................................................................... 7 

Results of Mini Purge Box Purging Experiments ................................................................ 8 

Summary: Mini Purge Box Experiments ............................................................................11

Refuge Alternative Airlock Purging with Carbon Monoxide........................................................11

Description of Mobile Refuge Alternative Airlocks..............................................................12

Simulation of Occupants for Testing Procedures...............................................................14

Test Procedure for CO Purging of Refuge Alternative Airlocks ..........................................14

Data Analysis and Test Results for CO Purging Experiments ............................................19 

Summary: Refuge Alternative Airlock Purging with Carbon Monoxide ...............................25

Airlock Purging with Sulfur Hexafluoride....................................................................................26

Results of Airlock Purging with Sulfur Hexafluoride ...........................................................26

Summary: Refuge Alternative Airlock Purging with Sulfur Hexafluoride .............................29

Airlock Contamination Research ...............................................................................................29

Historical Data on Measured Post-Disaster CO Concentrations ........................................29

Test Procedure for Airlock Contamination Research .........................................................31

Adjustment for SF6 in the Breath of Airlock Occupants ......................................................34

Airlock Contamination Research Results ...........................................................................36

Airlock Contaminant Concentration versus Outside Contaminant Concentration...............36

Summary: Airlock Contamination Research ......................................................................40

Discussion and Recommendations ...........................................................................................41

Acknowledgements ...................................................................................................................43

References ...............................................................................................................................44

Appendix A – Diffusion and Tracer Gases .................................................................................47

Appendix B – Protocol for Airlock Contamination Research ......................................................49
Figures 

Figure 1. Mini purge box with pressure relief system and sampling ports. .................................. 7 

Figure 2. Mini purge box test results at 0.53 psig relief pressure and three different purge air 

flow rates. .................................................................................................................................. 9 

Figure 3. Mini purge box test results at 0.13 psig relief pressure and three different purge air 

flow rates. .................................................................................................................................. 9 

Figure 4. Airlock in a tent-type mobile refuge alternative. ..........................................................12

Figure 5. Airlock in a rigid steel mobile refuge alternative. .........................................................13

Figure 6. Plan view of air flow schematic for the tent-type refuge alternative airlock 

during purging. ..........................................................................................................................15

Figure 7. Plan view of air flow schematic for the rigid steel refuge alternative airlock during 

purging......................................................................................................................................15

Figure 8. Purge plumbing diagrams. Top diagram is for the tent-type refuge alternative 

airlock. Bottom diagram is for the rigid steel refuge alternative airlock. .....................................16

Figure 9. Representative example of data for carbon monoxide (CO) purging of airlock for a

tent-type refuge alternative, test TT-25-1. .................................................................................22

Figure 10. Representative example of data for carbon monoxide (CO) purging of airlock for 

a rigid steel refuge alternative, test RS-36-0. ............................................................................22

Figure 11. Graph of time required to purge versus air flow rate.................................................24

Figure 12. Graph of total volume of air required to purge versus air flow rate............................24

Figure 13. Graph of exchange ratio versus air flow for the tent-type and rigid steel mobile 

refuge alternative airlocks. ........................................................................................................25

Figure 14. Refuge alternatives in OMSHR’s reverberation room in building 154 at the

OMSHR Pittsburgh site, Bruceton, PA. .....................................................................................32

Figure 15. Vacutainer sample tube and needle assembly used to collect SF6air samples.........33

Figure 16. Reverberation room showing mobile refuge alternative locations, fan locations, 

and sample recording locations 34

Figure 17. Graph of the ratio of SF6 concentration (surrogate for carbon monoxide) in the 

airlock and outside the airlock versus the number of subjects entering the airlock (from 

data in Table 13).......................................................................................................................37

Figure 18. SF6 contamination ratio versus number of persons entering tent-type refuge 

alternative airlock at varying entry times....................................................................................40

.................................................................................................
Tables 

Table 1. Symptoms of CO exposure .......................................................................................... 5 

Table 2. Instrument/apparatus specifications for mini purge box testing..................................... 8 

Table 3. Test results for purging at 0.53 and 0.13 psig relief pressures.....................................10

Table 4. Comparison of average CO concentration reduction for continuous purging air flow 

versus on/off purging air flow ....................................................................................................10

Table 5. Instrumentation and apparatus used during purging of CO from mobile refuge 

alternative airlocks ....................................................................................................................17

Table 6. Airlock volumes used during exchange ratio calculations ............................................19

Table 7. Tabulated data for tent-type RA airlock purging of CO with various number of 

occupants, purge air flow rates, and relief pressure of 0.18 psig ...............................................20

Table 8. Tabulated data for rigid steel RA airlock purging of CO with various numbers of 

occupants, purge air flow rates, and relief pressures.................................................................21

Table 9. Results from SF6 purging of tent-type RA airlock.........................................................27

Table 10. Comparison of purging efficiency for the CO and SF6 purging, one occupant, 86 

SCFM .......................................................................................................................................28

Table 11. Test differences or variables that may be responsible for differences in purging

efficiency ...................................................................................................................................28

Table 12. Example of CO concentrations measured post-disaster at the Upper Big Branch Mine .30

Table 13. Data for comparing contaminant ratio and number of subjects entering the airlock ...38

Table 14. Data from tent-type airlock entrance where time varies .............................................39
ACRONYMS AND ABBREVIATIONS 

ATDSR Agency for Toxic Substances and Disease Registry

CH4 methane

CO carbon monoxide

H2O water

I.D. inside diameter

IDLH immediately dangerous to life or health

MEO Mine Emergency Operations

MSHA Mine Safety and Health Administration

NIOSH National Institute for Occupational Safety and Health

NIST National Institute of Standards and Technology

NMA National Mining Association

NPT national pipe thread

OMSHR Office of Mine Safety and Health Research

OSHA Occupational Safety and Health Administration

PEL permissible exposure limit

RA refuge alternative

REL recommended exposure limit 

SCSR self-contained self-rescuer

SF6 sulfur hexafluoride

TWA time-weighted average

USBM United States Bureau of Mines
UNIT OF MEASURE ABBREVIATIONS 

cu ft cubic foot

cfm cubic feet per minute

°C degrees Celsius

°F degrees Fahrenheit

°R degrees Rankine

ft foot

gm gram

in inch

L liters

ml milliliter

min minute

ppb parts per billion

ppm parts per million

% percent

lb pound

psi pounds per square inch

psia pounds per square inch atmosphere

psig pounds per square inch gage

sec second

sq ft square foot

sq in square inch

SCFH standard cubic feet per hour

SCFM standard cubic feet per minute

Investigation of Purging and Airlock Contamination 

of Mobile Refuge Alternatives

1

Eric R. Bauer, Ph.D., P.E.

1

Senior Mining Engineer (retired), Human Factors Branch, NIOSH Office of Mine Safety and Health 

Research, Pittsburgh, PA. 

2

, Timothy J. Matty

2

Engineering Technician, Human Factors Branch, NIOSH Office of Mine Safety and Health Research, 

Pittsburgh, PA. 

3 

, and Edward D. Thimons

3

Formerly Senior Physical Scientist, NIOSH Office of Mine Safety and Health Research; currently Senior 

Technical Advisor, URS Corporation, Pittsburgh, PA. 

Executive Summary 

Background and Methods

The National Institute for Occupational Safety and Health (NIOSH) Office of Mine Safety 

nd Health Research (OMSHR) has conducted research to evaluate the effectiveness of purging 

of mine refuge alternatives (RAs). Two questions were addressed experimentally: (1) Does the 

current generation of mobile refuge alternatives meet the requirements of 30 CFR

a

4

4

 Code of Federal Regulations. See CFR in references. 

§ 7.508 (c) (2) 

which requires RAs to be capable of purging the internal atmosphere from 400 ppm of carbon 

monoxide (CO) to 25 ppm? (2) What is the relationship between the concentration of noxious 

gases in the mine atmosphere external to the refuge alternative and the concentration that will be 

present inside the refuge alternative following entry of miners but prior to purging? The goal of 

the second question was to evaluate the appropriateness of the 400-ppm criterion, given that 

ambient post-accident mine concentrations of CO can be in the thousands of ppm. 

A tent-type and a rigid steel mobile refuge alternative were used to investigate the first 

question5

5

 There are multiple manufacturers for both tent-type and rigid steel RAs. For the purposes of this study, 

one of each type was used. 

. Carbon monoxide (CO) and sulfur hexafluoride (SF6) were used as contaminant gases 

as part of this study, and the individual experiments were conducted with the purging area of the 

RA occupied by zero, one, or seven simulated (when CO was used) or live (when SF6 was used) 

occupants. 

To investigate the second question, the aforementioned RAs were used along with a third 

airlock constructed for and employed in the experiments. The volume and size of the entry door 

into the constructed airlock were roughly in the middle of the range of values for the rigid and 

tent-type RAs. The RAs and constructed airlock were placed in a large sealed reverberation 

room, and SF6 gas was released into the reverberation room as a surrogate for CO. Experiments 

were conducted to determine the gas concentrations inside the airlock after groups of test 

subjects (representing miners) had entered. 

 
2

Summary of Findings 

The experimental findings indicate that the current generation of mobile refuge alternatives 

employs techniques that are capable of reducing a CO concentration of 400 ppm within the 

volume to be purged, as required by the 30 CFR § 7.508 (c) (2) regulation. This answers the first 

research question in the affirmative. 

Test findings indicate that when the airlock entry door is opened, ambient air with a higher 

CO concentration will begin to move into the airlock. Significantly, as miners move through the 

airlock they will expedite the turbulent diffusion (sometimes referred to as advective diffusion) 

of CO into the airlock. The level of CO inside the airlock will continue to increase until the door 

is closed. The ratio of the CO concentration inside the airlock at that point in time to the ambient 

concentration of CO outside the RA—i.e. the contamination factor—is shown in Figure 17 for 

the RAs/airlocks investigated in this study. 

Consider the following example to demonstrate the significance of this finding. The 

contamination factor with five persons entering the airlock is 0.5 in the tent-type RA, as shown 

in Figure 17. Thus, in order for the internal concentration to be at 400 ppm or less, the ambient 

CO concentration cannot exceed 800 ppm. For the same five miners in the rigid steel RA, the 

contamination factor is 0.2, which corresponds to a maximum outside ambient concentration of 

2,000 ppm. As a point of comparison, the CO concentration after the explosion at Upper Big 

Branch Mine in 2010 was approximately 10,000 ppm. 

As a consequence of this contamination during entry, the CO concentration inside the airlock 

could be many times greater than 25 ppm, after four purging cycles have been completed. As the 

miners move from the airlock into the main body of the RA, some of this contaminated air will 

be carried into the primary (long-term) refuge space. When the second group of miners enters the 

airlock, the air will contain a residual amount of CO from the previous group that used the 

airlock, and therefore after four purging cycles, the level of CO that will be carried into the main 

chamber could be higher than for the previous group. These findings raise the concern that an 

unanticipated and potentially toxic level of CO could exist in the airlock after the purging cycles 

have been completed, and in the main chamber after miners have entered from the airlock. 

Summary of Discussion and Recommendations 

The findings of this study demonstrate that the starting concentration of CO in the airlock of 

a mobile RA can be significantly greater than 400 ppm—e.g., in the thousands rather than the 

hundreds of ppm—and that a portion of the remaining CO will be carried into the main chamber. 

It should be emphasized that the findings of this study cannot be used to quantify the level of 

contamination that might occur in all commercially available RAs, nor can they be used to 

establish the level of contamination that could occur in the main chamber of the mobile refuge 

alternative. The findings can be used to conclude that: contamination will occur; that there is an 

immediate need to assess the hazard that it presents; and that guidance to manufacturers and 

miners is available by way of this report. Recommendations for conducting a hazard assessment 

are identified here. 

For an effective hazard assessment, a defensible worst-case ambient level of post-accident 

CO must be established. Given the many variables of an explosion scenario, this will be an 

inexact endeavor that can be informed by science and the records from past mine disasters, but 
3

one that ultimately requires a policy decision. Once this decision is made, it will be possible to 

assess more completely this hazard, and to provide design requirements to manufacturers as well 

as training and operating procedures to miners. 

The following recommendations are intended as guidance to assess and mitigate hazards 

presented by airlock contamination6

6

 These recommendations apply generally to built-in-place RAs as well as mobile RAs, although this 

report has focused on the latter. 

. 

 An ambient CO contamination level must be established for assessment purposes, 

based on the disaster scenario, and then used to design mobile refuge alternatives and 

to evaluate them in the approval process. 

 The expected contamination factors for a specific RA design should be determined 

experimentally under a prescribed procedure, which could be the one used in this 

study, and charts similar to Figure 17 should be developed and applied. 

 The expected CO concentration inside the airlock should be calculated, using the 

information established in the previous two recommendations. 

 Ideally, the airlock should be capable of reducing the expected CO concentration to 

an acceptable level, such as 25 ppm .

7

 The beginning and end points, i.e. 400 and 25, define a performance characteristic for the purging 

system. Given the finding that the starting concentration can exceed 400, it may be appropriate to reevaluate the end point. 

However, this may be nearly impossible in 

many cases, given the expected level of contamination, practical limitations on 

purging air capacity, and time constraints. Given this potential shortcoming, which is 

unlikely to be overcome simply through a re-engineering of the purging process, 

additional measures must be taken to protect those who would take refuge. 

