Large Fire Research Facility

(Building 205) Exhaust Hood Heat

Release Rate Measurement System

David W. Stroup

Laurean DeLauter

Jack Lee

Gary Roadarmel

NISTIR 6509

Large Fire Research Facility

(Building 205) Exhaust Hood Heat

Release Rate Measurement System

David W. Stroup

Laurean DeLauter

Jack Lee

Gary Roadarmel

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-8641

U.S. Department of Commerce

William M. Daley, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of

Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director

NISTIR 6509

July 2000
 - iii - 

TABLE OF CONTENTS 

 Page

LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv 

LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv 

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Experimental Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Heat Release Rate Calculation Methodology. . . . . . . . . . . . . . . . . . . . . . . . 2 

Calibration Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

50 kW Hood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

500 kW Hood (Furniture Calorimeter) . . . . . . . . . . . . . . . . . . . . . . . 7 

Main Hood. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 

Uncertainty Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 
 - iv - 

List of Tables 

 Page

Table 1. Sample Natural Gas Heat of Combustion Calculation . . . . . . . . . . 13 

Table 2. Summary of Uncertainties for Gas Analysis Equipment . . . . . . . . . 14 

Table 3. Uncertainty Components for Heat Release Rate Calculations . . . . . . 14 

List of Figures 

 Page

Figure 1. Plan view showing exhaust hood locations in Large Fire Research Facility 15 

Figure 2. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for first test of 50 kW hood with fan operating 

at half speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Figure 3. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for second test of 50 kW hood with fan operating 

at half speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Figure 4. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus for third test of 50 kW hood with fan operating at half 

speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Figure 5. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for first test of 50 kW hood with fan operating 

at three-quarter speed . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Figure 6. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for second test of 50 kW hood with fan 

operating at three-quarter speed . . . . . . . . . . . . . . . . . . . . . . 18 

Figure 7. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for third test of 50 kW hood with fan 

operating at three-quarter speed . . . . . . . . . . . . . . . . . . . . . . 18 

Figure 8. Plot of measured versus theoretical heat release rates for all tests using 

the 50 kW hood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 

Figure 9. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for first test of 500 kW hood (furniture 

calorimeter) with fan operating at three-quarter speed . . . . . . . . . . 19 

Figure 10. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for second test of 500 kW hood (furniture 

calorimeter) with fan operating at three-quarter speed . . . . . . . . . . 20 

Figure 11. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for third test of 500 kW hood (furniture 
 - v - 

List of Figures (continued) 

 Page

calorimeter) with fan operating at three-quarter speed . . . . . . . . . . 20 

Figure 12. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

release rates versus time for first test of 500 kW hood (furniture 

calorimeter) with fan operating at full speed . . . . . . . . . . . . . . . . 21 

Figure 13. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

Release rates versus time for second test of 500 kW hood (furniture 

calorimeter) with fan operating at full speed . . . . . . . . . . . . . . . . 21 

Figure 14. Plot of measured (solid line) and theoretical (solid, straight lines) heat 

Release rates versus time for third test of 500 kW hood (furniture 

calorimeter) with fan operating at full speed . . . . . . . . . . . . . . . . 22 

Figure 15. Plot of measured versus theoretical heat release rates for all tests using 

the 500 kW hood (furniture calorimeter) . . . . . . . . . . . . . . . . . . 22 

Figure 16. Plot of measured (solid line) and theoretical (dotted lines) for transient 

ìfastî and ìslowî t-squared tests using the 500 kW hood (furniture 

calorimeter) with fan operating at full speed . . . . . . . . . . . . . . . . 23 

Figure 17. Plot of measured (solid line) and theoretical (solid, straight line) heat 

Release rates versus time for first test of main hood . . . . . . . . . . . . 24 

Figure 18. Plot of measured (solid line) and theoretical (dotted line) heat 

release rates versus time for second test of main hood . . . . . . . . . . . 24 

Figure 19. Plot of measured (solid line) and theoretical (dotted line) heat 

release rates versus time for third test of main hood . . . . . . . . . . . . 25 

Figure 20. Plot of measured versus theoretical heat release rates for all tests using 

the main hood . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 

Figure 21. Plot of measured (solid line) and theoretical (dotted line) for a transient 

ìmediumî t-squared test using the main hood . . . . . . . . . . . . . . . 26 

Figure 22. Plot of measured (solid line) and theoretical (dotted lines) for transient 

ìfastî and ìslowî t-squared tests using the main hood . . . . . . . . . . . 26 
 - 1 - 

LARGE FIRE RESEARCH FACILITY (BUILDING 205) 

EXHAUST HOOD HEAT RELEASE RATE 

MEASUREMENT SYSTEM 

David W. Stroup, Laurean DeLauter, 

Jack Lee and Gary Roadarmel 

Abstract 

Rate of heat release is a critical parameter for evaluation of the fire properties of a material. The 

Large Fire Research Facility at the National Institute of Standards and Technology operates three 

hoods capable of determining heat release rate from oxygen consumption measurements. This 

report describes the methodology and apparatus used to measure heat release rate. In addition, 

the results from several calibration tests conducted in each hood are discussed. The heat release 

rates determined from hood measurements range from 120 % to approximately 60 % of the 

theoretical values for the natural gas calibration fires. 

