NIST Technical Note 1809

Exploratory Study of Airflow from 

SCBA Exposed to Elevated 

Temperatures

Michelle K. Donnelly

Anthony Putorti Jr. 
NIST Technical Note 1809

Exploratory Study of Airflow from 

SCBA Exposed to Elevated 

Temperatures

Michelle K. Donnelly

Anthony Putorti, Jr.

Fire Research Division

Engineering Laboratory

September 2013

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
Disclaimer

Certain commercial entities, equipment, instruments, standards or materials may be identified in this document in 

order to describe an experimental procedure, equipment, or concept adequately. Such identification is not intended 

to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended 

to imply that the entities, standards, materials, or equipment are necessarily the best available for the purpose. 

The policy of the National Institute of Standards and Technology is to s use metric units in all of its published 

materials. To aid in the understanding of this report, in most cases, measurements are reported in both metric and

U.S. customary units.

National Institute of Standards and Technology Technical Note 1809

Natl. Inst. Stand. Technol. Tech. Note 1807, 30 pages (September 2013) 

CODEN: NTNOEF
iii

Abstract

This report describes an exploratory set of experiments that investigated the impact of 

external airflow temperature on the temperature of Self-Contained Breathing Apparatus (SCBA)

supply air, and the potential for the supply air to be heated when subjected to an elevated 

temperature environment during fire fighting operations. For these experiments, an entire SCBA 

assembly was placed inside an elevated temperature flow loop. The SCBA facepiece was fitted 

onto a mannequin headform, and a computer controlled breathing simulator provided artificial 

breathing. The SCBA was exposed to airflows with temperatures ranging between 100 oC and 

200 oC (212 oF and 392 oF) for time durations up to 1200 s (20 min). The temperature of the air 

from the SCBA was measured in the mannequin’s mouth. The results of these experiments 

demonstrate that the supply air temperature increases when the SCBA is exposed to external 

conditions of elevated temperatures. The increase in temperature of the supply air was greater 

for the tests at the higher external airflow temperatures, and the SCBA supply air temperature 

increased as the duration of exposure to the elevated temperatures increased. A simple energy 

balance model was developed to characterize the heat transfer process for the breathing air 

exiting the SCBA cylinder during thermal exposure. This model is used to predict the 

approximate temperatures of the breathing air as a function of time and external airflow 

temperature. The model closely predicts the experimental measurements for the three airflow 

temperatures used in these experiments.

 
iv

Table of Contents

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

Background .......................................................................................................................................1

Testing Procedures.............................................................................................................................2

Experimental Uncertainty ..................................................................................................................9

Results.............................................................................................................................................10

Discussion........................................................................................................................................18

Conclusions .....................................................................................................................................21

References.......................................................................................................................................22

Acknowledgements..........................................................................................................................22

Appendix .........................................................................................................................................23
1

Introduction

Fire fighters often face hazardous environments where there are reduced oxygen 

concentrations or the presence of dangerous contaminants in the atmosphere. To enter and work 

in these hazardous conditions, fire fighters use a Self-Contained Breathing Apparatus (SCBA). 

SCBA are portable breathing devices used to provide an individual with a supply of clean 

breathing air. SCBA are essential gear for search and rescue, fire extinguishment, and other fire

fighting operations.

Elevated temperatures are another danger typically present in a fire fighting environment. 

Exposure to elevated temperatures introduces the potential for temperature increases to the 

breathing air supplied by the SCBA. Fire fighters have provided anecdotal accounts of the 

breathing air from their SCBA being heated to uncomfortable temperatures. Questions have 

been raised, based on recent Line of Duty Death incidents, of the potential for fire fighters to 

suffer respiratory tract injuries due to breathing hot air from SCBAs. In a given thermal 

environment, key questions are:

1) How hot can the air in an SCBA become?

2) How quickly can the air in an SCBA heat up?

An exploratory investigation was conducted to examine the impact of environmental

temperatures on the temperature of the SCBA supplied air. The experiments provide a

preliminary look at the potential for the supply air in an SCBA to be heated when subjected to an 

elevated temperature environment. For the experiments discussed in this report, an entire SCBA 

assembly was placed inside an elevated temperature flow loop apparatus and the resulting SCBA 

air temperature was measured within the facepiece.