7

A redesign of the purging process of mobile refuge alternatives was beyond the scope of this 

study, and over the longer term, design changes may yield solutions to the airlock contamination 

problem. However, in the short-term, the following activities are recommended: 

 Operational guidance to miners for purging should be based on a prescribed number 

of air changes, and not based on achieving a target concentration of 25 ppm or less. 

 Operational guidance to miners should include continued use of their self-contained 

self-rescuers (SCSRs) until they are in the main chamber of the mobile refuge 

alternative and they have determined that the concentration of CO in the main 

chamber is at an acceptable level. 

 A maximum acceptable concentration of CO in the main chamber must be specified. 

Given the significant difference in volume between the airlock and the main chamber, 

it is likely that the CO in the heavily contaminated air within the airlock would be 

diluted to an acceptable level in the main chamber. Notwithstanding, this must be 

confirmed by engineering analysis of RAs under the specified conditions, including 

the number of miners, the number of groups that will use the airlock, and the 

specified contamination level. 

 
4

The design of the airlock itself was also not a part of this research study. The two mobile 

refuge alternatives used in this study are popular commercial models and each has a differently 

designed airlock (door size and airlock volume). Based on limited observations, the size of the 

airlock door affects the speed at which miners can enter the airlock, which directly affects the 

amount of CO that moves into the airlock during entry. Over the longer term, there may be 

opportunities for manufacturers to incorporate design changes in airlocks to reduce the level of 

CO contamination. 

Introduction 

Generally, in-mine refuge alternatives (RAs, also referred to as mobile refuge alternatives 

and built-in-place shelters) must have the ability to purge or otherwise remove contaminated air 

from the airlock and/or main chamber caused by personnel entering during emergency 

conditions, i.e. post-disaster. Effective purging of a refuge alternative airlock is essential if a 

contaminant-free main chamber is to be realized. In 30 CFR § 7.508, the Mine Safety and Health 

Administration (MSHA) specifies the following purging criteria: (1) Purging or other effective 

procedures shall be provided for the airlock to dilute carbon monoxide (CO) to 25 ppm and 

methane (CH4) to 1.0% or less as persons enter, within 20 minutes of deploying the refuge 

alternative; and (2) For testing the component’s ability to remove CO, a stable concentration of 

400 ppm, ±5%, CO is used as the starting point for purging evaluation 30 CFR § 7.508 (c) (2). 

Also, 30 CFR § 7.506 states that an automatic means be provided to ensure that the pressure is 

relieved at 0.18 psi, or as specified by the manufacturer [30 CFR § 7.508 (c) (2)]. This criterion 

applies to overpressure of the RA and to the pressure relief provided during purging. Other 

regulations that pertain to purging and removal of harmful gases can be found in 30 CFR § 7 and 75. 

The Occupational Safety and Health Administration (OSHA) has set a permissible exposure 

limit (PEL) for CO at a time-weighted average (TWA) of 50 ppm [NIOSH 2013]. Using a 

conservative 50% dilution and a starting concentration of 400 ppm, it would require three purges 

to reduce the concentration to 50 ppm. A fourth purge would be required to reduce the CO 

concentration from 50 to 25 ppm, which is below the OSHA PEL. A goal of this study was to 

confirm experimentally whether this level of purging is achievable with the purging mechanism 

used by RA manufacturers. 

The principle that governs the effectiveness of purging is dilution. Dilution is a reduction in 

the concentration of a chemical (gas, vapor, or solution) resulting from adding uncontaminated 

gas, vapor, or solution. When this principle is applied to the purging of refuge alternative 

airlocks, it is assumed that the concentration of a contaminant will be halved as one full airlock 

volume of uncontaminated air is added. As described in Appendix A, this approach is overly 

simplistic for RA airlocks, but it is the approach that the mining industry, RA manufacturers, and 

regulatory agencies are using to design purging systems. This technical oversimplification is 

based on MSHA 30 CFR § 7 and 75, and offers a significant safety factor. If this dilution holds 

true, at a starting point of 400 ppm CO, it will take four complete air exchanges to reduce the 

concentration to 25 ppm, or to 1/16th of the original concentration. This would occur as follows: 

the first volume of air drops the concentration from 400 to 200 ppm; the second from 200 to 100 

ppm; the third from 100 to 50 ppm; and the fourth complete air exchange from 50 to 25 ppm. 

Additional purging and purge air would be required if the CO level in the airlock is greater than 

400 ppm and complete contaminant purging is desired. 
5

The purging research performed as part of this study was designed to answer the question: 

Does the current generation of mobile refuge alternatives employ technology capable of purging 

the internal atmosphere from 400 ppm of carbon monoxide (CO) to 25 ppm as required by 30 

CFR § 7.508? Researchers investigated purging through multiple approaches. First, purging 

experiments were conducted in a mini purge box to gain a better understanding of the purging 

phenomena and to familiarize researchers with the instrumentation to be employed in the studies 

using actual RAs. In these experiments, researchers investigated dilution and the effectiveness of 

purging by varying the air flow, air quantity, and pressure relief setting. Next, purging 

experiments in actual RA airlocks using CO and/or SF6 contaminant gas were conducted. 

Background on Carbon Monoxide Toxicity 

To understand the effects of carbon monoxide as it applies to post-disaster scenarios in 

mines, some scientific background is in order. Carbon monoxide is a colorless and odorless gas 

that is produced by incomplete combustion of carbonaceous material and is the primary toxic 

contaminant in post-disaster (methane and/or coal dust explosions and fires) mine air. As noted 

later in this report, CO concentrations of 10,000 ppm and higher are not uncommon in postdisaster ambient mine air. In addition, previous U.S. Bureau of Mines (USBM) explosion and 

fire research has recorded CO concentrations of 90,000 + ppm [Hofer et al. 1996]. 

The National Institute for Occupational Safety and Health (NIOSH) and OSHA have 

established guidelines for CO exposure. The NIOSH recommended exposure limit (REL) is a 

time-weighted average (TWA) of 35 ppm. For NIOSH RELs, “TWA” indicates a time-weighted 

average concentration for up to a 10-hour workday during a 40-hour work week. The OSHA 

permissible exposure limit (PEL) is a TWA of 50 ppm. TWA concentrations for OSHA PELs 

must not be exceeded during any 8-hour work shift of a 40-hour work week. NIOSH has also 

established “immediately dangerous to life or health” (IDLHs) concentrations criteria. For CO, 

the IDLH is 1,200 ppm. Table 1 lists the symptoms of CO exposure. 

Table 1. Symptoms of CO exposure 

Concentration, 

ppm Symptoms

35 Headache and dizziness within six to eight hours of constant exposure.

100 Slight headache in two to three hours.

200 Slight headache within two to three hours; loss of judgment.

400 Frontal headache within one to two hours.

800 Dizziness, nausea, and convulsions within 45 min; insensible within 2 hours.

1,600 Headache, tachycardia, dizziness, and nausea within 20 min; death in less than 2 hours.

3,200 Headache, dizziness, and nausea in five to ten minutes. Death within 30 minutes.

6,400 Headache and dizziness in one to two minutes. Convulsions, respiratory arrest, and 

death in less than 20 minutes.

12,800 Unconsciousness after 2–3 breaths. Death in less than three minutes.
6

During a normal post-disaster escape scenario, most miners would have deployed a selfcontained self-rescuer (SCSR) at the first sign of disaster or smoke and thus should have isolated 

their lungs from any contaminants, including CO. Because all current SCSRs isolate the wearer’s 

lungs from the outside environment, the contaminant level is immaterial, unless the wearer 

removes the mouthpiece for some reason, most likely to communicate with other miners. 

Therefore, as long as miners keep correctly wearing their SCSRs, allow no leakage of outside air, 

and do not remove the mouthpiece until the CO level is 50 ppm or less, they will prevent CO 

poisoning. According to the Agency for Toxic Substances and Disease Registry (ATSDR), acute 

carbon monoxide poisoning can occur with steady-state exposure (e.g., > 500 minutes) to 300 

ppm, or exposure to 1,000 ppm for approximately 80–90 minutes [ATSDR 2012]. 

Mini Purge Box Experiments 

The primary purpose of the purging experiments in the mini purge box was to refine and 

validate the instrumentation and methodology for the follow-up purging experiments to be 

conducted in the refuge alternatives. The mini purge box experiments were designed to 

determine the appropriate process for injecting a 400-ppm concentration of CO into a ventilated 

enclosure, to understand the purging air flow rates required to obtain the required contaminant 

reductions, and determine the contaminated air and CO sampling requirements. These 

experiments helped to refine appropriate data collection procedures and analysis, and contributed 

to a better understanding of the results of subsequent refuge alternative airlock purging 

experiments. 

Description of Mini Purge Box 

A sealed test fixture was constructed from 0.25-in-thick aluminum plate and welded corners, 

with interior dimensions of 2 ft x 2 ft x 2 ft for a volume of 8 cu ft. An acrylic glass top was 

fabricated, then sealed and fastened in place to allow for observation of the inside of the box. 

The enclosure included a purge air inlet, contaminant gas charging inlet, relief exhaust port, and 

contaminant level sampling ports. The purging inlet used an air flow meter to control the purging 

rate along with a ball valve to shut off the air flow. The charging port was tied into the purging 

port with a ball valve to close off the port. The relief exhaust port was located diagonally from 

the inlet port on the left side of the box in the upper right corner. The relief exhaust was made up 

of an adjustable low pressure relief valve with a range of 0.13 to 1.3 psig and a pressure gage. 

Relief pressures less than 0.13 psig can be obtained by removing and replacing the original 

pressure relief valve with a lower pressure ball valve to regulate and reduce the back pressure. 

Three contaminant gas concentration sample ports were located diagonally on the front side with 

the lowest set 6 in x 6 in off the lower right corner, the second in the middle of the front panel, 

and the third 6 in x 6 in off the upper left corner (Figure 1). The sample ports had extension tubes 

installed to reach the center of the enclosure parallel to the port. 
7

Photo by NIOSH 

Figure 1. Mini purge box with pressure relief system and sampling ports.

Test Set-up for Mini Purge Box Experiments

Carbon monoxide (CO) was used as the contaminant gas for the mini purge box tests. The 

CO was supplied from a pressurized cylinder containing 99.9% CO. The purging air was 

supplied by an air compressor through an air pressure regulator, dryer, and filter. The pressure 

regulator was set at 30 psi. The purging air flow rate of 0.833 cfm and relief pressure of 0.53 psig 

were set and maintained throughout the test. 

The experiments were begun by injecting CO into the mini purge box. Once the level of CO 

exceeded 400 ppm (at most, 550 ppm) the charging port was closed. Then the air and CO inside 

the mini purge box were allowed to mix for 5 min. Readings of the CO concentration from the 

three sampling locations after 5 min showed that no layering was observed, showing uniform 

mixing. The purging port was then opened to its required flow rate for the given test. 

Contaminated air exited through the relief valve until the concentration of CO decreased to 400 

ppm. 

At this point, test measurements were begun and the CO concentration was continuously 

recorded, while purge air volume and elapsed time were recorded manually. Sampling of the 

contaminant gas level was completed using an Industrial Scientific iTX 4 gas monitor and iSP 

sample pump. Instrumentation used for the tests included an air flow meter, gas level detector, 

and stop watch. Table 2 lists the instrumentation specifications. 
8

Table 2. Instrument/apparatus specifications for mini purge box testing 

Apparatus Specifications

Air flow meter

Manufacturer: Dwyer Instruments

Model: RMC-102-SSV and RMC-103-SSV 

Range: 10–100 SCFH and 20–200 SCFH 

Accuracy: 2% of Full Scale

Gas level detector

Manufacturer: Industrial Scientific Corporation

Model: ITX with CO monitoring configuration

Range: 0 to 999 ppm

Accuracy: 1 ppm, +-5% of reading

Stop watch Manufacturer: H Heuer Instruments Pty Ltd.

Model: Trackstar 7-jewels

Lower pressure relief valve

Manufacturer: Stra-Val Machine Company

Model: RVi20-05T

Range: 0.13 to 1.3 psig

Lower pressure gage

Manufacturer: NOSHOK, Inc.

Model: 25-200-30

Range: 0–30 in H2O

Accuracy: NIST-Certified Calibration

Results of Mini Purge Box Purging Experiments 

The initial shake-down tests with the mini purge box showed that to reduce the CO 

concentration from 400 ppm to 25 ppm, a box volume exchange rate of approximately 3.2 to 1 

was needed—that is, 3.2 complete air volumes were required to cause a four-fold reduction in 

the CO concentration. 30 CFR § 7.508 (a) (1) requires purging to be completed within 20 min of 

refuge alternative deployment. The 20-min purging requirement is for all occupants to enter, 

whether they enter all at once or in groups. If the airlock design is such that miners are required 

to enter as groups, each purge must be an equal percentage of 20 min with the total for all groups 

being 20 min or less. 

Next, experiments were run to determine the air flow rate required to reduce the level of CO 

in the mini purge box from 400 ppm to 25 ppm in 20, 15, and 10 min. Using the approximate 

volume exchange rate of 3.2 to 1 as determined previously, an air flow rate was calculated to 

reach the test criteria of 400 to 25 ppm in 20, 15, and 10 min at a relief setting of 0.53 psig. The 

tests were repeated with small changes to the air flow rate until the required times (20, 15, and 10 

min) were obtained. This adjusted air flow rate was then maintained and the test was repeated 

three times to verify results (Figure 2). To evaluate the effect of a lower relief setting—i.e. one 

that is closer to what is suggested in 30 CFR § 7.506—tests at the same air flows were repeated 

for a relief setting of 0.13 psig (Figure 3). Note: In Figures 2 and 3, several of the graph lines 

overlap which makes it difficult to see each line separately. 
9

Figure 2. Mini purge box test results at 0.53 psig relief pressure and three 

different purge air flow rates. 