Key Words: calorimetry; data analysis; fire data; fire measurements; fire tests; heat release rate; 

large scale fire tests; oxygen consumption 
 - 2 - 

Introduction 

Measurement of the rate at which a burning item releases heat is a critical parameter in fire 

protection engineering. The heat release rate can be used in the characterization of the hazard 

represented by a given fuel package. When used as input to a computer fire model, the heat 

release rate can provide information on fire size, fire growth rate, available egress time, and 

suppression system impact. The Large Fire Research Facility at the National Institute of 

Standards and Technology (NIST) has three exhaust hoods for use in determining heat release 

rate. These hoods operate based on the principle of oxygen consumption calorimetry. 

Experimental Configuration 

The arrangement of the three exhaust hoods in the NIST Large Fire Research Facility is shown in 

Figure 1. The 50 kW hood is nominally a 1.22 m square sloping upward at approximately a 45 

degree angle to a 0.23 m diameter duct. The 500 kW hood (furniture calorimeter) is a square 

approximately 3 m on a side. It slopes upward at a 45 degree angle to a 0.5 m diameter duct. 

The main hood is 4 m by 5 m and slopes upward to a 1.2 m square duct. 

During a fire test, data from the various sensors are acquired using a computer-based data 

acquisition system. The fire test data are recorded on magnetic media for further data reduction 

and interpretation after the test. Data acquisition and reduction in the Large Fire Research 

Facility are accomplished using in-house developed computer software. 

Heat Release Rate Calculation Methodology 

Using the principle of oxygen consumption, it is possible to calculate the heat release rate of 

burning materials when the products of combustion are collected in an exhaust hood. Thornton 

[1] found, in 1917, that many organic materials produced almost the same amount of heat per 

unit mass of oxygen consumed. Hinkley et al. [2] suggested using oxygen concentration in 

exhaust gases to determine the heat release rate of wood cribs in 1968. Parker [3] used this 

technique to determine the heat release rate of specimens in the ASTM E84 tunnel test. Huggett 

[4] calculated an average value for the heat release from a fire involving typical organic fuels to 

be 13.1 MJ per kilogram of oxygen consumed. According to reference 4, heat release rates 

calculated based on oxygen consumption compare "favorably" with those derived from 

conventional calorimetric measurements. 

The calculation of heat release rate of fires, burning in normal air, using oxygen consumption 

requires a minimum of two measurements, the flow rate of the products of combustion through 

the exhaust system and the oxygen concentration in the exhaust products. Parker [5, 6] presents 
 - 3 - 

several sets of equations for calculating heat release rate using oxygen consumption. The 

appropriateness of each set of equations depends on the combustion products being measured. A 

paper by Janssens [7] proposes a form of the equations for calculating heat release rate 

specifically for full-scale fire test applications. These equations use mass flow rates instead of 

volumetric flow rates. Volumetric flow rates can lead to confusion because of the need to choose 

an arbitrary reference temperature and pressure. 

Typical measurements obtained in the exhaust stream from a fire test include oxygen 

concentration, carbon dioxide concentration, carbon monoxide concentration, gas temperature, 

and pressure. A data reduction program, called RAPID [8], implements equations to calculate 

gas velocity, mass flow rate, and the associated heat release rate. 

The gas velocity is determined from the output of a velocity probe and an adjacent thermocouple. 

Bi-directional probes are used to obtain velocity data in the Large Fire Research Facility hoods. 

These velocity probes provide a voltage output proportional to the difference between the static 

and dynamic pressures exerted by the flow. A bi-directional probe yields velocities within 10 % 

over an angular range of ±50 degrees of the probe axis in any direction [9]. Two pressure 

transducers, 0 Pa to 150 Pa and 0 Pa to 500 Pa, were used to monitor pressure. The equation 

used in RAPID to convert the probe voltage output to gas velocity is: 

where v = gas velocity (m/s) 

 Cpt = probe conversion (Pa/volt) 

 R = probe voltage reading (volt) 

 T = gas temperature (K) 

For the 50 kW hood and the 500 kW hood (furniture calorimeter), a single velocity probe and 

associated thermocouple are placed in each duct approximately along the centerline of the duct. 