Background

SCBAs used by the fire service in the United States are designed to meet the standard 

requirements set forth by the National Fire Protection Association (NFPA). NFPA 1981: 

Standard on Open-Circuit Self-Contained Breathing Apparatus (SCBA) for Emergency Services, 

applies “to all open-circuit SCBA...used by emergency services organizations for respiratory 

protection of its personnel during fire fighting, rescue, hazardous materials, terrorist incident, and 

similar operations where products of combustion, oxygen deficiency, particulates, toxic products 

or other IDLH (immediately dangerous to life or health) atmospheres exist or could exist at the 

incident scene [1].” NFPA 1981describes various design and performance requirements, 

including rigorous test specifications, which an SCBA must meet or exceed to obtain 

certification. Included among the requirements are specifications for heat and flame resistance 
2

performance, detailed in NFPA 1981, Chapter 8.11 Heat and Flame Test. Test methods include

subjecting the SCBA to an oven that is pre-heated to a temperature of 95 oC (203 oF) for 

15 minutes, while operating at a ventilation rate of nominally 40 L/min (1.4 cfm). Immediately 

following the elevated temperature exposure, the ventilation rate was increased to 103 L/min (3.6 

cfm), and the SCBA subjected to 10 s of direct flame impingement. After these exposures, “no 

component of the SCBA shall separate or fail in such a manner that would cause the SCBA to be 

worn and used in a position not specified by the manufacturer’s instructions,” and “no 

components of the SCBA shall have an afterflame of more than 2.2 seconds.” In addition, “the 

SCBA facepiece pressure shall not be less than 0.0 mm (0.0 in.) water column and shall not be 

greater than 89 mm (3.5 in.) water column above ambient pressure from the time the test begins 

until the time the test is concluded” [1]. These requirements are designed to ensure that SCBA 

manufactured for use by fire fighters perform adequately in a fire fighting environment.

Injuries can occur to the human respiratory tract due to the inhalation of hot gases. When 

the temperature of the tissue in the respiratory tract increases above 44 oC (111 oF), it can result 

in damage to the tissue [2]. The rise in temperature of the tissue is a complex interaction that 

varies based on the composition of the gas and the heat capacity of each individual’s tissue. 

Depending on the exposure time, burns to the larynx may occur by breathing dry air at 

temperatures around 120 oC (248 oF) [3]. The addition of humidity, steam, or smoke can 

increase the thermal capacity or latent heat of the air. Such air at temperatures of 100 oC (212 oF) 

can cause burns if inhaled [3]. The condition of the air, including the temperature and humidity,

must be sufficient to cause facial burns in order for thermal burns to the respiratory tract to

occur [3]. 

Testing Procedures

For this exploratory investigation, all tests were completed using a Scott Air-Pak 75 SCBA

assembly. The SCBA included a carbon-fiber reinforced aluminum-lined compressed air 

cylinder, with a maximum working pressure of 31.03 MPa (4500 psi), rated for 45 min. The 

cylinder was certified to U. S. Department of Transportation (DOT) Special Permit (SP) DOT-SP 

10915, 18th revision requiring compliance with DOT Basic Requirements for Fully Wrapped 

Carbon-Fiber Reinforced Aluminum Lined Cylinders, DOT- CFFC, 5th Revision, March 2007.

Prior to testing, the cylinder underwent the required hydrostatic testing as specified in DOT-SP 

10915. The SCBA assembly used a Scott AV-3000 facepiece.

An elevated temperature flow loop was used to supply the testing environment for the 

SCBA equipment. Figure 1 shows a photograph of the NIST flow loop apparatus. The blue 

arrows show the direction of the air flow. A 50 kW air duct heater located in the flow loop is 

used to heat the air to the desired temperature to provide convective heat flow. An adjustable 
3

rate electric blower located below the heater circulates the air through the flow loop. The test 

section of the flow loop provides space for the SCBA equipment to be placed during the high 

temperature exposure tests. The test section of the flow loop has a 0.91 m by 0.91 m (3 ft by 

3 ft) cross sectional area. Thermocouples and bi-directional probes located in the test section 

provide air temperature and velocity measurements. A return airflow duct carries the air back to 

the blower, where it is recirculated through the loop. A movable platform located below the test 

section of the flow loop is used to setup and secure the SCBA equipment and to raise the 

equipment into the flow loop.