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

CO Concentration, ppm 

Time, min 

Test #1, 1.36 cfm Test #2, 1.36 cfm Test #3, 1.36 cfm

Test #4, 1.75 cfm Test #5, 1.75 cfm Test #6, 1.75 cfm

Test #7, 2.54 cfm Test #8, 2.54 cfm Test #9, 2.54 cfm

20 min to drop concentration to 25 ppm 

15 min to drop concentration to 25 ppm 

10 min to drop concentration to 25 ppm 

Figure 3. Mini purge box test results at 0.13 psig relief pressure and three 

different purge air flow rates. 

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25

CO Concentration, ppm 

Time, min 

Test #10, 1.36 cfm Test #11, 1.36 cfm Test #12, 1.36 cfm

Test #13, 1.75 cfm Test #14, 1.75 cfm Test #15, 1.75 cfm

Test #16, 2.54 cfm Test #17, 2.54 cfm Test #18, 2.54 cfm

18.5 min to drop concentration to 25 ppm 

14.5 min to drop concentration to 25 ppm 

9.5 min to drop concentration to 25 ppm 
10

Table 3 presents the results of these tests and shows that by increasing the air flow rate, the 

time to purge from 400 to 25 ppm was reduced, while at the same time the exchange rate, or 

volume of air required to purge, was relatively unchanged. Reducing the relief setting from 0.53 

psig to 0.13 psig resulted in only a minor reduction of the time to purge and exchange rate. These 

purging experiments were conducted to gain a better understanding of the purging phenomena 

and to familiarize researchers with the instrumentation to be employed in the later studies using 

actual RAs. Therefore, purge times shorter than 10 minutes were not conducted. 

Table 3. Test results for purging at 0.53 and 0.13 psig relief pressures 

Purge Air 

Flow, cfm

Time to Purge 

from 400 to 25 

ppm CO, min

Exchange 

Rate1

Relief 

Setting, 

psig

1.36 20 3.4 0.53

1.75 15 3.3 0.53

2.54 10 3.2 0.53

1.36 18.5 3.1 0.13

1.75 14.5 3.2 0.13

2.54 9.5 3.0 0.13

1

 Exchange Rate = (Time to reach 25 ppm (min) x Air Flow Rate (cfm)) / Enclosure Volume (cu ft)

Because followup RA purging tests in actual RAs would be done using sulfur hexafluoride 

(SF6) tracer gas and this testing would require that the purge air be shut off for 20 sec after each 

complete volume of purge air was added, mini purge box experiments were run at a flow of 1.71 

cfm with the purge air being shut off after each full air exchange and with a continuous flow. 

These tests were performed at a pressure relief valve setting of 0.0 psig. Table 4 shows the test 

results and indicates that there was not a measureable difference in the purging results whether 

the flow was continuous or intermittent. 

Table 4. Comparison of average CO concentration reduction for continuous purging air 

flow versus on/off purging air flow 

a. Purge air steady (0.0 psig relief setting). Purge air flow was 1.71 cfm, 

and the time for one complete airlock air volume was 4.7 min. 

Purge 

Number1

Average Reduction in CO Concentration 

for Each Airlock Air Volume, %

1 57

2 59

3 55

4 25

1

 Purge air flow time for purge number 4 was significantly less than the time for the previous purges 

because a concentration of 25 ppm was reached prior to the fourth complete air volume exchange. 
11

b. Purge air on/off (20 sec, 0.0 psig relief setting). Purge air flow was 1.71 cfm, 

and the time for one complete airlock air volume was 4.7 min. 

Purge 

Number1

Average Reduction in CO Concentration 

for Each Airlock Air Volume, %

1 56

2 56

3 54

4 36

1

 Purge air flow time for purge number 4 was significantly less than the time for the previous purges 

because a concentration of 25 ppm was reached prior to the fourth complete air volume exchange. 

Summary: Mini Purge Box Experiments 

The mini purge box experiments were successful in that they accomplished their intended goal 

of allowing NIOSH researchers to gain a better understanding of the purging phenomena and to 

familiarize them with the instrumentation to be employed in the studies using actual RAs. The 

experiments revealed that a CO concentration of 400 ppm could be diluted to 25 ppm in less than 

four air exchanges. In addition, it was shown that by increasing the purge air flow rate, the time to 

purge from 400 to 25 ppm could be reduced while maintaining a similar exchange rate. It was also 

found that reducing the relief setting from 0.53 psig to 0.13 psig resulted in only a minor reduction 

of the time to purge and exchange rate. Finally, the results show that there is not a measureable 

difference in the purging results whether the purge air flow was continuous or intermittent. 

Refuge Alternative Airlock Purging with Carbon Monoxide 

CO purging tests were performed on two RA airlocks in accordance with MSHA standards 

for testing of RA airlocks. According to 30 CFR §7.508, the airlock purge system must be 

capable of reducing the concentration of CO from 400 to 25 ppm within 20 min of the RA being 

deployed. Within this 20-min time period, the designed maximum number of occupants for the 

RA must be able to enter the airlock, purge, and enter the main portion of the chamber. This 

could require multiple purges if the airlock is not designed to allow all occupants to enter at one 

time. If the purging process is designed appropriately, the exchange ratio volume should not be 

greater than 4 to reduce the CO concentration from 400 to 25 ppm. 

Tests were performed to determine if it was possible to purge the airlocks in 20 min or less. 

Two different refuge alternative airlocks were used in the testing: an inflatable tent-type RA 

airlock and a rigid steel RA airlock. The tests consisted of purging the airlocks at various air flow 

rates to determine the exchange ratio and time to reach a CO concentration of 25 ppm. Three 

different variations of the tests were performed on one airlock and two on the other. 

The tent-type RA airlock was evaluated with the airlock empty of any occupants, then with 

one simulated occupant, and finally with seven simulated occupants. The rigid steel RA airlock 

was evaluated with no occupants and one simulated occupant. The evaluation employing only 

one simulated occupant might be considered the worst-case scenario because the airlock volume 

is reduced by the least amount. 
12

The airlock was loaded with a concentration of up to 425 ppm and allowed to stabilize for 

five minutes. Then a set purging air flow was allowed to enter the airlock. Three CO recorders 

were used to monitor the CO concentration in the airlock. The sampling points were located at 

various heights within the airlock to ascertain if uniform mixing of the contaminant occurred. 

When the CO concentration was reduced to 400 ppm the test was initiated and data recording 

begun. Once the CO level dropped below 25 ppm on all the CO recorders, the test was stopped. 

The test was then repeated at different purging air flow rates. 

Description of Mobile Refuge Alternative Airlocks 

Two actual mobile RA airlocks were used for these purging tests. One was a 7-man capacity 

airlock that was part of a 35-man inflatable tent-type RA (Figure 4). The manufacturer specified 

the airlock volume as 57 cu ft. The RA was designated a training model in that the tent was 

inflated using a blower system rather than by compressed air bottles. The second airlock was an 

8-man capacity airlock and was part of a rigid steel RA (Figure 5). This RA was also designated 

as a training model, and had a total capacity of 8 occupants. It was never intended to have an 

airlock that was purged. The manufacturer designed the RA to purge the airlock and main 

chamber simultaneously rather than just the airlock. The airlock was measured to have a free 

volume of 153.5 cu ft. Both airlocks came supplied with a purging relief system, but neither were 

supplied with purge air supply inlet ports. 

Figure 4. Airlock in a tent-type mobile refuge alternative. Photo by NIOSH 
13

Figure 5. Airlock in a rigid steel mobile refuge alternative. 

The tent-type RA airlock had two relief ports that were gravity style with an exhaust area of 

6.8 sq in each and a relief pressure of approximately 0.18 psig. One relief port exhausted air from 

the airlock to the outside while the second was installed to purge from the airlock back into the 

chamber.8

8

 The RA manufacturer confirmed that this second port was installed backwards. Its intended purpose 

was to allow the exhaust of air from the main chamber into the airlock in the event of overpressure in the 

main chamber. This relief port would be closed during purging and, as such, would have no impact on the 

purging tests. Therefore, it was taped shut. 

The rigid steel RA was supplied with two, spring-operated relief ports with an exhaust area 

of 8.29 sq in each and relief pressures of approximately 0.5 psig. In the airlock, one relief port 

permitted air to flow from the main chamber into the airlock while the other purged from the 

airlock to the outside environment. NIOSH modified the pressure relief port that vented from the 

airlock to the outside by changing the springs to allow for a lower relief pressure. These ports 

were installed in this manner because the RA manufacturer intended that all eight occupants 

would enter the airlock and proceed directly into the chamber without stopping. After both of the 

airlock doors were closed, the entire chamber would be purged through the airlock to the outside. 

Tests were performed at given air flow rates in both purging rooms to determine the time 

required to reduce the CO concentration in the airlock from 400 to 25 ppm. In addition, the 

airlock volume exchange ratio at the given air flow rates was determined. Both airlocks were 

tested at six different air flow rates with no occupants and then the tests were repeated with one 

simulated occupant. The tent-type refuge alternative airlock was also tested with seven simulated 

occupants. Because test results showed that purging from 400 to 25 ppm in four or fewer air 

exchanges was easy to accomplish, there was no attempt at testing various designs of purging 

port configurations, vent size, or locations. 

 

Photo by NIOSH 
14

Simulation of Occupants for Testing Procedures 

Occupants were simulated by fabricating containers (boxes) from wood and sealed with 

plastic and tape. Each box was identical and had a volume of 2.83 cu ft. This volume was 

derived from a study conducted by Ward on the body volume of adult men [Ward 1967], and 

from surveys conducted by McWilliams et al. [NIOSH 2012] and by the National Mining 

Association [NMA 2011] on the average age of coal miners. Boxes of the appropriate volume 

were used to simulate actual human beings. 

Test Procedure for CO Purging of Refuge Alternative Airlocks 

Because the tent-type mobile RA airlock was not supplied with a purge air inlet, the entrance 

handle was removed and a supply port, three CO sampling ports, and a door latch were 

fabricated to fit the existing entrance handle opening. The rigid steel RA airlock was modified 

also, allowing for the installation of the purge air inlet port and three CO sampling ports. The 

purge air inlet port was plumbed to discharge just above floor level of the rigid steel RA airlock, 

whereas the purge air inlet port discharged through the door at mid-height in the tent-type RA 

airlock. Because the purge configuration for all RAs varies, no attempt was made to match the 

purge inlet configurations for these two airlocks. However, an attempt was made to keep the inlet 

and outlet as far apart as possible, given the modification limitations provided by the two RA 

manufacturers. The purge air inlet port for the tent-type RA airlock was 1⁄2-in NPT pipe with a 

flat deflector plate on the discharge side. The purge air inlet port for the rigid steel RA airlock 

consisted of a 1⁄2-in NPT pipe inlet into the chamber and 1⁄2-in I.D. tubing inside the airlock, 

discharging horizontally 1 in off the floor parallel to the main chamber entrance door. Simplified 

purge air flow schematics for the two RA airlocks are illustrated in Figures 6 and 7. 

The purge air for the tent-type RA airlock was supplied from a high-pressure cylinder bank 

of 12 pressurized air cylinders connected to a common manifold and pressure regulator. The 

plumbing from the supply air to the airlock purging ports consisted of an air pressure regulator, 

air flow shutoff valve, flow control valve, air flow meter, pressure gage, temperature probe, and 

CO charging port with valve. Because of the large volume of air required to perform the tests on 

the rigid steel RA airlock, a portable air compressor was used with a maximum air flow capacity 

of 125 SCFM at 125 psi. This required a moisture/oil separator between the air pressure regulator 

and the shutoff valve. 

Figure 8 illustrates the plumbing diagrams for the tests. All purge plumbing for the tent-type 

and rigid steel RA airlocks utilized the same plumbing from the flow control valve to the purging 

port inlets. The only difference was from the shutoff valve to the air supply because of the 

different purging air supplies. For all tests, CO was used as the charging (contaminant) gas and 

was released from a pressurized cylinder containing 99.9% CO. 

The gas sample ports had extension tubes installed to reach the center of the airlocks, with all 

tubes of equal length to keep the sample time constant and equal. The two sample ports for the 

tent-type RA airlock were located 4 inches off the floor and ceiling with a third located in the 

middle. The rigid steel RA airlock had the sample tubes located 6 in off the floor and ceiling 

with the third in the middle. Each sample port was connected to an Industrial Scientific iSP 

motorized sample pump that used an Industrial Scientific iTX multi-gas monitor/recorder. The 

airlocks had a low-pressure gage installed to record the internal pressure of the airlock during 

purging. Table 5 lists the instruments and specifications for this testing. 
15

40 in

Figure 6. Plan view of air flow schematic for the tent-type refuge alternative airlock during purging. 

Vent

Airlock Entrance

Vent

Main Chamber Entrance

Airlock

Main Chamber

Purge Air

Figure 7. Plan view of air flow schematic for the rigid steel refuge alternative airlock during purging. 

84 in

89.5 in

46-52 in

Airlock Entrance

Main Chamber Entrance

Purge Air

Airlock

Main Chamber

Vent
16

Figure 8. Purge plumbing diagrams. Top diagram is for the tent-type refuge alternative airlock. 

Bottom diagram is for the rigid steel refuge alternative airlock. 