The probe and thermocouple are located at a point beyond flow straightening vanes and 

approximately 8 duct diameters downstream from the duct entry to ensure a reasonably uniform 

velocity and temperature profile across the duct cross-section. The main hood contains an array 

of 9 velocity probes and thermocouples. Unlike the circular cross sections of the 50 kW and 

500 kW hoods, the cross section of the main hood duct is square, and it is too large to ensure a 

uniform flow profile. Therefore, the data acquisition program calculates the velocity at each of 

the 9 points and uses the average to calculate the mass flow rate. 

The mass flow rate of gas in the exhaust duct is determined using the calculated velocity and the 

measured gas temperature. The equation for mass flow rate is: 

v = 0.0698 C pt R T

T

T

m C v A amb

e = ρ amb &

(1) 

(2) 
 - 4 - 

where me = gas mass flow rate (kg/s) 

 C = empirical flow coefficient 

v = calculated gas velocity (m/s) 

 A = cross sectional area of duct perpendicular to the flow (m2) 

ρamb = density of ambient air (kg/m3) 

 Tamb = temperature of ambient air (K) 

 T = gas temperature (K) 

The duct for the 50 kW hood is circular with a diameter of 0.23 m and a cross sectional area of 

0.041 m2. The 500 kW duct is also circular with a diameter of 0.48 m and a cross sectional area 

of 0.18 m2. The flow coefficient used for these two ducts is 0.824. The duct for the main hood 

(3 MW) is a 1.22 m square with a cross sectional area of 1.49 m2. A flow coefficient of 0.9 is 

used for this hood. Both of these flow coefficient were determined empirically during 

undocumented tests. 

Using the principle of oxygen consumption, the heat release rate can be obtained from the 

following set of equations [6]: 

where 

q = heat release rate (kW) 

Tamb = temperature of ambient air (293.13 K) 

Pamb = pressure of ambient air (101.3 kPa) 

RHamb = relative humidity of ambient air (50 %) 

α = combustion expansion factor (1.105) 

E = heat released per unit mass of oxygen consumed (13.1 MJ/kg of O2) 

ECO = heat released per unit mass of oxygen consumed for combustion of CO to CO2 (≈

17.6 MJ/kg of O2) 

Mr(E) = relative molecular mass of exhaust gas (≈ 28.97 kg/kmol) 

Mr(O2) = relative molecular mass of oxygen (≈ 32 kg/kmol) 

( ) ( )

( )

( ) ( ) 2 0 0

2 2

2

1

2 1 1

1

H O O

r

e r

O

CO

CO X X

M E

m M O

X

X

q E E E −  + − 











 − = − − φ α

φ φ & &

( ) ( )

O ( ) O CO CO

O CO CO O CO

X X X X

X X X X X

− − −

− − − − =

2 2 2

2 2 2 2

1

1 1

0

0 0

φ

( )

amb

amb amb amb amb

H O P

RH T T T

X

2 4

0 0.6107 0.06052 0.0002088 0.00007376

100 2

+ − + =

(3) 

(4) 

(5) 
 - 5 - 

X0

O2

 = measured mole fraction of O2 in the incoming air* 

X0

CO2

 = measured mole fraction of CO2 in the incoming air* 

X0

CO = measured mole fraction of CO in the incoming air* 

XO2

 = measured mole fraction of O2 in the exhaust gases* 

XCO2

 = measured mole fraction of CO2 in the exhaust gases* 

XCO = measured mole fraction of CO in the exhaust gases* 

* as measured in the gas analyzer located downstream of the cold traps and desicant 

The mole fractions of oxygen, carbon dioxide, and carbon monoxide in the exhaust stream are 

measured continuously using a paramagnetic oxygen analyzer, a nondispersive infrared carbon 

dioxide analyzer, and a nondispersive infrared carbon monoxide analyzer, respectively. The 

values for incoming air are determined from concentration measurements of the exhaust gases at 

the start of each fire test. 

Gas sampling is accomplished by withdrawing exhaust gas through a sample probe located in 

each hood duct near the associated velocity probe(s). The gas sample is passed through two 

filters to remove soot particles and dried by pulling it through two cold traps and two tubes of 

desiccants. The sampling line has a nominal internal diameter of 9.5 mm. Each moisture trap 

tube is 270 mm long with a diameter of 30 mm for an approximate volume of 200 cm3

. The cold 

traps are approximately 190 mm long with an internal diameter of 60 mm for an approximate 

volume of 500 cm3 for each trap. The sampling system volume is minimized to avoid damping 

out important peak values in the heat release rate measurements. The moisture trap tubes are 

tightly packed with chemical absorbent, and the volume of each cold trap is reduced by 70 % 

with the addition of glass beads. 

This sampling train does induce some delay in the measurements. The heat release rate 

calculation is adjusted to account for the ìpumpingî delay time associated with each gas 

measurement (oxygen, carbon dioxide, carbon monoxide). The delay times are determined by 

injecting carbon dioxide into each hood and measuring the time from injection until onset of 

changes in oxygen and carbon dioxide measurements are observed. No correction is made for 

the response times (time constants) of the gas analyzers. 