Figure 1 Side view of NIST Flow Loop
4

To perform the elevated temperature exposure tests, the SCBA cylinder and frame 

assembly was secured in the upright position by a support post mounted to the flow loop 

equipment testing platform. A mannequin headform was mounted on the testing platform, next 

to the support post, and the SCBA facepiece was positioned on the headform. Figure 2 shows a 

photograph of the SCBA equipment setup on the testing platform. For exposure testing 

purposes, the cylinder and the facepiece were positioned side by side, so that both faced into the 

oncoming airflow when raised into the flow loop. A protective fire fighting hood made of 

aramid was placed on the mannequin headform over the SCBA facepiece, as would typically be 

worn by a fire fighter.

Figure 2 SCBA equipment on the flow loop testing platform
5

The mannequin headform used for this investigation is of the same type as specified in 

NFPA 1981, Section 8.1 Airflow Performance Test, 8.1.4.1 [1]. A photograph of the mannequin 

headform is shown in figure 3. The headform is equipped with a nominally 38 mm (1.5 in) 

breathing passageway through its mouth, with an opening on the underside of the headform. 

This passageway may be connected to a mechanical breathing apparatus to allow for simulated 

breathing. When an SCBA facepiece is placed on the headform, air may be drawn through the 

SCBA to simulate breathing while wearing an SCBA. A pressure measurement probe is located 

in the headform, in the location of the left eye. This pressure probe was connected to a pressure 

transducer to monitor the pressure inside the SCBA facepiece.

Temperatures were measured using type-K bare-bead thermocouples. The sampling rate 

for all temperature measurements was 1 Hz. Two thermocouples were located inside the mouth 

opening of the mannequin head, to measure the temperature of the airflow through the mouth. 

This measurement is representative of the temperature of the air breathed by a fire fighter 

wearing an SCBA. The thermocouples inside the mannequin’s mouth can be seen in the 

photograph of the headform in Figure 3. The thermocouples were located at the mid-height of 

the mouth, 1.0 cm ± 0.2 cm (0.39 in.) in from the right and left sides, and 1.0 cm ± 0.2 cm 

(0.39 in.) deep inside the passageway of the mouth.

 

Figure 3 Mannequin headform with thermocouples inside mouth.
6

Two thermocouples were attached to the SCBA facepiece. One was placed on the 

outside of the face piece, positioned along the centerline of the lens, 3.0 cm (1.2 in) below the 

silicon rubber gasket. The other thermocouple was placed on the inner surface of the lens, 

directly opposite the first thermocouple, centered and 3.0 cm (1.2 in) below the silicon rubber 

gasket. The thermocouples were secured in place with fiberglass tape. Figure 4 shows a 

photograph of the mannequin headform wearing the SCBA, along with the aramid heat-resistant

protective hood. The thermocouples are visible on the facepiece. A thermocouple was also 

located on the outside of the SCBA cylinder, in order to record the temperature of the outer 

cylinder wall. This thermocouple was located along the centerline at the midway point of the 

cylinder, For some tests, an additional thermocouple was placed on the outside of the SCBA air 

hose, 25.4 cm (10.0 in.) below the connection between the hose and the regulator, at the 

midpoint of the hose facing the airflow.