Shut Off Valve

Pipe Union

Pressure Regulator

Flow Control Valve

Flow Meter

To CO Charging Cylinder

To Airlock Purge Port

Pipe Union

To Air Compressor

Moisture/Oil Separator

Pressure Gage

CO Charge Gas Valve

Temperature Thermocouple

Shut Off Valve

Pipe Union

Pressure Regulator Flow Control Valve

Flow Meter

Pressure Gage

CO Charge Gas Valve

To CO Charging Cylinder

To Airlock Purge Port

Pipe Union

To Air Cylinder

Manifold

Temperature Thermocouple

Rigid Steel Refuge Alternative Airlock Purge Plumbing Diagram

Tent-Type Refuge Alternative Airlock Purge Plumbing Diagram
17

Table 5. Instrumentation and apparatus used during purging of CO 

from mobile refuge alternative airlocks 

Apparatus Test Parameter Measured Specification

Air flow meter Purge air flow rate

Manufacturer: Dwyer Instruments, Inc.

Model: VFC-122

Range: 0–50 SCFM

Accuracy: 2% of Full Scale

Gas level detector CO contaminant concentration 

and total time of test

Manufacturer: Industrial Scientific Corporation

Model: ITX with CO monitoring configuration

Range: 0 to 999 ppm

Accuracy: 1 ppm, ±5% of reading

Recording: Set at 1 sample every 2 sec

Included air temperature monitoring

Gas sampling pump CO contaminant concentration

Manufacturer: Industrial Scientific Corporation

Model: iSP

Sample Distance: up to 100 ft

Stop watch CO stabilization time Manufacturer: H Heuer Instruments Pty Ltd.

Model: Trackstar 7-jewels

Air pressure gage

Purge air pressure on airlock 

side of flow meter to correct 

air flow

Manufacturer: Ashcroft, Inc.

Model: 35W1005PH 02L 60#

Range: 0–60 psi

Accuracy: 1%, NIST-Certified Calibration

Lower pressure 

gage

Airlock internal pressure

Manufacturer: NOSHOK, Inc.

Model: 25-200-30-H2O

Range: 0–30 in. H2O

Accuracy: NIST-Certified Calibration

Air temperature 

thermocouple

Purge air temperature on 

airlock side of flow meter to 

correct air flow

Manufacturer: OMEGA Engineering, Inc.

Model: JMQSS-125G-6

Type: J

Thermocouple 

meter

Purge air temperature on 

airlock side of flow meter to 

correct air flow

Manufacturer: OMEGA Engineering, Inc.

Model: HH23A

Accuracy: NIST-Certified, 0.1% accuracy, .1 

degree resolution in degree F & C
18

Tests were completed with no occupants, one simulated occupant, and seven simulated 

occupants for the tent-type RA airlock, and no occupants and one simulated occupant for the 

rigid steel RA airlock.9

9

Although it would have been logical to conduct tests with seven and eight simulated persons in the rigid 

steel RA as well, compressor malfunctions, including excessive amounts of oil in the air, prevented the 

two additional tests from being completed. The excessive oil might have damaged the CO monitoring 

equipment and necessitated frequent cleaning of the flow control and monitoring equipment.

Each test was designated with an identification label for documentation. All tests performed 

on the tent-type refuge alternative airlock began with “TT,” while the tests in the rigid steel RA 

airlock began with the letters “RS.” This RA designation was followed by the uncorrected air 

flow setting, and finally an indication of whether the air lock was occupied or unoccupied and 

the number of occupants. For example, TT-20-0 refers to a test in the tent-type RA airlock, 20 

cfm uncorrected air flow, and unoccupied, while RS-30-1 refers to a test in the rigid steel RA 

airlock, 30 cfm uncorrected air flow, and one simulated occupant. For each test, the air flow rate, 

pressure, temperature, and airlock internal pressure were documented. The serial numbers of the 

iTX multi-gas monitors were recorded along with their sample locations, recording start time, 

and the purging start time. Any other occurrences that were observed during the tests were also 

noted on a data sheet. Prior to each test, the three iTX multi-gas monitors were set to record at a 

sample rate of one sample every 2 sec. The time clocks for the three monitors were synchronized 

as well. Then each of the iTXs were zeroed and checked against a calibration gas of 100 ppm CO 

to verify accuracy. If any inaccuracy was found the units were recalibrated. 

Initially, the air flow shutoff valve was opened all the way and the air flow rate for the given 

test was set using the flow control valve. Once set, the flow control valve was not adjusted 

during the tests. The air flow for starting and stopping the purging was controlled by the shutoff 

valve. For charging the airlock with CO, approximately 10 cfm of air was allowed to enter the 

chamber while the CO was added to the air. Once the CO concentration inside the airlock 

reached approximately 450 ppm, the CO was shut off and the air flow continued until the multigas monitors indicated a CO concentration of between 400 and 420 ppm. The multi-gas monitors 

were then set to begin recording the CO concentration. 

The CO concentration was allowed to stabilize in the air lock for 5 min before purging was 

initiated. After the 5-min stabilization period the air flow shutoff valve was opened, allowing the 

purging air to enter the airlock at the previously set flow rate. Once the purging began, the air 

flow, outlet pressure, air flow temperature, and airlock internal low pressure were recorded. The 

iTX multi-gas monitor’s recording was stopped once all the monitors were reading below 25 

ppm of CO. At the completion of each test, the air flow control valve was reset to the next 

desired air flow rate and the procedure repeated until all six tests for the given condition were 

completed. The data were then downloaded from the iTX multi-gas monitors and placed into a 

spreadsheet for analysis. The air flow meter was corrected at this time to SCFM which was used 

during the data analysis.

 
19

Data Analysis and Test Results for CO Purging Experiments 

Using the recorded data from the air flow meter, flow meter outlet pressure gage, and air flow 

temperature, the air flow meter readings were corrected back to standard units (SCFM). The 

equation to correct for nonstandard operating conditions is: 

 √ 

(1)

where: Q2 = Standard flow corrected for pressure and temperature, SCFM; 

Q1 = Actual or observed flow meter reading, cfm; 

P1 = Actual pressure (14.7 psia + gage pressure), psig; 

P2 = Standard pressure (14.7 psia, which is 0 psig), psig; 

T1 = Actual temperature (460°R + temp °F), °F; and 

T2 = Standard temperature (530°R, which is 70°F), °F. 

The data were downloaded from the iTX multi-gas monitors and placed into an analysis 

spreadsheet with their designated sample locations (high, middle, low). The recorded data was 

analyzed and the longest time required to purge from 400 to 25 ppm level of CO was recorded. 

The airlock volume used during the analysis was the actual empty volume minus any 

simulated occupants. From the corrected air flow rate, volume of the airlock, and time to reach 

the 25 ppm CO level, the exchange ratio was determined. Volumes used during the exchange 

ratio calculations are shown in Table 6. The exchange ratio calculation follows: 

 

(

 

 ) 

 

(2)

where: t1 = Time to reach 25 ppm CO, min; 

f1 = Corrected air flow rate, SCFM; and 

v1 = Volume of airlock with no occupants, one occupant, or seven occupants, cu ft. 

Table 6. Airlock volumes used during exchange ratio calculations 

Chamber Type Number of Occupants Airlock Volume, cu ft

Tent-type 0 57.0

Tent-type 1 54.2

Tent-type 7 37.2

Rigid steel 0 153.5

Rigid steel 1 150.7
20

Once all the individual series of tests with the same conditions and various air flow rates 

were completed, the resultant data was combined and tabulated as shown in Tables 7 and 8. This 

data was used to plot all three iTX multi-gas monitor readings of the CO concentration and the 

time required to reach a CO concentration of 25 ppm. A representative example of the data is 

plotted in Figures 9 and 10 for the tent-type and rigid steel RA airlocks, respectively. Figures 9 

and 10 illustrate that the three gas sample locations gave nearly identical readings (making it 

difficult to see the readings for “Low” on the line graphs in these figures), which indicated that 

complete mixing was occurring within the airlocks for the case of no occupants and one 

occupant. The tests of the 7 simulated occupants in the tent-type RA airlock display an initial 

variance between the three iTX multi-gas monitors, signifying that some layering was taking 

place during the initial stages of purging, but eventually the readings converged, indicating that 

complete mixing did occur as the purging progressed. 

Table 7. Tabulated data for tent-type RA airlock purging of CO with various number of 

occupants, purge air flow rates, and relief pressure of 0.18 psig

Test1

Air Flow Setting

Uncorrected, 

cfm2

Air Flow 

Corrected, 

SCFM3,4

Number of 

Simulated 

Occupants

Exchange Ratio 

Corrected for 

Occupants5

Time to 

Purge, 

sec

Volume of 

Air Used to 

Purge, cu ft

TT-20-0 20 24.67 0 2.63 365 150.1

TT-25-0 25 34.04 0 2.98 300 170.2

TT-30-0 30 47.93 0 3.36 240 191.7

TT-35-0 35 63.08 0 3.41 185 194.5

TT-40-0 40 84.52 0 3.80 154 216.9

TT-45-0 45 105.15 0 4.24 138 241.8

TT-20-1 20 24.85 1 2.74 358 148.3

TT-25-1 25 34.70 1 3.02 282 163.1

TT-30-1 30 48.95 1 3.29 218 177.9

TT-35-1 35 67.27 1 3.65 176 197.3

TT-40-1 40 85.93 1 3.76 142 203.4

TT-45-1 45 105.66 1 4.30 132 232.5

TT-20-7 20 26.14 7 2.83 242 105.4

TT-25-7 25 37.55 7 3.43 204 127.7

TT-30-7 30 53.61 7 4.27 178 159.0

TT-35-7 35 70.95 7 5.91 180 212.9

TT-40-7 40 99.15 7 8.26 186 307.4

TT-45-7 44 114.10 7 10.32 202 384.1

1

Files ending in 0 are no occupants, 1 is one simulated occupant, and 7 is seven simulated occupants. 

2Air flow setting on flow control valve. 

3Corrected using Equation (1). 

4Actual pressure ranged from 6 psi for the 20 cfm air flow to 73 psi for the 44 cfm air flow test. In addition, 

temperatures ranged from 10°F to 50.5°F because the purge air was supplied from compressed air bottles. 

5All exchange ratios were corrected for volume (number of occupants) using Equation (2). 
21

Table 8. Tabulated data for rigid steel RA airlock purging of CO with various numbers of 

occupants, purge air flow rates, and relief pressures 

1 Test

Air Flow 

Setting 

Uncorrected, 

2 cfm

Air Flow 

Corrected, 

3,4 SCFM

Number 

of 

Simulated 

Occupants 

Exchange Ratio 

Corrected for 

5 Occupants

Time to 

Purge, sec 

Relief 

Pressure, 

6 in H2O

Volume of 

Air Used to 

Purge, cu ft 

RS-24-0 22 27.69 0 2.83 942 2.0 434.7 

RS-30-0 26 35.79 0 3.03 780 2.5 465.3 

RS-34-0 30 47.08 0 3.09 606 3.0 475.5 

RS-36-0 34 60.56 0 3.28 500 3.5 504.7 

RS-40-0 38 76.69 0 3.30 396 3.5 506.2 

RS-44-0 42 95.42 0 3.56 344 4.0 547.1 

RS-22-1 22 28.08 1 2.84 916 1.5 428.7 

RS-26-1 26 36.45 1 3.04 756 2.0 459.3 

RS-30-1 30 47.44 1 3.06 584 2.5 461.7 

RS-34-1 34 60.38 1 3.44 516 3.0 519.3 

RS-38-1 38 76.31 1 3.36 398 3.5 506.2 

RS-42-1 42 96.43 1 3.60 338 3.5 543.2 

1

Files ending in 0 are no occupants, and 1 is one simulated occupant. 

2Air flow setting on flow control valve. 

3Corrected using Equation (1). 

4Actual pressure ranged from 8.5 psi for the 22 cfm air flow to 62.5 psi for the 42 cfm air flow test. In addition, 

temperatures ranged from 62°F to 71°F because purge air was supplied from a diesel-powered compressor located 

outside the building.

5All exchange ratios were corrected for volume (number of occupants) using Equation (2). 

6

Indicated on the low pressure gage that measured the airlock internal pressure. Internal pressure increased as the 

flow rate increased because the relief valve was spring operated. Increased pressure is required to compress the 

spring, resulting in a larger relief outlet opening that allows more purge air to flow out of the airlock. 
22

Figure 9. Representative example of data for carbon monoxide (CO) purging of airlock

for a tent-type refuge alternative, test TT-25-1. 

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

CO, ppm 

Time, sec 

Low Middle High

Exchange ratio of 3.02:1 

282-sec purge time 

Figure 10. Representative example of data for carbon monoxide (CO) purging of airlock 

for a rigid steel refuge alternative, test RS-36-0.

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

375

400

425

450

0 50 100 150 200 250 300 350 400 450 500

CO, ppm 

Time, sec 

Low Middle High

Exchange ratio of 3.28:1 

500-sec purge time 
23

The data were analyzed to compare exchange ratios, air flow rates, time to purge to 25 ppm,

and total volume of air required to purge. A regression analysis was performed and included for

each condition plotted. 

The graphs of purge time (400 to 25 ppm CO) versus air flow rate (Figure 11), the total 

volume of air required to purge (400 to 25 ppm CO) versus air flow rate (Figure 12), and the 

exchange ratio (volume of air required to purge from 400 to 25 ppm CO/purge room volume) 

versus air flow rate (Figure 13) show several interesting findings: 

 Figure 11 indicates that at an airflow rate of about 70 SCFM, purging efficiency 

decreases with seven occupants in the tent-type RA. No such effect was noticed for 

any of the other airlock/occupant combinations. 