Calibration Experiments 

A series of calibration experiments were conducted in the Large Fire Research Facility to 

examine the heat release rate calibration constants, proper functioning of the instrumentation, 

and system integrity. The calibration experiments consisted of a series of at least three fire tests 

conducted under each hood using natural gas fired burners. Different burners were used 

depending on the maximum fire size desired for each test. In each calibration test, the burner 

was operated at a minimum of three, approximately steady, heat release rates. 
 - 6 - 

The theoretical heat release was calculated from the volume flow data obtained during a 

calibration test and the heat of combustion of natural gas. Natural gas, while predominately 

methane, is composed of a number of different gases. The local gas utility provided component 

analyses of the natural gas on a daily basis. A sample gas analysis is presented in Table 1. Using 

the appropriate percentage for each component gas and the associated heat of combustion, the 

total heat of combustion for the natural gas on any day can be calculated. The heat of 

combustion was calculated on a wet basis, i.e., all of the water in a vapor state. The volume flow 

rate of gas was determined using a diaphragm test meter. Revolutions of the meter were timed 

with a stopwatch, and the average of five readings was used to determine the flow. Gas pressure 

was measured using a pressure gage with a resolution of ± 3.5 kPa. 

The ducts from the 50 kW and 500 kW hoods are connected to a variable speed fan. The fan 

speed can be adjusted continuously. When using the 50 kW hood and the 0.23 m diameter duct, 

the fan speed is limited to 75 % of its maximum value. Fan speeds higher than this value 

produce duct pressures greater than the maximum allowable for one of the two pressure 

transducers. Calibration tests were conducted for the 50 kW and 500 kW hoods at two different 

fan speeds to examine the impact on the calculated heat release rate. 

Results 

50 kW Hood 

A series of six tests were conducted under the 50 kW hood. Three tests were conducted with the 

fan operating at % of maximum (motor speed of 1500 rpm), and three were conducted with the 

fan at 50 % of maximum (motor speed of 1000 rpm). A 0.2 m square burner, filled with sand, 

was used for calibration of the 50 kW hood. At ignition, the calibration fire was immediately 

ramped up to a steady state heat release rate of approximately 20 kW. At the end of 5 min, the 

heat release rate was increased to 40 kW and maintained for 5 min. The heat release rate was 

then increased and maintained at 85 kW for an additional 5 min. After this 5 min period, the 

heat release rate was decreased to 40 kW and held for 5 min. Finally, the heat release rate was 

again decreased to 20 kW for 5 min before being shutoff. 

Figures 2 - 7 present heat release rate results calculated from the measurements obtained during 

the 6 calibration fire tests. The first three graphs are for the 75 % fan speed and the second set of 

three figures is for the 50 % speed. The system provides results within 15 % for the heat release 

rates regardless of fan speed. 

Figure 8 presents a comparison between the theoretical and measured heat release rates. The 

theoretical heat release rate is obtained by multiplying the heat of combustion for natural gas by 

the measured flow rate of the gas. The dotted line in the Figure represents exact agreement. The 
 - 7 - 

maximum difference between the calculated results and the theoretical values is about 15 % with 

an average value of 4 %. 

500 kW Hood (Furniture Calorimeter) 

A series of six tests were conducted under the 500 kW (furniture calorimeter) hood. Three tests 

were conducted with the fan operating at maximum speed, and three were conducted at 75 % of 

maximum speed. Since the exhaust duct from the 500 kW hood is larger than that from the 

50 kW hood, the pressure transducers remain within range regardless of the fan speed. The fan 

was operated at two speeds considered typical for potential applications of the system. The 

system should be capable of determining the heat release rate at any fan speed if accurate 

velocity measurements are obtained. 

A 0.3 m square sand burner was used for calibration of the 500 kW hood. At ignition, the 

calibration fire was immediately ramped up to a steady state heat release rate of approximately 

100 kW. At the end of 5 min, the heat release rate was increased to 200 kW and maintained for 

5 min. The heat release rate was then increased and maintained at about 450 kW for an 

additional 5 min. Finally, the heat release rate was again decreased to 200 kW for 5 min before 

being shutoff. 

Figures 9 - 14 present heat release rate results calculated from the measurements obtained during 

the 6 calibration fire tests. The first three graphs are for the fan operating at maximum speed and 

the second set of three figures is for the 75 % speed. As expected, the system successfully 

calculates the heat release rate regardless of fan speed. 

Figure 15 presents a comparison between the theoretical and measured heat release rates. The 

dotted line in the Figure represents exact agreement. The maximum difference between the 

calculated results and the theoretical values is approximately 40 %. The average difference is 

about 13 %. The heat release rate in most cases is calculated to be less than the theoretical heat 

release rate with the disagreement increasing as the heat release rate increases. 