 

Figure 4 Thermocouples placed on SCBA facepiece
7

Figure 5 ASL 5000 Breathing Simulator System

The mechanical breathing operation for the mannequin head was provided using the 

Active Servo Lung 5000 (ASL 5000) Breathing Simulator. The ASL 5000 is a computer 

controlled system designed to provide the mannequin head with precisely controlled and 

repeatable artificial breathing. An electronic drive motor operates a piston to control airflow into 

and out of a cylinder to simulate breathing. A diagram of the ASL 5000 setup is shown in 

Figure 5. An Auxiliary Gas Exchange Cylinder with a bellows inside was used in-line between 

the breathing simulator and the mannequin head to protect the internal components of the 

breathing simulator from possible elevated temperatures. The ASL 5000 software was used to 

designate a controlled breathing profile with a sinusoidal waveform at a rate of nominally 

40 L/min (1.4 cfm) for all tests. This breathing volume work rate is the lower of the two testing 

work rates as described in NFPA 1981 [1].
8

The SCBA cylinder was filled to its maximum capacity of 31 MPa (4500 psi) with 

compressed breathing air prior to the start of each test. The temperature of the cylinder was 

allowed to equilibrate to ambient room temperature. The full cylinder was installed in the SCBA 

frame assembly, and the whole SCBA assembly was secured to the support on the flow loop 

equipment testing platform, as shown in Figure 2. The SCBA face piece was positioned on the 

mannequin headform and secured in place with the facepiece straps. The facepiece was then 

connected to the rest of the SCBA assembly. Prior to each test, a breathing check was performed 

on the apparatus using the ASL 5000 breathing simulator. This check ensured that everything 

was properly connected, that the SCBA, mannequin, and breathing machine were operating 

correctly, and that there was no leakage from the SCBA or the facepiece. 

For each test, an airflow temperature was selected and the testing section of the flow loop 

was pre-heated to the desired temperature. Tests on the SCBA were conducted at air flow 

temperatures of nominally 100 oC (212 oF), 150 oC (302 oF) and 200 oC (392 oF). The air speed 

inside the flow loop was set to the maximum blower speed of 1.4 m/s ± 0.3 m/s (3.0 mph). 

When the testing section of the flow loop reached the desired temperature, the equipment testing 

platform was then raised into the flow loop, exposing the entire SCBA assembly to the heated 

airflow. During the elevated temperature exposure, temperatures were recorded at all 

thermocouple locations at a data rate of 1 Hz. 
9

Experimental Uncertainty

Uncertainties identified for the measurements in this report are described below and are a 

result of evaluating the Type A and Type B standard uncertainty as specified in [4]. Type A 

uncertainties are evaluated using statistical methods, and the Type B uncertainties are determined 

by estimating the upper and lower limits of uncertainty for the measurements. 

The temperature measurements for these tests were made using bare-bead thermocouples, 

with bead diameters of nominally 1 mm. The thermocouples used type-K standard thermocouple 

wire manufactured by Omega Engineering. The manufacturer lists the standard uncertainty for 

this type of thermocouple wire as ± 2.2 oC for the temperature ranges encountered in this 

investigation [5]. The thermocouples are subjected to some measurement uncertainty due to 

radiative heating from the flow loop. The combined standard uncertainty for the thermocouple 

measurements is estimated to be ± 8 %. The total expanded uncertainty for the thermocouple 

measurements with a coverage factor of two and a confidence level of 95 % is ± 16 %.

There are uncertainties associated with the reported measured location for the 

instrumentation used in these tests, including the placement position of the thermocouples. The 

component standard uncertainty for each location measurement is estimated to be ± 7 %. The 

combined standard uncertainty for the position of the instrumentation is ± 12 % and the total 

expanded uncertainty with a coverage factor of two and a confidence level of 95 % is ± 24 %. 

The uncertainty in the flow velocity measurement is dependent on the accuracy of the 

instrumentation and the variations of measurement due to flow repeatability. The standard 

uncertainty due to accuracy of the velocity probes is ± 10 % [6]. The uncertainty due to the 

repeatability of the flow measurement is ± 11 %. The combined standard uncertainty of the flow 

velocity measurements is ± 15 %. The total expanded uncertainty with a coverage factor of two 

and a confidence level of 95 % is ± 30 %.
10

Results

A total of seven elevated temperature tests were performed on the SCBA. The first test 

was conducted with the SCBA exposed to an airflow temperature of nominally 100 oC (212 oF) 

for 1200 s (20 minutes). Four tests were conducted at airflows of nominally 150 oC (302 oF), 

two tests for 900 s (15 minutes) and two for 1200 s (20 minutes). The final two tests were 

performed with the SCBA equipment exposed to an airflow temperature of 200 oC (392 oF) for 