 Figure 11 also illustrates that purge time from 400 ppm to 25 ppm CO is reduced as 

the air flow is increased, except for the seven-occupant tent-type RA airlock above 70 

SCFM. 

 Figure 12 depicts the relationship between purge air flow and total air volume 

required to purge the airlocks. It reveals that as the air flow increases the total volume 

of air to reduce CO from 400 to 25 ppm increases. 

A final characteristic of purging and an important part of these tests is the exchange ratio as 

determined by the ratio of the volume of purge air to the airlock volume. “Effective” airlock 

purging occurs when the exchange ratio is 4 or less. Airlock purging is designed to reduce the 

th contaminant to 1/16 (400 to 25 ppm CO) of its original concentration in four or fewer air 

exchanges. Figure 13 illustrates that for the tent-type airlock and rigid steel airlock with zero or 

one occupant, the exchange ratio was less than 4 for nearly all air flow rates. It was above 4 for 

the tent-type airlock (regardless of the number of occupants) when the air flow exceeded about 

90 SCFM and, with seven simulated occupants, the exchange ratio was greater than 4 once the 

air flow exceeded approximately 50 SCFM. Even though the time to purge remained nearly 

constant at air flows above 50 SCFM as illustrated on Figure 13, a significantly larger volume of 

air is needed to purge the CO from 400 to 25 ppm. 
24

Figure 11. Graph of time required to purge versus air flow rate.

0

100

200

300

400

500

600

700

800

900

1000

20 30 40 50 60 70 80 90 100 110 120

PurgeTime, 400 ppm to 25 ppm CO, sec 

Air Flow, SCFM 

Tent-Type No Occupants Rigid Steel No Occupants Tent-Type Seven Occupants

Tent-Type One Occupant Rigid Steel One Occupant

Figure 12. Graph of total volume of air required to purge versus air flow rate.

50

100

150

200

250

300

350

400

450

500

550

600

20 30 40 50 60 70 80 90 100 110 120

Total Volume of Air, cu ft 

Purge Air Flow, SCFM 

Tent-Type No Occupants Rigid Steel No Occupants Tent-Type Seven Occupants

Tent-Type One Occupant Rigid Steel One Occupant
25

Figure 13. Graph of exchange ratio versus air flow for the tent-type and 

rigid steel mobile refuge alternative airlocks. 

2

3

4

5

6

7

8

9

10

20 30 40 50 60 70 80 90 100 110

Exchange Ratio 

(Purge Air Volume/Airlock Volume)

Air Flow, SCFM

Tent-Type All Rigid Steel All Tent-Type 7 Occupants

Summary: Refuge Alternative Airlock Purging with Carbon Monoxide 

The airlock purging with carbon monoxide research was completed in two different RA 

airlocks with zero, one, or seven simulated occupants. In general, purging was shown to be 

effective and occurred in fewer than four air exchanges. The exception was when seven 

simulated occupants were placed in the tent-type RA airlock with an empty volume of 57 cu ft. 

In this case, the exchange ratio was greater than 4 when air flows above approximately 50 SCFM 

were used for purging. 

It was also determined that the time to purge from 400 to 25 ppm decreased as the air flow 

increased, but at the expense of requiring greater quantities of purge air and higher exchange 

rates. Finally, regression analyses of the data indicated that the tests were very repeatable and 

thus might be used for estimating purge times, purge air volumes, and exchange ratios for other 

RA airlocks with different volumes and numbers of occupants 
26

Airlock Purging with Sulfur Hexafluoride 

The final series of tests to determine if the current generation of mobile RAs employ 

technology capable of purging the internal atmosphere from 400 to 25 ppm CO as required by 30 

CFR § 7.508 were completed using sulfur hexafluoride (SF6) as the contaminant gas and purging 

with live occupants inside the tent-type mobile RA airlock. 

A large room containing the RA was filled with a uniform low concentration of SF6 and the 

SF6 entered the airlock when the airlock door was opened and researchers, representing miners, 

entered the airlock. After 5 min to allow for complete contaminant gas mixing in the airlock, the 

airlock was purged at the air flow rates used in the CO purging experiments. The only difference 

was that after each complete airlock air volume of purge air was injected into the airlock, the 

purge air was shut off for approximately 10 to 15 sec so that SF6 concentration samples could be 

collected by the occupants in the airlock. During the original CO purging experiments, the purge 

air was continuously injected. This on/off procedure was continued for six complete airlock air 

volume exchanges. 

Results of Airlock Purging with Sulfur Hexafluoride 

Table 9 includes the results of the SF6 purging of the tent-type RA airlock with one or five 

live occupants inside. While not identical for both cases, Table 9 shows that the first four purges 

were more efficient than what occurred during the CO purging experiments. The reduction, or 

dilution, was above 50%, ranging from 52% to 73%. The purging of SF6 with live occupants in 

the airlock was more efficient but more variable than when the tests were conducted with 

simulated occupants and CO as the contaminant gas. 

Table 10 presents a comparison of purging efficiency with one simulated or live occupant, 

purge air flow of approximately 86 SCFM, and CO or SF6 contaminant gas. Table 10 illustrates 

that the CO purging was very consistent at reducing the contaminant concentration (46%–52%) 

for each complete air volume exchange, while during the SF6 purging the efficiency was greater 

in most instances but more variable as well. 

Some of the test variables that could explain the differences observed between the CO 

purging and SF6 purging are included in Table 11. Both test approaches show that purging can be 

effectively achieved, reducing CO purging room concentrations from 400 to 25 ppm in four or 

fewer complete air exchanges. 
27

Table 9. Results from SF6 purging of tent-type RA airlock 

a. One person entering (Test 1) 

1 Purge Average SF6 

Concentration, ppb 

Reduction of SF6 

Concentration, % 

Cumulative SF6 

Reduction, % 

Four Volume 

Exchange 

Goal, ppb 

Start 107.11 0 0 6.69 

1 40.70 62 62 6.69 

2 10.91 73 90 6.69 

3 5.05 54 95 6.69 

4 2.88 43 97 6.69 

5 2.28 21 98 6.69 

6 2.03 11 98 6.69 

1

Purges 1 through 6 denote one complete airlock volume of purge air. 

b. One person entering (Test 2) 

Purge1 Average SF6 

Concentration, ppb

Reduction of SF6 

Concentration, %

Cumulative SF6 

Reduction, %

Four Volume 

Exchange 

Goal, ppb

Start 137.37 0 0 8.59

1 56.79 59 59 8.59

2 16.19 72 88 8.59

3 5.32 67 96 8.59

4 2.42 55 98 8.59

5 1.81 25 99 8.59

6 1.61 11 99 8.59

1

Purges 1 through 6 denote one complete airlock volume of purge air. 

c. Five persons entering 

1 Purge Average SF6 

Concentration, ppb 

Reduction of SF6 

Concentration, % 

Cumulative SF6 

Reduction, % 

Four Volume 

Exchange 

Goal, ppb 

Start 149.25 0 0 9.33 

1 57.98 61 61 9.33 

2 18.92 67 87 9.33 

3 6.33 67 96 9.33 

4 3.03 52 98 9.33 

5 2.27 25 98 9.33 

6 2.01 11 99 9.33 

1

Purges 1 through 6 denote one complete airlock volume of purge air. 
28

Table 10. Comparison of purging efficiency for the CO and SF6 purging, 

one occupant, 86 SCFM 

Contaminant Gas 

Carbon monoxide 

1 Purge Initial 

Concentration (Ci) 

Contaminant 

Concentration after 

each Purge (Cp) 

Contaminant 

Concentration 

2 Reduction, %

1 405 ppm 189 ppm 53 

Carbon monoxide 2 405 ppm 87 ppm 54 

Carbon monoxide 3 405 ppm 42 ppm 52 

Carbon monoxide 

Sulfur hexafluoride 

4 405 ppm 22 ppm 48 

1 107 ppb 41 ppb 62 

Sulfur hexafluoride 2 107 ppb 11 ppb 73 

Sulfur hexafluoride 3 107 ppb 5 ppb 55 

Sulfur hexafluoride 4 107 ppb 3 ppb 40 

1

Purges 1 through 4 denote one complete airlock volume of purge air. 

2

Percentage of Reduction = (1 – Cp/Ci) x 100, where Ci is initial concentration and Cp is the subsequent purge 

concentration. Ci and Cp change for each subsequent percentage of reduction calculation. 

Table 11. Test differences or variables that may be responsible for differences 

in purging efficiency 

Test 

(Contaminant Gas) Test Observations Test Differences/Variables 

Carbon monoxide 

purging 

Consistent contaminant 

reduction 

Consistently efficient purging 

Simulated occupant(s) 

No extraneous air movement 

Continuous purge air flow 

Instantaneous data recording 

Reliable and accurate gas sampling 

Controlled testing 

Constant purge relief pressure 

Sulfur hexafluoride 

purging 

Variable contaminant reduction 

Improved purging efficiency 

One or five human occupant(s) 

Possible occupant-generated air movement 

On/Off purge air flow 

Sample collection lag 

Vacutainer sampling slightly less accurate 

Semi-controlled testing (human subject 

involved) 

Variable purge relief pressure 
29

Summary: Refuge Alternative Airlock Purging with Sulfur Hexafluoride 

The airlock purging with sulfur hexafluoride research revealed that the purging of the tenttype RA airlock with live occupants and SF6 as the contaminant gas was more efficient, but also 

more variable, than the purging of CO with simulated occupants. The percentage of contaminant 

reduction for the first three purges with live occupants in the airlock ranged from 52% to 73% as 

compared to 52% to 54% when the simulated occupants were in the airlock. Some reasons have 

been proposed but not verified as to why the purging with SF6 as the contaminant gas and live 

occupants was more efficient but also more variable. However, the key finding is that the SF6 

testing, as with the CO testing in both the mini purge box and the actual RAs, showed that 

purging can be effectively achieved, which reduces CO concentrations in a purging room from 

400 to 25 ppm in four or fewer complete air exchanges. 

Airlock Contamination Research 

Miners enter most mobile refuge alternatives (RAs) through an airlock room. If carbon 

monoxide (CO) or other harmful gases from the ambient post-disaster mine air enter the airlock 

as the miners enter, purging must be conducted to reduce the contaminant to an acceptable level. 

In the case of CO, it must be purged to a concentration of 25 ppm or less before the miners enter 

the main chamber of the RA. For RA approval, 30 CFR § 7.508 (c) (2) states that “For testing the 

component’s ability to remove carbon monoxide, the structure shall be filled with a test gas of 

either purified synthetic air or purified nitrogen that contains 400 ppm carbon monoxide, ± 5 

percent.” This has been interpreted to mean that the highest concentration of CO that will need to 

be purged from a RA airlock after miners have entered is 400 ppm, but the ± 5 percent criteria 

indicates that the actual maximum concentration is 420 ppm. 

Historical Data on Measured Post-Disaster CO Concentrations 

A review of historical data related to ambient CO concentrations in underground coal mines 

following mine disasters shows a wide range of measured CO concentrations. The resulting CO 

concentrations depend upon the type of disaster (mine fire, CH4 explosion, or coal dust 

explosion), the extent of the fire or explosion, and whether or not the ventilation system is 

disrupted or destroyed by the event. RAs need to be designed to function and handle the 

expected range of CO concentrations—thus the higher CO concentrations that could be present 

following a disaster must be considered when evaluating the use of mobile RAs. 

Some examples of high CO concentrations can be found in the measurements recorded 

during some of the major coal mine disasters. Gas measurements taken by the Mine Safety and 

Health Administration (MSHA) in the first borehole at the Upper Big Branch Mine following the 

2010 explosion showed CO concentrations at the borehole, which was not far from a portable 

RA, as high as 14,250 ppm CO (1.4% CO) [Wharry 2013]. Table 12 presents some of the CO 

concentrations measured in the Upper Big Branch Mine in the hours and days after the disaster 

occurred. It reveals that CO concentrations can vary widely throughout the mine and can be 

above 400 ppm even four days post-disaster. 
30

Table 12. Example of CO concentrations measured post-disaster 

at the Upper Big Branch Mine 

Mine Location 1 Day/Time CO, ppm Reference 

Bandy Bleeder Fan First Night/9:42 pm 4,350 Command Center Log, 

D-0042A, p. 13 [MSHA 2010a] 

HG22 Mouth First Night/10:07 pm 122 Command Center Log, 

D-0105, p. 7 [MSHA 2010b] 

Glory Hole Heading First Night/10:22 pm 774 Command Center Log, 

D-0042A, p. 16 [MSHA 2010a] 

HG22 Switch First Night/11:00 pm 40–80 Interview Transcript, 

I-0024a, p. 21 [MHS&T 2010b] 

HG 22, 3 Breaks inby 

Miner Panel First Night/11:30 pm 8,676 Command Center Log, D-0042A, p. 19 [MSHA 2010a] 

HG22, Xcut 22, at 

Section Mantrip First Night/12:15 am 9,999 Command Center Log, D-0042A, p. 21 [MSHA 2010a] 

Crossover Belt 2 First Night/NTR 280 Interview Transcript, I-0259, 

pp. 28–29 [MHS&T 2012] 

LW Tail First Night/NTR 

9,999 

(Solarises 

pegged) 

Interview Transcript, I-0259, 

p. 50 [MHS&T 2012] 

LW Headgate Second Day/NTR 5–6 Interview Transcript, I-0045, 

p. 38 [MHS&T 2010a] 

Areas “A”, “B”, “D” Fourth Day/7–8:00 pm 382–1,169 Command Center Log, D-0105, 

pp. 26-28 [MSHA 2010b] 

FAB 135, Behind 

Temp Seal No. 3 Seventh Day/3:00 pm 1,592 MEO Log Book, D-0032A, p. 60 [MSHA 2010c] 

1

 Date/Time is relative to the time of the explosion which occurred at 3:02 pm. 