A second set of experiments was conducted under the 500 kW hood using a growing fire 

designed to test the transient response of the measurement system. The fire was produced using 

a 0.7 m by 1 m rectangular burner, composed of four elements with individual dimensions of 

0.35 m by 0.5 m. The burner was 0.31 m (1 ft) high and was connected to six technical grade 

(minimum 98 % pure) methane cylinders through a manifold and computer controlled valve 

system. The computer was programmed to monitor the flow of methane gas through four mass 

flow controllers arranged in parallel. 

The system could be programmed to reproduce any fire with a growth rate proportional to time 

from ignition to the second power, t-squared [10]. The heat release rate curve produced by the 

burner is one that follows the fire growth rates used in the 1993 edition of National Fire 

Protection Association (NFPA) 72 National Fire Alarm Code Appendix B [11]. The fast, 

medium, and slow fires described by this NFPA standard are based on a wide variety of fires that 
 - 8 - 

grow with the square of time and are sometimes referred to as t-squared (t2) fires. A fast 

developing fire is one that would take less than 150 s from the time of open burning until the fire 

reaches a heat release rate of 1055 kW. A medium developing fire is one that would take more 

than an additional 150 s, but less than 600 s from the time of open burning until the fire reaches a 

heat release rate of 1055 kW. A slow developing fire is one that would take 600 s or more from 

the time of open burning until the fire reaches a heat release rate of 1055 kW. A mathematical 

representation for these curves is as follows: 

where: q = heat release rate (kW) 

α = coefficient (kW/s2) 

 t = time (s) 

For the purpose of this experimental series, a ìfastî fire reached 1055 kW in 150 s, which 

corresponds to an α equal to 0.0468 kW/s2

. A ìmediumî fire reached 1055 kW in 300 s, which 

corresponds to an α equal to 0.0117 kW/s2

. A ìslowî fire reached 1055 kW in 600 s, which 

corresponds to an α equal to 0.00293 kW/s2. 

The "t-squared" burner was used to produce two heat release rate curves during a test under the 

500 kW hood. The theoretical fire growth curves (dotted lines) for a fast and a slow fire are 

overlaid with the data obtained from the test in Figure 16. The measurement system is able to 

reflect the transient nature of the fire growth curves. 

Main Hood 

A series of five tests were conducted under the main hood. Three of the tests were conducted 

with a 0.3 m square sand burner producing steady state heat release rates, and two tests were 

conducted using the "t-squared" burner. 

The 0.3 m square sand burner was used for the steady heat release rate experiments. At ignition, 

the calibration fire was immediately ramped up to a steady state heat release rate of 

approximately 425 kW. At the end of 4 min, the heat release rate was increased to 550 kW and 

maintained for 4 min. After 8 min from ignition, the burner was shut off. 

Figures 17 - 19 present heat release rate results calculated from the measurements obtained 

during the 3 calibration fire tests. Figure 20 presents a comparison between the theoretical and 

measured heat release rates. The dotted line in the figure represents exact agreement. The 

maximum difference between the calculated results and the theoretical values is approximately 

20 % with an average of 7 %. 

The second set of experiments under the main hood used a growing fire designed to test the 

responsiveness of the measurement system. Two tests were conducted using the "t-squared" 

2

q& = α t (6) 
 - 9 - 

burner. The first test used the burner to produce a medium growth rate fire. Fast and slow fires 

were produced in the second test. 

The calculated heat release rate results (solid lines) for the two tests and the three theoretical heat 

release rate curves (dotted lines) are overlaid in Figures 21 and 22. The measurement system is 

able to reflect the transient nature of the fire growth curves. 

Uncertainty Analysis 

The results presented in this report are subject to a number of uncertainties. Uncertainty can be 

categorized as either Type A or Type B. Type A uncertainties can be evaluated using standard 

statistical methods. Type B uncertainties must be evaluated based on scientific judgment using 

any available relevant information such as previous measurements, similar experience, 

equipment specifications, calibration data, and handbook values [12]. To assess Type B 

uncertainty, the upper and lower bounds for each quantity of interest must be estimated such that 

the probability of the true value being between the bounds is approximately 100 %. The two 

types of uncertainty are combined into a "combined standard uncertainty" using the law of 

propagation of uncertainty [12]. Finally, the "expanded uncertainty" which corresponds to the 

95 % confidence interval (2σ) is determined by multiplying the combined standard uncertainty 

by a coverage factor, k, of 2. 

The estimated accuracy of the heat release data obtained from the exhaust hoods in the Large 

Fire Research Facility is influenced by a number of factors. Each of these factors has some 

uncertainty associated with it. Uncertainty exists in both the calculated heat release rate and the 

known "theoretical" value. 