1200 s (20 minutes). Because of the safety controls in place during these experiments, 200 oC 

was the maximum temperature at which these experiments could be conducted in the NIST flow 

loop apparatus. When multiple tests were conducted at the same air flow temperature, the results 

were very repeatable. Measured temperatures were within ±5 oC for repeat tests, which is less 

than the uncertainty in the measurements. Table 1 lists the tests performed, the nominal airflow 

temperature for each test, and the low and high temperatures recorded in the mannequin mouth at 

different times during the high temperature exposure. A temperature range is reported because 

the measured temperatures varied over the 10 s interval about each time, depending on whether 

the mannequin was inhaling or exhaling. The mean ambient temperature measured in the 

mannequins mouth prior to each test was 24 oC (75 oF) with at standard deviation of 1 oC.

Table 1. Gas Temperature Measured at Mouth of the Mannequin

Test 

Number

Nominal 

Airflow 

Temperature

Mouth Temperature Range as Function of Time Exposure

Low and High over 10 s interval 

300 s 600 s 900 s 1200 s

1 100 oC 34 oC to 41 oC 42 oC to 50 oC 46 oC to 57 oC 51 oC to 60 oC

2 150 oC 45 oC to 51 oC 60 oC to 65 oC 65 oC to 73 oC N/A

3 150 oC 44 oC to 50 oC 55 oC to 65 oC 62 oC to 73 oC N/A

4 150 oC 45 oC to 50 oC 55 oC to 65 oC 63 oC to 73 oC 70 oC to 80 oC

5 150 oC 42 oC to 51 oC 55 oC to 68 oC 63 oC to 79 oC 68 oC to 85 oC

6 200 oC 47 oC to 61 oC 65 oC to 84 oC 76 oC to 99 oC 84 oC to 110 oC

7 200 oC 48 oC to 60 oC 66 oC to 85 oC 78 oC to 101 oC 86 oC to 110 oC
11

Figure 6 shows a graph of the average temperature measurements as a function of 

exposure time for each of the thermocouples located inside the mannequin mouth during the test 

of SCBA equipment exposed to a nominally 100 oC (212 oF) airflow. This was the first test 

completed in this series, and it demonstrates an increase of temperature in the air supplied by the 

SCBA when it is exposed to elevated temperatures. For this test, the temperatures that were 

measured in the mouth began increasing after just 35 s of exposure to the 100 oC (212 oF) 

environment. The temperature inside the mouth increased steadily, reaching an average 

temperature of 40 oC (104 oF) after approximately 400 s. After 1200 s (20 min) of continuous 

exposure to 100 oC (212 oF) airflow, the average temperatures measured in the mannequin mouth 

reached approximately 55 oC (131 oF).

20

30

40

50

60

70

0 200 400 600 800 1000 1200

Mouth Left Side 

Mouth Right Side 

Average Temperature (oC)

Time (s)

Airflow 100 oC

Figure 6 Average Temperatures measured in the mannequin mouth for 100 oC exposures.
12

The graph in Figure 7 shows the average temperatures measured inside the mannequin 

mouth for a test in which the SCBA was exposed to a nominally 150 oC (302 oF) airflow. 

Temperatures measured in the mannequin mouth rose at a faster rate than for the 100 oC (212 oF) 

exposure, reaching an average of 40 oC (104 oF) after just 210 s, and reaching an average of 

52 oC (126 oF) after 400 s. Average temperatures reached above 75 oC (167 oF) after 1200 s of 

continuous exposure at 150 oC (302 oF).

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Mouth Left Side

Mouth Right Side

Average Temperature (oC)

Time (s)

Airflow 150 oC

Figure 7 Average Temperatures measured in the mannequin mouth for 150 oC exposure.
13

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200

Mouth Left Side

Mouth Right Side

Average Temperature (oC)

Time (s)

Airflow 200 oC

Figure 8 Average Temperatures measured in the mannequin mouth for 200 oC exposure.