2

 NTR = No time reported 

In another example, in the Willow Creek Mine fire in 1998, the concentration of CO reached 

approximately 38,000 ppm. These concentrations were higher than normally found in most mine 

disasters in the U.S. due to the presence of liquid hydrocarbons. During the fire recovery at 

Willow Creek while under apparatus, the ambient air contained less than 2% O2, 60 to 65% CH4, 

and approximately 2,500 to 3,000 ppm CO [Trackemas 2013]. Mine rescue teams taking gas 

measurements during rescue and recovery after the Willow Creek Mine disaster in 2000 reported 

CO concentrations up to 4.9% (almost 50,000 ppm) [MSHA 2001]. 
31

After the Sago Mine disaster, CO concentrations were measured at the drift mouth and 

through Borehole No. 1 which was drilled into the section where the miners were barricaded 

[MSHA 2007]. At the drift, the highest concentration recorded was 2,600 ppm approximately 8 

hrs after the initial explosion. At Borehole No. 1, the highest concentration was 1,052 ppm, which 

was measured shortly after the borehole was completed approximately 23 hrs after the event. 

The above concentrations are in line with CO data obtained from coal mine dust explosion 

tests conducted in the Bruceton Experimental Mine of the USBM. MSHA publication IR 1231 

cites 63 gas samples collected from 37 full-scale coal mine dust explosions in the Bruceton 

Experimental Mine [Hofer et al. 1996]. Of the 63 gas samples collected, only 11 showed CO 

mine ambient concentrations less than 1% (10,000 ppm), while the remaining samples showed 

CO concentrations ranging from 1% to 9.7% (10,000 to 97,000 ppm). These are representative of 

the CO concentrations that could occur in a mine following a coal dust explosion if the mine 

ventilation system was not providing replacement air to the affected area of the mine. 

The above examples demonstrate that it is safe to assume that ambient CO concentrations in 

coal mines following a mine disaster could easily exceed 1%, or 10,000 ppm. 

Test Procedure for Airlock Contamination Research 

In order to determine the potential resulting CO concentration inside an RA airlock, the ratio 

of inside to outside contaminant concentration must be estimated. The NIOSH Office of Mine 

Safety and Health Research (OMSHR) conducted sulfur hexafluoride (SF6) tracer gas 

experiments to estimate this ratio under a variety of conditions, including different numbers of 

miners entering the airlock, different door open times, and different size airlocks and airlock 

doors. Under these variety of conditions, an estimate of the ratio of the resulting airlock CO 

concentration to the CO concentration in the ambient mine atmosphere outside of the airlock was 

made. 

The primary objective of this research was to estimate the possible concentration of 

contaminated gas that could be found in a mobile refuge alternative airlock after occupants enter 

and prior to purging of the airlock relative to the concentration in the external ambient mine 

environment. This is important to know because it dictates the quantity of purge air and time 

needed to reduce the contaminant (presumably CO) level to 25 ppm prior to occupants entering 

the main chamber of the RA. To estimate the concentration of contaminated air that could enter 

an RA airlock as miners enter, a series of experiments were conducted using sulfur hexafluoride 

(SF6) tracer gas and live occupants entering an airlock. A protocol, “CO Entering Purging Room 

with Miners Entry (Test Plan 1),” was developed and externally reviewed (Appendix B). SF6 was 

used as the post-disaster mine air contaminant instead of CO for obvious safety reasons, and 

because the two gases behave similarly at the concentrations used during the testing. As noted by 

Dr. Peter Lagus [Lagus 2013], an expert in tracer gas use and detection: 

. . . any gas at less than 1% concentration in air acts just as the air acts. . . . My 

thirty some years of experience in using various tracer gases, especially SF6, has 

continually proven this to be true. We have seen it repeatedly in our studies of air 

movement in nuclear power plants when we study the impact of opening doors in 

clean rooms. When a volume of air moves, a small concentration of heavier 

molecules does not move differently. 
32

As further evidence of the similar behavior of CO and SF6 in air, Wallace and Hobbs [1977] 

state: 

In contrast to molecular diffusion, the mixing due to the motions of macroscale air 

parcels does not discriminate on the basis of molecular weight. Within the range 

of levels where this process predominates, atmospheric composition tends to be 

independent of height. 

All the tests involved in this research followed the same basic procedure and were conducted 

in the reverberation room in Building 154 at the NIOSH OMSHR facility in Bruceton, PA 

(Figure 14). Initially, the reverberation room and RA airlocks were ventilated to eliminate all 

traces of contaminant. Air samples were collected using vacutainer sample bottles and sample 

collection needles (Figure 15) in the reverberation room at a height of approximately 4 ft and in 

the RA airlock to verify that no contaminant concentration was present. 

Photo by NIOSH 

Figure 14. Refuge alternatives in OMSHR’s reverberation room in building 154 at the OMSHR 

Pittsburgh site, Bruceton, PA. 
33

Photo by NIOSH 

Figure 15. Vacutainer sample tube and needle assembly used to collect SF6air samples. 

After the airlock door was closed, a quantity of SF6 designed to obtain a concentration of 

approximately 150 ppb SF6 in the reverberation room was released into the air stream of one 

of the fans. The concentration of SF6 was kept below 200 ppb because this was the upper range 

of the chromatograph being used. Higher levels would have required dilution with pure air prior 

to analyzing each sample. Also, the NIOSH recommended exposure limit (REL) for SF6 is a 

time-weighted average (TWA) of 1,000 ppb; thus the tests concentrations were kept well below 

that level. 

The air in the reverberation room was mixed using a number of fans for 30 minutes to ensure 

that a uniform distribution was obtained (Figure 16). Air samples were collected at designated 

locations at intervals of 10, 20, and 30 min after SF6 release to verify that a uniform SF6 

concentration was obtained. Prior to subjects entering the airlock, all fans were turned off. Next, 

the airlock door was opened and a number of subjects (1, 3, 5, 7, or 8) entered the airlock then 

shut the airlock door. The total time the airlock door was open was noted. After 5 minutes, 

samples of the air inside and outside the airlock were collected. The subjects were then allowed 

to exit the airlock at which time ventilation of the airlock and reverberation room was initiated. 

When the test was complete the sample vacutainer tubes were taken to be analyzed by OMSHR 

technicans using a calibrated chromatograph. 

It should be noted that the entering of live occupants into the airlocks during this testing was 

not conducted to exactly replicate how miners would enter post-disaster. NIOSH personnel who 

served as live occupants for this testing wore street clothes only; they were not wearing mining 

boots, coveralls, coats, hard hats, belts and cap lamps, nor carrying other work-related items. In 

addition, NIOSH personnel were not wearing SCSRs as would likely be the case for miners 

entering a refuge alternative post-disaster. NIOSH personnel did not “drill” or practice 

beforehand to expedite their entering, but did discuss positioning, especially when seven or eight 

persons were entering to try to mininmize the congestion and time. Finally, because the same 

personnel were used in many of the tests, subjects likely became more efficient and took less 

time as the testing progressed. 
34 

Figure 16. Reverberation room showing mobile refuge alternative locations, fan locations, and 

sample recording locations. Sample locations 1, 2, 3, and 4 are for tent-type refuge alternative 

airlock test, locations 5, 6, and 7 are for the rigid steel refuge alternative airlock test, and location 

8 for the mock-up airlock tests. Note: Drawing is not to scale. 

4 3

1

2

7

8

6

5

Key 

Sample Location 

Fan 

Sample No. 

Reverberation Room

Main Door

Exit 

Tent-Type RA

Mock-up Airlock

Airlock

Rigid Steel RA

Airlock

Exit 

Exit 

Adjustment for SF6 in the Breath of Airlock Occupants

During an actual post-disaster entrance into an airlock by miners, they would likely be 

wearing SCSRs to isolate their lungs from the contaminated ambient mine air. Also, after 

entering the airlock, they would not contribute to the contaminant level in the airlock. In contrast,

during the contaminated air inflow testing in this study, the lungs of the subjects entering the 

airlock were not isolated from the ambient atmosphere contaminated by SF6. In this case, there 

would be a contribution to the airlock contamination level from the occupants breathing due to 

the SF6 in their lungs.
35

The SF6 that can be introduced into the airlock from an individual’s lungs must be accounted 

for in the analysis. To account for this, some general information as to an average individual’s 

lung capacity and body volume are needed. In general, an average person’s lung capacity is 

about 6 L (0.212 cu ft) of air. Assuming that the average person entering the airlock weighs 180 

lb, their body volume is about 2.8 cu ft. Therefore, the solid volume of the individual is their 

total volume minus their open lung space volume, or 2.588 cu ft (2.8 cu ft – 0.212 cu ft). 

If the volume of an empty airlock is V cu ft, then 0.212 cu ft/(V – 2.588 cu ft) is the ratio 

between an individual’s lung volume and the resulting volume of the airlock after the individual 

enters the airlock. If Y is the concentration of SF6 in the ambient air (reverberation room), then 

the amount of SF6 that results in the airlock from the breath of one individual that enters can be 

calculated as: 

 (3) 

where: I = Amount of SF6 that results in the airlock from the breath of one individual, ppb;

Y = Concentration of SF6 in the ambient air, ppb; and 

V = Volume of empty airlock, cu ft 

As an example, if the ambient SF6 concentration in the reverberation room outside the airlock 

is 150 ppb (Y), and the empty volume of an airlock is 57 cu ft (V), then the contribution to the 

airlock SF6 concentration from one person entering the airlock with SF6 in their lungs is 0.58 ppb 

(150 x 0.212)/(57 – 2.588). 

If the number of individuals entering the airlock is N, then the equation to be used to 

calculate their contribution is: 

 (4) 

where: TI = Total SF6 contribution from all individuals, ppb; 

I = Amount of SF6 in the airlock from the breath of one individual, ppb; and 

N = Number of individuals entering the airlock. 

Based on this equation, if seven individuals enter the 57 cu ft volume airlock and the SF6

concentration in the reverberation room is 150 ppb, then their contribution to the SF6

concentration in the airlock is 5.72 ppb ((150 x 0.212) x 7)/(57 -(7 x 2.588 cu ft)). 

The calculation completed above is the worst-case scenario given that it involves the lowest 

volume airlock (57 cu ft) and the maximum occupancy (7 persons) that NIOSH tested. Larger 

airlocks and/or fewer occupants will reduce this impact. For example, seven persons entering the 

rigid steel airlock (155 cu ft) would introduce an increase of only 1.63 ppb into the airlock. 

These contributions from occupants’ lungs to the SF6 concentrations in the airlocks have been 

subtracted from the resulting concentrations. If miners were to remove their SCSRs before 

entering the airlock, then they might also bring ambient CO into the airlock in their lungs. The 

above analysis could be used to evaluate miners’ CO contribution post-disaster if their lungs 

were not isolated by wearing an SCSR. 
36

Airlock Contamination Research Results

The airlock contamination testing was completed in three different size airlocks. The first 

was the airlock in a tent-type mobile refuge alternative. The RA and airlock occupancies were 35 

and 7 persons, respectively. The airlock volume at full chamber deployment was 57 cu ft. In 

addition, the airlock door opening was 25.5-in high x 24-in wide, or 4.25 sq ft. The second 

airlock was part of a rigid steel RA. Both the RA and airlock had a design capacity of 8 persons. 

In this case the airlock volume was approximately 153.5 cu ft. For this airlock the door opening 

was 46.25-in high x 29.5-in wide, or 9.5 sq ft. Finally, a mock-up airlock was constructed from 

wood with a volume of 102 cu ft and door opening of 36-in high x 26.75-in wide, or 6.7 sq ft. 

The mock-up airlock volume and door size were designed to fall halfway between the other two 

RA airlocks and doors. The reason for this was to have an airlock and door sized such that it 

would be easier for the occupants to enter than the smaller airlock but slightly more difficult to 

enter than the larger airlock. The size of the door opening and the time for occupants to enter are 

believed to be important factors in how much contaminated air enters the airlock. 

Air samples were collected at three specific times: prior to releasing SF6 to ascertain the prerelease contaminant levels in the airlock and reverberation room, if any; after SF6 release to 

ascertain the quantity and distribution of the tracer gas in the reverberation room; and after 

subjects entered the airlock to determine the airlock and outside contaminant concentrations. The 

samples were then sent for chromatograph analysis of SF6 concentration. After the samples were 

analyzed for SF6 concentration, they were recorded in an Excel file from which the analysis was 

completed. 