The first source of uncertainty in the calculated heat release rate is the exhaust system. In order 

to obtain accurate data for determining heat release rate, the exhaust flow must be well mixed 

and turbulent when it reaches the sampling point. In addition, the velocity profile should be 

nearly uniform across the duct. The relative locations of the sampling probes (gas concentration, 

temperature, velocity) can produce uncertainty in the measured quantities. Probe locations can 

influence each other by altering the flow, and probes can be oriented improperly relative to the 

flow. Long term experience in the facility suggest these issues result in an expanded uncertainty 

of ±5 % for velocity measurements and ±5 % for temperature and gas concentration 

measurements. 

Another source of uncertainty is the accuracy of the instruments themselves. Several factors 

such as noise, drift, ambient temperature, gas stream temperature and pressure, and 

electromagnetic interference can influence the accuracy of the gas analysis instruments. The 

impact of these factors on the instrument readings, as specified by the respective manufacturers, 

is summarized in Table 2. Calibration of the instruments can affect the accuracy. Each 

instrument is calibrated prior to the start of a test using zero and span gases with known 
 - 10 - 

concentrations. The known concentrations have been verified by the gas supplier and are 

independently reconfirmed periodically. The uncertainty due to incorrect calibration is estimated 

to be less than 2 %. Velocities obtained using bidirectional probes are estimated be within 5 % 

[9]. 

Other sources of uncertainty include flow coefficients, expansion factors, sample delay times, 

and dilution of peak values. The currently used flow coefficients and expansion factors are 

based on some undocumented calibration tests and theoretical analysis. The data reduction 

software provides some capability to investigate the impact of changes in these values on the 

calculated heat release rate. Varying the flow coefficient by ±50 % has approximately a ±50 % 

affect on the calculated heat release rate. Increasing the expansion factor by 50 % produces 

approximately a 50 % decrease in heat release rate. Decreasing the expansion factor to a value 

less than 1 has a negligible impact on the calculated heat release rate. A certain amount of time 

is required for a gas sample to reach an analyzer and for the analyzer to respond. This time delay 

must be accounted for to obtain accurate heat release rate measurements. The delay time ranges 

from 10 s to 20 s. The impact of varying the time adjustments was investigated using the data 

reduction software. Various changes in time delays had less than a 5 % impact on calculated 

heat release rate. In many fire tests, the heat release rate will vary rapidly. In order to properly 

measure rapid changes, the sampling system volume must be minimized to avoid excessive 

dampening. The transient heat release rate calibration tests indicate that the system can 

satisfactorily identify fluctuations in heat release rate associated with ìfastî t-squared fires 

(α = 0.05 kW/s). More work is needed to investigate these factors in more detail. 

Several factors can influence the accuracy of the known theoretical heat release rate. 

Uncertainty associated with the assumed heat of combustion can impact the theoretical as well as 

the measured heat release rate. The flow meter measurement accuracy is estimated to be ±10 %. 

The impact of variations in combustion efficiency is estimated to be ±5 %. Finally, pressure 

variations are expected to yield an uncertainty of ±2 %. The uncertainties associated with the 

instruments used to determine heat release accuracy are summarized in Table 3. 

While some of these uncertainties can result in significant errors, they can be minimized to 

acceptable levels through careful calibration. The two most significant sources of error are more 

difficult to quantify and eliminate. Leaks in the exhaust gas sampling train can result in 

significantly underestimating the heat release rate. Similarly, loss of exhaust gas from the 

collection hood can produce errors. The sampling train is examined for leaks before and after 

each test. Assessing the integrity of the sampling train during a test is more difficult. Various 

methods are being evaluated for improving the integrity of the sampling train and eliminating 

potential loss of exhaust gas from the collection hoods. 
 - 11 - 

Summary 

A series of calibration experiments were conducted under the exhaust hoods in the Large Fire 

Research Facility at the National Institute of Standards and Technology. The experiments were 

conducted to examine the ability of the system to determine heat release rate from oxygen 

consumption measurements. The tests included both steady state and transient fire growth rates. 

The results indicate a wide variation in the level of agreement between measured and theoretical 

heat release rate. Heat release rates from the fires were determined from oxygen consumption 

and velocity measurements. The 50 kW hood showed the best level of agreement with a 

maximum of 13 % difference. The 500 kW hood provided the high level of difference with a 

maximum of 40 %. The main hood had a maximum difference between measured and 

theoretical heat release rate of 20 %. In most cases, the measured values obtained using the 500 

kW hood were lower than the theoretical heat release rates with the disagreement increasing at 

the higher heat release rates. The heat release rate values measured in the main hood were 

generally higher than the calculated values. These values represent the best estimates of rate of 

heat release measurements in the three calorimetry hoods in the Large Fire Research Facility for 

a natural gas calibration fire. 

A preliminary analysis of the uncertainty associated with the measurement of heat release rate 

was conducted as part of this work. This analysis identified a number of issues which can 

influence the accuracy of the heat release rate measurements. A future study will investigate 

these issues in greater detail. 