For this exploratory investigation, the maximum external airflow exposure for the 

equipment was nominally 200 oC (392 oF). Figure 8 shows the average temperatures measured 

inside the mannequin mouth for one of the 200 oC (392 oF) exposure tests. At this exposure,

temperatures on both sides of the mouth reached an average of 40 oC (104 oF) after just 175 s. 

By 400 s, average mouth temperatures exceeded 60 oC (140 oF). For both of the tests completed 

at an airflow exposure of 200 oC, the temperature measured on the left side of the mouth was 

slightly higher than on the right side of the mouth, by up to 5 oC. This difference is likely due to 

the exact placement of the thermocouples inside the mannequin’s mouth, and is within the 

uncertainty of the thermocouple measurements.
14

Figure 9 Average Gas Temperatures at SCBA Mouth as a Function of External Exposure Temperature 

For this investigation, the SCBAs were tested at three external airflow exposure 

temperatures, 100 oC (212 oF), 150 oC (302 oF) and 200 oC (392 oF). Figure 9 shows the average 

temperatures measured in the mannequin mouth as a function of the airflow exposure 

temperature. The averages of the temperatures were computed at 1 min, 1.5 min, 2 min, and 

3 min, for each of the airflow exposure temperatures. At 3 min of exposure at 100 oC (212 oF) 

airflow temperature, the average mannequin mouth temperature was 32 oC (90 oF), just 8 oC 

above of the ambient temperature of 24 oC (75 oF). At 3 min of exposure at the 200 oC (392 oF) 

airflow, the average mannequin mouth temperature was 42 oC (108 oF), which is 18 oC above the 

ambient temperature.
15

Figure 10 Average Gas Temperatures at SCBA Mouth for SCBA Exposure Times beyond 5 minutes

Figure 10 shows the average temperatures measured in the mannequin mouth as a 

function of the airflow exposure temperature, for SCBA equipment operation at time durations of 

5 min, 10 min, 15 min, and 20 min. For 5 min of exposure to the 100 oC (212 oF) airflow 

temperature, the average mannequin mouth temperature was 37 oC (99 oF), and at 20 min of 

exposure at the 100 oC airflow, the average mouth temperature was 55 oC (131 oF). A 5 min 

exposure at 200 oC (392 oF) airflow temperature resulted in an average mannequin mouth 

temperature of 53 oC (127 oF). At 20 min of exposure at the 200 oC airflow, the mouth 

temperature was 96 oC (205 oF). This represents a 72 oC increase over the average ambient 

temperature.
16

Figure 11 shows a plot of the temperatures measured on the surface of the facepiece for 

the three airflow temperatures to which the SCBA equipment was exposed. The temperature of 

the facepiece rose quickly during the initial phase of each test, and then had a moderate rate of 

increase for the remainder of the exposure time. 

Figure 11 Temperature of the Outer Surface of the Facepiece

0

50

100

150

200

0 200 400 600 800 1000 1200

100 oC exposure

150 oC exposure

200 oC exposure

Temperature (oC)

Time (s)

 Airflow Temperatures
17

Figure 12 Temperature of the Outer Surface of the Cylinder for Increasing Airflow Temperatures

Temperature measurements were taken on the outside surface of the cylinder, at the midpoint of the cylinder on the side facing the oncoming airflow. The results of these measurements 

are shown in Figure 12. After a quick initial rise, the temperature on the outer surface of the 

cylinder continued to increase through the exposure time. The heating rate was slightly more for 

the 200 oC (392 oF) exposure, as show in Figure 12. Uncertainties in temperature measurements 

are ± 16 %.

0

20

40

60

80

100

120

140

160

0 200 400 600 800 1000 1200

100 oC exposure

150 oC exposure

200 oC exposure

Temperature (oC)

Time (s)

 Airflow Temperatures
18

Discussion

The results from these experiments clearly show that the external temperature of the 

environment in which an SCBA is operating has an effect on the breathing air that emerges from 

the SCBA. The temperature measurements inside the mannequin’s mouth show that when the 

SCBA is exposed to an external environment with an elevated temperature, the temperature of 

the air supplied by the SCBA can increase. The amount of temperature increase depends on both 

the environmental airflow temperature and the time duration of exposure to the airflow.