Airlock Contaminant Concentration versus Outside Contaminant Concentration 

Experiments were conducted for 1, 3, 5, 7, and 8 subjects entering the airlocks, depending on 

the design capacity. Since the outcome desired was to estimate the potential airlock 

contamination as subjects entered, the ratio of contaminant concentration inside the airlock to the 

concentration outside the airlock was calculated, then this ratio versus the number of subjects 

entering the airlock was plotted (Figure 17). Table 13 summarizes the data used to generate the 

plot shown in Figure 17. The results indicate that for the smallest airlock that was part of the 

tent-type RA, from 20% to 58% of the outside contaminant concentration entered the airlock, 

depending on the number of subjects entering the airlock. For the largest airlock (the steel RA 

airlock), from 5% to 32% of the outside contaminant concentration entered. For the mock-up 

airlock, 7% to 19% of the outside contaminant concentration entered. 
37

Figure 17. Graph of the ratio of SF6 concentration (surrogate for carbon monoxide) in the airlock 

and outside the airlock versus the number of subjects entering the airlock (from data in Table 13). 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

1 2 3 4 5 6 7 8 

Ratio In/Out SF6 Concentrations, ppb 

Number of Persons Entering Airlock 

Tent-Type Rigid Steel Mock-up Airlock 
38

Table 13. Data for comparing contaminant ratio and number of 

subjects entering the airlock 

Chamber

Number of 

Subjects 

Entering 

Airlock

Time to 

Enter, sec

Door 

Size,

sq ft

Airlock 

Volume 

(Empty), 

cu ft

Airlock 

Volume 

(Occupied), 

cu ft

SF6 Contaminant 

Ratio

(Inside Airlock 

vs. Outside)1,2

Tent-type 1 20 4.25 57 54.2 0.20

Tent-type 1 20 4.25 57 54.2 0.20

Tent-type 1 20 4.25 57 54.2 0.26

Tent-type 1 20 4.25 57 54.2 0.28

Tent-type 1 20 4.25 57 54.2 0.29

Tent-type 3 36 4.25 57 48.5 0.39

Tent-type 5 38 4.25 57 42.9 0.48

Tent-type 7 70 4.25 57 37.2 0.58

Rigid steel 1 15 9.50 153.5 150.7 0.05

Rigid steel 3 28 9.50 153.5 145.0 0.15

Rigid steel 5 15 9.50 153.5 139.4 0.18

Rigid steel 7 31 9.50 153.5 133.7 0.25

Rigid steel 8 42 9.50 153.5 130.9 0.32

Mock-up

airlock 1 12 6.70 102 99.2 0.07

Mock-up

airlock 3 17 6.70 102 93.5 0.12

Mock-up

airlock 5 21 6.70 102 87.9 0.16

Mock-up

airlock 7 25 6.70 102 82.2 0.19

1

 Inside airlock and outside SF6 concentrations calculated from several samples taken inside or outside the airlock. 

2

Includes consideration for contaminant in occupant’s breath.

Next, additional tests were completed that allowed the subjects to enter the tent-type airlock 

as quickly as possible. The data collected and used for generating the plot (Figure 18) are 

included in Table 14. Table 14 indicates that, in general, as the time to enter decreases the 

contaminant ratio decreases, while Figure 18 illustrates that the contaminant ratio can vary by as 

much as 18%. 
39

Table 14. Data from tent-type airlock entrance where time varies 

No. of Subjects 

Entering

Time to

Enter, sec

SF6 Contaminant Ratio 

(Inside Airlock vs. Outside)

1 20 0.20

1 20 0.20

1 20 0.26

1 20 0.28

1 20 0.29

1 40 0.36

1 60 0.38

1 80 0.43

3 36 0.36

3 36 0.37

3 36 0.39

3 14 0.39

3 16 0.25

5 38 0.40

5 38 0.42

5 38 0.48

5 25 0.39

5 28 0.32

7 70 0.55

7 70 0.56

7 70 0.58

7 29 0.44

7 55 0.43
40

Figure 18. SF6 contamination ratio versus number of persons entering 

tent-type refuge alternative airlock at varying entry times. 

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

1 2 3 4 5 6 7

Ratio In/Out SF6 Concentrations, ppb 

Number of Persons Entering Tent-Type RA Airlock 

Summary: Airlock Contamination Research 

The airlock contamination research revealed that the contaminated air concentration inside an

RA airlock after refuging miners enter could be as high as 58% of the exterior concentration, 

especially when the airlock volume and door size are small and restrict or slow the miners’ entry.

The percentage of outside contaminant entering a small airlock that was part of a tent-type 

mobile refuge alternative ranged from 20% to 58%, while for a larger airlock in a rigid steel RA 

it ranged from 5% to 32%. Analyses suggest that the number of miners entering the airlock and 

the time the airlock door is open are the most important factors with respect to the contaminant 

concentration, although airlock volume, door size, and physical configuration are likely factors 

as well. Factors that should be considered when designing RA airlocks and doors include stepover height and width, airlock height and length, door width and height, and door closing 

mechanisms. Airlocks and doors should be designed to maximize miners’ abilities to enter the 

airlock and close the outside door as quickly as possible. 
41

Discussion and Recommendations

The purging research, which included CO testing in the mini purge box and the actual tenttype and rigid steel RA along with the SF6 testing in the tent-type RA, indicates a number of 

important findings: 

 The current generation of mobile RAs does employ techniques that are capable of 

reducing a CO concentration of 400 ppm to 25 ppm in a purging room as is required 

by the 30 CFR § 7.508 regulation. 

 Purge time from 400 ppm to 25 ppm CO is reduced as the air flow is increased, 

except in the case of a seven-occupant tent-type RA airlock at flows above 70 SCFM. 

 As the purge air flow increases, the total volume of air to reduce CO from 400 to 25 

ppm increases. 

 “Effective” airlock purging occurs when the exchange ratio is 4 or less. Airlock 

purging is designed to reduce the contaminant to 1/16th (400 to 25 ppm CO) of its 

original concentration in four or fewer air exchanges. This research showed that for 

the tent-type airlock and rigid steel airlock with zero or one occupant, the exchange 

ratio was less than 4 for nearly all air flow rates. It was above 4 for the tent-type 

airlock (regardless of the number of occupants) when the air flow exceeded about 90 

SCFM; with seven simulated occupants, the exchange ratio was greater than 4 once 

the air flow exceeded approximately 50 SCFM. 

 Testing shows that up to 60% of the outside contaminant concentration could enter 

the airlock as the miners enter. Depending on the size of the airlock, size of the 

airlock door, number of miners entering the airlock, and total time the airlock door is 

open, the percentage of outside contaminant found in the airlock after miners entered 

ranged from a low of 5% for one person entering a large, rigid steel RA airlock to 

60% for seven persons entering a small, tent-type RA airlock. This implies that 

depending on the post-disaster CO concentration found outside of a mobile refuge 

alternative, the concentration inside the airlock after miners enter could be 

significantly more than the 400 ppm level that all mobile refuge alternatives are 

designed to dilute to 25 ppm. 

Establishing the parameters of an explosion scenario, including the post-explosion 

environment, is an inexact endeavor because the circumstances of both vary widely from event

to event. If a set of worst-case parameters is selected from historical disasters, then it quickly 

becomes a nearly impossible problem to design, build, and deploy a mobile refuge alternative 

with the requisite characteristics. It is noteworthy that prior to the deployment of RAs in 2007, 

all coal miners relied on the practice of barricading, which offered virtually no protection from 

contaminated air. Nonetheless, the findings of this research and new information on likely 

contaminant concentrations within the airlock indicate a need to define the disaster conditions 

under which refuge alternatives are expected to provide safe refuge, and then to re-examine the

design parameters for mobile refuge alternatives.
42

Sound scientific research can inform decisions to require certain specifications and operational 

characteristics of mobile refuge alternatives, but given the difficulty of defining a “disaster scenario,” 

these decisions are ultimately policy in nature. Accordingly, the following recommendations are 

intended as guidance to determine appropriate purging requirements for refuge alternatives. 

 An ambient CO contamination level should be established, based on the disaster 

scenario, and used to design mobile refuge alternatives and to evaluate them in the 

approval process. 

 The expected contamination factors for a specific RA design should be determined 

experimentally under a prescribed procedure, or alternatively a chart similar to Figure 

17 could be applied. 

 To compute the expected CO concentration inside the airlock, the information 

resulting from the first two recommendations should be used in conjunction with the 

expected number of miners per group entering the airlock. 

 Ideally, the airlock should be capable of reducing the expected CO concentration to 

10 an acceptable level, such as 25 ppm . 

10 The beginning and end points, i.e. 400 and 25, define a performance characteristic for the purging 

system. Given the finding that the starting concentration can exceed 400, it may be appropriate to reevaluate the end point. 

However, this may be nearly impossible in 

many cases, given the expected level of contamination, practical limitations on 

purging air capacity, and time constraints. Given this potential shortcoming, which is 

unlikely to be overcome simply through a re-engineering of the purging process, 

additional measures must be taken to protect those who would take refuge. 

A redesign of mobile refuge alternatives, or the operational procedures for deploying and 

using them, was beyond the scope of this study. As a starting point to addressing this 

shortcoming, the following activities are recommended: 

 Operational guidance to miners for purging should be based on a prescribed number 

of air changes, and not based on achieving a target concentration of 25 ppm or less. 

 Operational guidance to miners should include continued use of their self-contained 

self-rescuers (SCSRs) until they are in the main chamber of the mobile refuge 

alternative and they have determined that the concentration of CO in the main 

chamber is at an acceptable level. 

 A maximum acceptable concentration of CO in the main chamber must be specified. 

Given the significant difference in volume between the airlock and the main chamber, it 

is likely that the CO in the heavily contaminated air within the airlock would be diluted 

to an acceptable level in the main chamber. This must be confirmed by engineering 

analysis of RAs and under the specified conditions, including the number of miners, the 

number of groups that will use the airlock, and the specified contamination level. 

 
43

The design of the airlock itself was also not a part of this research study. The two mobile 

refuge alternatives tested are popular commercial models and each has a differently designed 

airlock (door size and airlock volume). Based on limited observations, the size of the airlock 

door affects the speed at which miners can enter the airlock, which directly affects the amount of 

CO that moves into the airlock during entry. Over the longer term, there may be opportunities for

manufacturers to incorporate design changes in airlocks and the details of entry procedures into 

the airlock to reduce the level of CO contamination. 

Acknowledgements

The authors would like to acknowledge the assistance of OMSHR technicians Mary Ellen 

Nelson, Al Cook, and Jon Hummer during the fabrication, equipment procurement, and testing 

and data collection phases of the research. The analysis of air samples by Cindy Hollerich and 

Andrew Mazzella, OMSHR technicians from the Fires and Explosions Branch, was an integral 

part of the research and is greatly appreciated by the authors. The authors would also like to 

thank the numerous OMSHR employees who graciously volunteered to enter the airlocks and 

collect data during the air inflow testing and purging experiments. Finally, thanks to the refuge 

chamber manufacturers for their assistance in making refuge alternatives available. 
44

References

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office 

of the Federal Register. 

ASTM [2011]. Standard E741-11, Standard Test Method for Determining Air Change in a Single 

Zone by Means of a Tracer Gas Dilution. ASTM International, West Conshohocken, PA, 2003, 

DOI: 10.1520/E0741-11. 

[http://www.astm.org/search/fullsite-search.html?query=Standard%20E741&]. 

ATSDR [2012]. Toxicological profile for carbon monoxide. Atlanta, GA: U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention (CDC), Agency for 

Toxic Substances and Disease Registry, June, p. 35.

[http://www.atsdr.cdc.gov/toxprofiles/tp.asp?id=1145&tid=253].

Hofer LJE, Giardino DA, Zeller LF [1996]. The evaluation of gases from mine explosions by 

elemental balancing. Arlington, VA: U.S. Department of Labor, Mine Safety and Health 

Administration. IR 1231, March, 31 pp. 

Lagus, P Dr. (plagus@tracergas.com) [2013]. Use of SF6 tracer and the ability of the tracer gas 

to simulate exactly other contaminant gas, such as CO. Multiple e-mail and telephone calls in 

September 2012 between Edward Thimons (ebt7@cdc.gov), Senior Technical Advisor, URS 

Corporation, NIOSH Office of Mine Safety and Health Research (OMSHR), and Dr. Peter 

Lagus, Lagus Applied Technology, Inc., 457 Corporate Drive, Escondido, CA 92029. 

[http://www.tracergas.com/].

MHS&T [2010a]. Interview Transcript I-0045 of interview of Virgil Brown, May, 24, 2010. 84 

pp. Charleston, WV: West Virginia Office of Miners’ Health, Safety and Training. 

[http://www.msha.gov/PerformanceCoal/Transcripts/May2011/I-0042%20.pdf].

MHS&T [2010b]. Interview Transcript I-0024a of interview of Barry Fletcher, May 17, 2010. 51 

pp. Charleston, WV: West Virginia Office of Miners’ Health, Safety and Training (MHS&T). 

[http://www.msha.gov/PerformanceCoal/Transcripts/May2011/I-0024b.pdf].

MHS&T [2012]. Interview Transcript I-0259 of interview of Jim Aurednik, December 1, 2012. 

50 pp. n the matter of: the investigation of the April 5, 2010 Mine Explosion at Upper Big 

Branch Mine. Charleston, WV: West Virginia Office of Miners’ Health, Safety and Training 

(MHS&T). [http://www.msha.gov/PerformanceCoal/Transcripts/May2011/I-0259.pdf].

MSHA [2001]. Report of Investigation, Underground Coal Mine Explosion, July 31 – August 1, 

2000, Willow Creek Mine. MSHA ID No. 42-02113, Plateau Mining Corporation, Helper, 

Carbon County, Utah. Arlington, VA: United States Department of Labor, Mine Safety and 

Health Administration, Coal Mine Safety and Health, July 17, 45 pp. 

[http://www.msha.gov/disasterhistory/WCREEK2000/willowcreekfinal/WCfinalcvr.htm].