References 

[1] Thornton, W., "The Relation of Oxygen to the Heat of Combustion of Organic 

Compounds," Philosophical Magazine and J. of Science, 33, 1917. 

[2] Hinkley, P., H. Wraight, and A. Wadley, "Rates of Heat Output and Heat Transfer in the 

Fire Propagation Test," Fire Research Note No. 709, Fire Research Station, 

Borehamwood, England, 1968. 

[3] Parker, W., "An Investigation of the Fire Environment in the ASTM E-84 Tunnel Test," 

NBS Technical Note 945, National Bureau of Standards, Gaithersburg, MD, 1977. 

[4] Huggett, C., "Estimation of the Rate of Heat Release by Means of Oxygen 

Consumption," J. of Fire and Flammability, 12, pp. 61-65, 1980. 

[5] Parker, W., "Calculations of the Heat Release Rate by Oxygen Consumption for Various 

Applications," NBSIR 81-2427, National Bureau of Standards, Gaithersburg, MD, March 

1982. 
 - 12 - 

[6] Parker, W., "Calculations of the Heat Release Rate by Oxygen Consumption for Various 

Applications," J. of Fire Sciences, 2, September/October 1984, pp. 380-395, 1984. 

[7] Janssens, M.L., "Measuring Rate of Heat Release by Oxygen Consumption," Fire 

Technol., 27, pp. 234-249, 1991. 

[8] Peacock, R.D., J.N. Breese, and C.L. Forney, "A Users Guide for RAPID, Version 2.3," 

NIST Special Publication 798, National Institute of Standards and Technology, 

Gaithersburg, MD, January 1991. 

[9] Emmons, H.W., "Vent Flows," The SFPE Handbook of Fire Protection Engineering, 

Second Edition, Society of Fire Protection Engineers, Bethesda, MD, pg. 2-43, 1995. 

[10] Vettori, R.L., "Effect of an Obstructed Ceiling on the Activation Time of a Residential 

Sprinkler," NISTIR 6253, National Institute of Standards and Technology, Gaithersburg, 

MD, November 1998. 

[11] NFPA 72, "Appendix B Engineering Guide for Automatic Fire Detector Spacing," 

National Fire Alarm Code, National Fire Protection Association, Quincy, MA, 1996. 

[12] Taylor, B.N. and C.E. Kuyatt, "Guidelines for Evaluating and Expressing the Uncertainty 

of NIST Measurement Results," NIST Technical Note 1297, 1994 Edition, National 

Institute of Standards and Technology, Gaithersburg, MD, September 1994. 
 - 13 - 

Table 1. Sample Natural Gas Heat of Combustion Calculation 

Energy Content of Natural Gas 

Component Volume 

Fraction 

(%) 

Molecular 

Weight 

Molecular 

Weight 

Fraction 

Hc

(kcal/mol) 

Hc Fx

(kJ/L) 

Methane 94.93318 16.041 15.22823 191.759 30.947293 

Ethane 2.788895 30.07 0.838621 341.261 1.617960 

Propane 0.506528 44.09 0.223328 484.704 0.417377 

i-Butane 0.085614 58.12 0.049759 633.744 0.0922376 

n-Butane 0.104749 58.12 0.06088 635.384 0.113145 

i-Pentane 0.040068 72.15 0.028909 780.12 0.0531377 

n-Pentane 0.030024 72.15 0.021663 782.04 0.0399163 

C6 + 0.083655 107.2169 0.089692 1146.538 0.163053 

Nitrogen 0.496727 28 0.139084 0 0 

CO2 0.92905 44 0.408782 0 0 

Total 99.99849 17.08895 33.7990 

Volumetric Rates from Diaphragm Test Meter 

 Time (s) Volume (L) Pressure (kPa) 

1 18 141.6 92.7 

2 18 141.6 92.4 

3 17.9 141.6 92.4 

4 18 141.6 92.4 

Average 18 141.6 92.5 

Ambient Pressure = 101.3 kPa 

Ambient Temperature = 298.15 K 

Pressure Correction = (Pdtm + Pamb)/Pamb = (92.5 + 101.3)/101.3 = 1.91 

Heat Release Rate = Volumetric Flow Rate x Pressure Correction x Heat of Combustion 

Heat Release Rate = (141.6/18) x 1.91 x 33.8 

Heat Release Rate = 508 kW 
 - 14 - 

Table 2. Summary of Uncertainties for Gas Analysis Equipment 

Factor Oxygen Meter CO2 Meter CO Meter 

Noise <1 %(smallest span) <1 %(smallest span) 

Zero <±1 %(span) <±1 %(span) 

Calibration <±1 %(set point) <±1 %(set point) 

Repeatibility ±0.005 % 0.1 % to 1 %(span) 0.1 % to 1 %(span) 

Sample Flow <0.01 % Negligible Negligible 

Ambient Temp. <0.02 % <1 %(span) <1 %(span) 

Flow Presssure <0.15 %(set point) <0.15 %(set point) 

Power Supply <0.1 %(signal) <0.1 %(signal) 

RFI Interference ±0.1 % 

Table 3. Uncertainty Components for Heat Release Rate Measurements 

Components Measured Theoretical 

Sample Port 

Velocity 

Temp. and Gas Conc. 