The results shown in Figure 6 for the test at the 100 oC (212 oF) airflow, show that after 

400 seconds, the average gas temperature inside the mannequin’s mouth was above 40 oC 

(104 oF). This demonstrates that even for a relatively low exposure temperature, the breathing 

air inside the SCBA can increase 16 oC above the ambient temperature in less than seven

minutes. After 20 min of exposure to 100 oC (212 oF), the average temperature in the mannequin 

mouth reached approximately 55 oC (131 oF). This is a shorter time duration at 100 oC (212 oF), 

than that of Thermal Class I (25 min at 100 oC), where fire fighters may routinely be expected to 

work [7], indicating that fire fighters may often be exposed to elevated temperature SCBA 

breathing conditions during routine operations.

For the 150 oC (302 oF) airflow exposure, Figure 7, the average gas temperatures in the 

mouth of the mannequin reached 40 oC (104 oF) after just 210 s, significantly faster than for the 

100 oC exposure test. As expected, at the greater airflow temperature, the temperature of the air 

in the SCBA increased faster, especially during the initial heating.

Figures 9 and 10 show the average mannequin mouth temperature plotted as a function of 

the exposure temperature, for various time durations. Figure 9 shows that for short time 

exposures of 3 minutes or less at airflow exposure temperatures up to 200 oC (392 oF), the 

average air temperature in the mannequin mouth stayed at 42 oC or less. This temperature is 

18 oC above the ambient temperature of 24 oC. Although these results indicate an increase in the 

temperature of the air from the SCBA, for the time durations and temperatures shown in 

Figure 9, the average mouth temperature stayed well below 100 oC (212 oF), the temperature 

where burns to the internal tissue could occur [3]. 

In Figure 10, results are shown for longer airflow exposure times of 5 minutes to 

20 minutes. Some of the airflow temperature and time duration combinations that are shown in 

this plot may represent conditions that are not survivable for fire fighters; however the data are 

reported to show the results for operating the equipment at these conditions. For an external 

airflow temperature of 200 oC, and a time exposure of 1200 s (20.0 min), the average 

temperature in the mannequin’s mouth measured 96 oC (205 oF). Breathing humid air at 100 oC 

(212 oF) has been shown to cause thermal burns to the respiratory tract [3]. 
19

To characterize the heat transfer process for the SCBA cylinder during thermal exposure 

in the flow loop, a simplified model was developed. The model approximates the system by 

disregarding the tubing and facepiece sections, as well as the periodic cycles of the breathing 

apparatus, and treats the breathing air as a constant flow out of the cylinder. An energy balance 

is used to model the heat transfer into the system as the breathing air leaves the system. A 

detailed derivation of the model is given in the Appendix. Using the simplified model, the 

temperature of the air exiting the system, T1, which corresponds to the temperature of the 

breathing air measured in the mannequin’s mouth, can be represented with the following 

equation:

[ ] 





























 − − − + − + − =

+ −

∞ ∞

(St 1)

1,

1, 1

1 1 1 1,

1

1

1

St St (St 1) (St 1)

1

γ

γ

γ i

i

bi N

N K t

T T T T

The temperature, T1, in the equation above is solved as a function of the time, t. The other 

terms of the equation are defined in the Appendix, including the dimensionless heat transfer 

Stanton number, St1 [8]. This simplified model can be used to predict the approximate 

temperatures of the cylinder air exiting the system, which effectively represent the breathing air 

temperatures that would be measured in the mannequin’s mouth. A graph of the predicted 

temperatures of the air exiting the cylinder system as computed using this model is shown in 

Figure 13. The temperature, T1, was computed using the three external airflow exposure 

temperatures that were used in the experiments, 100 oC (212 oF), 150 oC (302 oF) and 200 oC 

(392 oF). A Stanton number of St1=3 is used for the calculations plotted in Figure 13, and the 

constant terms used are given in Table A2 of the Appendix. Graphs showing the comparisons of 

the model with the measured results are shown in Figures A2, A3, and A4 of the Appendix.
20

Figure 13 Computed temperature exiting the system, (mannequin mouth temperature), calculated using 

the simplified model, with St1 = 3

In this exploratory set of experiments, the intent was to investigate the possibility of 

temperature increases in the SCBA air supply and to measure temperature increase rates. It is 

recognized that during these experiments, the SCBA equipment was exposed to some 

temperatures and time duration combinations that may not be survivable for living persons, even 

absent of a temperature increase of SCBA breathing air. These tests were intended to collect 

temperature data for the SCBA equipment and to characterize temperature rate increases for the 

SCBA equipment and the air it would supply under various temperature conditions. 