MSHA [2007]. Report of Investigation, Fatal Underground Coal Mine Explosion, January 2, 

2006, Sago Mine. MSHA ID No. 46-08791, Wolf Run Mining Company, Tallmansville, Upshur 

County, West Virginia. Arlington, VA: United States Department of Labor, Mine Safety and 

Health Administration, Coal Mine Safety and Health, May 9, 198 pp. 

[http://www.msha.gov/FATALS/2006/Sago/sagoreport.asp].
45

MSHA [2010a]. Command Center Log for 4/5/2010 to 4/12/2010, Upper Big Branch Mine 

Disaster. State of West Virginia, D-0042A, 120 pp. 

[http://www.msha.gov/PerformanceCoal/Logs/D-0042A%20Command%20Center%20Log.pdf].

MSHA [2010b]. Command Center Log for 4/5/2010 to 4/13/2010, Upper Big Branch Mine 

Disaster. State of West Virginia, D-0105, 99 pp. [http://www.msha.gov/PerformanceCoal/Logs/D0105%20Command%20Center%20Log%20from%20State%20of%20WV.pdf].

MSHA [2010c]. Mine Emergency Operations (MEO) Log Book, D-0032A. Upper Big Branch 

Mine Disaster, April 2010, 68 pp. Arlington, VA: U.S. Department of Labor, Mine Safety and 

Health Administration. [http://www.msha.gov/PerformanceCoal/Logs/MEOBlue%20Goose%20Log/D-0032A%20MEO%20Log%20Book.April%202010.pdf].

NIOSH [2012]. National Survey of the Mining Population Part I: Employees. By McWilliams 

LJ, Lenart PJ, Lancaster JL, Zeiner JR Jr. Pittsburgh, PA: U.S. Department of Health and Human 

Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety 

and Health, DHHS (NIOSH) Publication No. 2012-152, IC 9527. 

[http://www.cdc.gov/niosh/mining/works/coversheet776.html].

NIOSH [2013]. NIOSH Pocket Guide to Chemical Hazards. Cincinnati, OH: U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention, National Institute for 

Occupational Safety and Health. [http://www.cdc.gov/niosh/npg/pgintrod.html].

NMA [2011]. Profile of the U.S. Coal Miner – 2010. National Mining Association, 101 

Constitution Ave. NW, Suite 500 East, Washington, DC 20001, 1 p. 

Trackemas J [2013]. Conversation in October 2013 between Jack Trackemas, Mining Engineer,

NIOSH Office of Mine Safety and Health Research (OMSHR), and Eric Bauer, Mining 

Engineer, NIOSH OMSHR. Topic of conversation was Jack Trackemas’ personal experience in

the fire recovery efforts at the Willow Creek Coal Mine in 2000, including the CO 

concentrations that were measured by the recovery teams.

Wallace JM, Hobbs st PV [1977]. Atmospheric Science: An Introductory Survey. 1 ed. New

York: Academic Press, p. 13.

Ward CL [1967]. Body volume of adult men. Unclassified, AD 657 316, USAF School of 

Aerospace Medicine, Brooks Air Force Base, Texas, June, 5 pp.

[http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=AD0657316].

Wharry MJ (wharry.matthew@dol.gov) [2013]. MSHA borehole monitoring data from mine 

disasters. E-mail message on February 21, 2013 from Matthew J. Wharry, General Engineer, 

U.S. Department of Labor, Mine Safety and Health Administration (MSHA), Pittsburgh Safety 

and Health Technology Center, Pittsburgh, PA, to Ed Thimons, Senior Technical Advisor, URS

Corporation, NIOSH Office of Mine Safety and Health (OMSHR).

47

Appendix A – Diffusion and Tracer Gases 

The dilution of a contaminant in an enclosed volume where only fresh air is injected and no 

contaminated air is exhausted follows the principle of dilution which is: 

 

(A1) 

where: C1 = Initial concentration, ppm; 

V1 = Initial volume, cu ft; 

C2 = Ending concentration, ppm; and 

V2 = Ending volume, cu ft. 

In the sealed volume case, one volume of air will reduce the concentration level by 50%. 

However, RA airlocks are not sealed volumes; they are a ventilated fixed volume, having exhaust 

relief ports designed to maintain a slight positive pressure in the airlock and main chamber and 

prevent any overpressure that might damage the RA. To determine the diffusion and/or airflow 

rates when using tracer gas, or any contaminant within a ventilated enclosure, appropriate 

equations are found in ASTM Standard E741-11, “Standard Test Method for Determining Air 

Change Rate in a Single Zone by Means of a Tracer Gas Dilution” [ASTM 2011].

To interpret data resulting from tracer gas methods, the mass balance of the tracer gas 

released within a volume under test is used. Assuming that the tracer gas mixes thoroughly 

within the test volume, the mass balance equation is

 

 

 (A2) 

where: V = the test volume, cu ft; 

C(t) = the tracer gas concentration (dimensionless); 

dC(t)/dt = the time derivative of concentration; 

Q(t) = the volumetric airflow rate out of the test volume, cfm; 

S(t) = the volumetric tracer gas injection rate, cfm; and 

t = time, min. 

The air exchange or infiltration rate, A, is given by 

 (A3) 

where: A = air changes per unit minute (min-1 or ACM). 

In the simplest case, the value of A represents the flow rate of “dilution air” entering the 

volume during the test interval. This analysis could be used to analyze the purging efficiency of 

actual RA airlocks when CO is the contaminant.

49

Appendix B – Protocol for Airlock Contamination Research 

1. Tests will be conducted in the NIOSH OMSHR reverberation room, Bldg. 154 (see Figure 

14). The reverberation room is 60-ft long x 33.8-ft wide x 22-ft high. The 44,602 cu ft of 

room volume will represent the ambient atmosphere in a coal mine following a mine 

explosion or fire. 

2. Sulfur hexafluoride (SF6) tracer gas will be released into the reverberation room to create an 

ambient SF6 concentration of approximately 100–150 ppb. The SF6 ambient concentration in 

the reverberation room will be used to simulate a CO concentration resulting in the ambient 

air of a coal mine following a mine disaster when miners might need to enter a refuge 

alternative (RA). SF6 is a colorless, tasteless, and odorless tracer gas that has a NIOSH 

recommended exposure limit (REL) and OSHA permissible exposure limit (PEL) of 1,000 

ppm. This gas has been used for many years to study mine and building ventilation systems. 

The range of 100 to 150 ppb was selected because this is within the upper range of detection 

of the NIOSH OMSHR chromatograph. 

3. Initially, two types of RAs will be tested. The first is an inflatable tent-type RA. NIOSH was 

able to obtain a 35-person chamber designed for groups of 7 miners to enter the airlock. The 

airlock is partly inside of the steel portion of the RA and partly inside the inflatable portion 

of the RA. The total volume of the airlock is approximately 57 cu ft with the inflatable 

portion pushed up to its full potential height. The second RA NIOSH obtained was a solid 

steel RA designed to hold 8 miners. This is actually a test RA developed by the 

manufacturer, but is similar to the RAs provided to mines. The area inside the airlock in this 

RA is 153.5 cu ft and is designed to hold all 8 miners during purging and prior to entering the 

main chamber. These RAs will be located inside the reverberation room during the testing. In 

addition, NIOSH may build mock-up airlocks to test contaminated air inflows. 

4. Figure 14 shows the reverberation room and the locations of the inflatable and steel RAs 

during the tests. Prior to the start of a test, all doors to the reverberation room will be opened 

and five mixing fans located around the room will be operated for at least one hour. Two 

reverberation room exhaust fans will also be operated during this time period. During this 

time the large garage door located at the outside entrance to the building containing the 

reverberation room will also be opened. The airlock door to the RA will be open and a fan 

will be directed into the airlock. This is done to insure that there is no, or minimal ambient 

SF6 in the reverberation room or the airlock at the start of the testing. After the one hour of 

fan operation, all doors to the reverberation room will be closed, the garage door to the 

building will be closed, and the two reverberation room exhaust fans will be turned off. 

5. With the reverberation room sealed (all doors and vents closed or sealed), the five mixing 

fans will be located around the room to effectively mix the air. After 5 minutes baseline SF6

vacutainer tube samples will be taken using 15-ml vacutainer bottles at a height of 

approximately 4 ft at each group of four sample locations when only one chamber is being 

evaluated, or at all seven (7) sampling locations shown in Figure 16 when simultaneous 

testing is being conducted. At least one additional baseline sample will be taken inside the 

airlock. These samples should show that no SF6 is present in the reverberation room or the 

airlock at the start of the testing. The airlock door will then be closed. 
50

6. All collected SF6 vacutainer sample bottles will be labeled on the sample bottle as to date, 

time, and location where sample was collected. 

7. One MIRAN SapphIRe Portable Ambient Air Analyzer, 205B-XL Series (Serial No. 205B75043-378, Calibration Date: July 30, 2012), SF6 analyzer will be placed at a specific 

location in the reverberation room to provide continuous real-time SF6 analysis and 

concentrations during all testing and to serve as a check for the chromatograph results. This 

will provide additional confirmation of the ability to establish and maintain a constant 

ambient SF6 concentration in the reverberation room and as a method for insuring that there 

are no excursions into higher than desired SF6 concentrations. This analyzer will be located 

next to sampling location 3 in Figure 16 when conducting tests on the inflatable chamber and 

at location 7 when conducting tests on the rigid steel chamber. 

8. Following the baseline sampling, approximately 1 gm of SF6 will be released from an SF6

lecture bottle into the reverberation room by releasing it into the flow in front of one of the 

fans. Upon completion of the release, the release person will place the SF6 lecture bottle into 

a plastic bag and seal the plastic bag to insure against any additional SF6 being accidentally 

released into the room due to a leak in the lecture bottle or valve fittings. The researcher 

releasing the SF6 will not represent any miner entering into the airlock during testing just to 

insure that no trace amount of SF6 is carried on the release individual’s clothing. This person 

may serve as data recorder or sample collector. 

9. At intervals of 10 min, 20 min, and 30 min after release of the SF6, ambient SF6 concentration

samples will be taken at one or both groups of four sample locations shown in Figure 16 at 

approximately 4-ft height off of the floor. This will result in a total of 12 or 24 samples. 

These samples will be taken to insure that a uniform ambient SF6 concentration is established 

around the reverberation room before subjects representing miners enter the purging room. 

10. Following the collection of these ambient room concentration samples, the 5 room fans will 

be turned off and subjects representing miners (1, 3, 5, or 7 individuals in the inflatable tenttype RA and 1, 3, 5, 7, or 8 individuals in the rigid steel RA) will then open the RA airlock 

door and enter the airlock. Once all subjects are in the airlock, they will close the airlock 

door. The total time for the subjects to open the airlock door, enter the airlock and close the 

airlock door will be recorded. The entering subject(s) will have 6 vacutainer sampling bottles 

with them. This will require a total of 7 tests (4 with the tent-type RA airlock with 1, 3, 5, 

and 7 subjects entering and 5 with the rigid steel RA airlock with 1, 3, 5, 7, or 8 subjects 

entering). It may be possible to run simultaneous tests on both the RAs if enough research 

personnel are available to enter the airlocks. 

11. Once inside the airlock, with the airlock door closed for 15 sec, airlock SF6 samples will be 

collected with vacutainers at 3 locations somewhat equally spaced within the airlock. 

Another set of 3 samples will be collected five minutes later at the same locations. This will 

be a total of 6 samples. The sampling will be divided among the subjects representing miners 

in the airlock. If only one subject enters, that individual will take all 3 samples. If more than 

one subject enters the sampling will be divided up as is most appropriate. At the same times 

that the samples are collected inside the airlock (15 seconds after closing the airlock door and 

5 minutes later), samples will be taken just outside of the airlock door. This will result in an 

additional 2 or 4 samples. SapphIRe analyzer readings will also be recorded at these 

sampling intervals. 
51

12. Following the collection of the 5-min post-entering SF6 samples both inside and outside of 

the airlock, the airlock occupants will open the airlock door, and exit the airlock. 

13. All reverberation room doors will be opened as well as the garage door to the outside of the 

building. The two reverberation room exhaust fans will be turned on and all of the five 

mixing fans in the reverberation room will continue to operate for several hours. In addition, 

one of the fans will be directed into the airlock through the open airlock door. This is done to 

remove all residual SF6 from the reverberation room and from the airlock. Previous testing 

has shown that a minimum of one hour of ventilating the reverberation room and the RA 

airlock will remove all SF6 and prepare the reverberation room and the RA airlock for the 

next test. 

14. All collected SF6 vacutainer sample tubes will be analyzed using the OMSHR SF6

chromatograph located in Building 118 at OMSHR Pittsburgh. This SF6 type of testing has 

been a standard analysis tool used in OMSHR research for many years. All SF6 vacutainer 

bottle samples will be analyzed on the chromatograph within one week of testing. Samples 

may also be sent for analysis to an independent outside testing firm. 

15. The ratio of the SF6 concentration in the RA airlock to the ambient SF6 concentration in the 

reverberation room can be used to determine what CO concentrations might occur in an 

airlock during the entry of miners in a mine disaster situation. For example, if it is found that 

the reverberation room SF6 concentration is 200 ppb and the airlock concentration after 

miners enter is 10 ppb, then the ratio of airlock SF6 to reverberation room SF6 is 0.05. If it is 

assumed that the post-disaster mine ambient CO concentration is 15,000 ppm (a 

concentration similar to what was found in the first borehole near the RA at Upper Big 

Branch), then the airlock CO concentration would be 0.05 X 15,000 ppm = 750 ppm CO 

following the entry of the miners. 
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