±5 % 

±1 % 

Bidirectional Probe ±10 % 

Calibration Gas <2 % 

Coefficients ±15 % 

Delay Times ±5 % 

Heat of Combustion ±5 % ±5 % 

Gas Flow Measurement ±10 % 

Pressure Measurement ±2 % 
 - 15 - 

Note: Locations are approximate 

Figure 1. Plan view showing exhaust hood locations in Large Fire Research Facility 

Main Hood 

Afterburner 500 kW Hood 

50 kW Hood

Fan 

Motor 
 - 16 - 

Figure 2. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for fire test 

of 50 kW hood with fan operating at half speed of 50 kW hood with fan operating at half speed 

Figure 3. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for fire test 

of 50 kW hood with fan operating at half speed of 50 kW hood with fan operating at half speed

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)
 - 17 - 

Figure 4. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus for third test of 

50 kW hood with fan operating at half speed 

Figure 5. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for first test 

of 50 kW hood with fan operating at three-quarter speed

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)
 - 18 - 

Figure 6. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for second 

test of 50 kW hood with fan operating at three-quarter speed 

Figure 7. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for third 

test of 50 kW hood with fan operating at three-quarter speed

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Time (s)

Heat Release Rate (kW)
 - 19 - 

Figure 8. Plot of measured versus theoretical heat release rates for all tests using the 50 kW hood 

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)

Figure 9. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for first test 

of 500 kW hood (furniture calorimeter) with fan operating at three-quarter speed 

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Measured Heat Release Rate (kW)

Theoretical Heat Release Rate (kW)
 - 20 - 

Figure 10. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for second 

test of 500 kW hood (furniture calorimeter) with fan operating at three-quarter speed 

Figure 11. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for third 

test of 500 kW hood (furniture calorimeter) with fan operating at three-quarter speed 

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)
 - 21 - 

Figure 12. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for first 

test of 500 kW hood (furniture calorimeter) with fan operating at full speed 

Figure 13. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for second 

test of 500 kW hood (furniture calorimeter) with fan operating at full speed 

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)
 - 22 - 

Figure 14. Plot of measured (solid line) and theoretical (solid, straight lines) heat release rates versus time for third 

test of 500 kW hood (furniture calorimeter) with fan operating at full speed 

Figure 15. Plot of measured versus theoretical heat release rates for all tests using the 500 kW hood (furniture 

calorimeter) 

0

50

100

150

200

250

300

350

400

450

500

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)

0

50

100

150

200

250

300

350

400

450

500

0 50 100 150 200 250 300 350 400 450 500

Measured Heat Release Rate (kW)

Theoretical Heat Release Rate (kW)
 - 23 - 

Figure 16. Plot of measured (solid line) and theoretical (dotted lines) for transient ìfastî and ìslowî t-squared tests 

using the 500 kW hood (furniture calorimeter) with fan operating at full speed 

0

100

200

300

400

500

600

0 100 200 300 400 500 600 700 800

Time (s)

Heat Release Rate (kW)
 - 24 - 

Figure 17. Plot of measured (solid line) and theoretical (solid, straight line) heat release rates versus time for first 

test of main hood 

Figure 18. Plot of measured (solid line) and theoretical (dotted line) heat release rates versus time for second test of 

main hood 

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600 700 800 900

Time (s)

Heat Release Rate (kW)

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

Time (s)

Heat Release Rate (kW)
 - 25 - 

Figure 19. Plot of measured (solid line) and theoretical (dotted line) heat release rates versus time for third test of 

main hood 

Figure 20. Plot of measured versus theoretical heat release rates for all tests using the main hood 

0

100

200

300

400

500

600

700

800

0 100 200 300 400 500 600

Time (s)

Heat Release Rate (kW)

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

Measured Heat Release Rate (kW)

Theoretical Heat Release Rate (kW)
 - 26 - 

Figure 21. Plot of measured (solid line) and theoretical (dotted line) for a transient ìmediumî t-squared test using the 

main hood 

Figure 22. Plot of measured (solid line) and theoretical (dotted lines) for transient ìfastî and ìslowî t-squared tests 

using the main hood 

0

100

200

300

400

500

600

700

800

900

0 50 100 150 200 250 300 350 400

Time (s)

Heat Release Rate (kW)

0

200

400

600

800

1000

1200

0 200 400 600 800 1000 1200 1400

Time (s)

Heat Release Rate (kW)