For this investigation, the SCBA unit was taken as a whole, and the entire assembly was 

inserted into the test section during each test. The air temperature ultimately measured in the 

mannequin’s mouth is the result of heating of the air as it passed through the entire SCBA, from 

20

40

60

80

100

120

0 200 400 600 800 1000 1200

100 oC exposure

150 oC exposure

200 oC exposure

Temperature (oC)

Time (s)

 Airflow Temperatures
21

inside the cylinder, through the regulators and tubing connections, and through the face piece, 

before reaching the mouth. During this exploratory look, no attempt was made to identify or 

quantify temperature increases that may have occurred in each section of the SCBA. Instead, the 

focus was on the final temperature that would be experienced by a user at the mouth inlet of the 

SCBA face piece. 

This exploratory investigation yielded initial results regarding the effect of the 

environmental temperature on the SCBA supply air temperature, but there are other variables 

that may impact the SCBA and would be useful to explore in future research. For future work, 

experiments could be conducted to examine the heat transfer at external flow velocities greater 

than the 1.4 m/s (3.0 mph) used in these experiments. Thermal flows in fire environments have 

been measured in the 6 m/s to 9 m/s (13 mph to 20 mph) flow range, and these flows could 

increase the heat transfer rate significantly. Another issue for future work could be to investigate 

the heat transfer contributions of the different components of the SCBA, and their impacts on the 

air supply temperature. Other variables to consider would be different breathing flow rates, and

the effects of pre-heating of a closed SCBA system before use. Radiant heating of the SCBA 

equipment, and flame impingement on the cylinder are two more issues that may affect the 

SCBA supply air temperature, and should be considered in future testing. 

Conclusions

This exploratory set of experiments investigated the impact of elevated external 

temperatures on the temperature of the breathing air supplied by a Self-Contained Breathing 

Apparatus. For these experiments, the SCBA assembly was placed inside an elevated 

temperature flow loop, and was exposed to airflows with temperatures between 100 oC (212 oF) 

and 200 oC (392 oF), for time durations up to 1200 s (20 min). The results show:

1) Breathing air supplied by the SCBA increases in temperature when exposed to an 

elevated temperature environment. 

2) The increase in temperatures of the SCBA supply air became greater as the 

external airflow temperatures increased. 

3) The SCBA supply air temperature increased as the duration of exposure to the 

elevated external temperature increased. 

4) For the maximum time exposure, 1200 s, and the maximum external airflow 

temperature, 200 oC, for this set of tests, the average SCBA supply air 

temperature measured 96 oC (205 oF). 
22

References

[1] National Fire Protection Association, "NFPA 1981: Standard on Open-Circuit Self-Contained 

Breathing Apparatus (SCBA) for Emergency Services," 2007 Edition, Quincy, MA, 2007.

[2] Lv, Y., Lui, J., Zhang, J., Theoretical evaluation on burn injury of human respiratory tract 

due to inhalation of hot gas at the early stage of fires, Cryogenic Laboratory, Technical Institute 

of Physics and Chemistry, Chinese Academy of Sciences, Beijing, PR China. November 2005.

[3] Purser, D. A., “Assessment of Hazards to Occupants from Smoke, Toxic Gases and Heat,” 

The SFPE Handbook of Fire Protection Engineering, Fourth Edition, Society of Fire Protection 

Engineers, National Fire Protection Association, One Batterymarch Park, Quincy, Ma. 02269 

(2008).

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

of NIST Measurement Results," National Institute of Standards and Technology, Gaithersburg, 

MD, NIST Technical Note 1297, 1994 edition, September 1994.

[5] Omega Engineering, Inc., The Omega Temperature Measurement Handbook and 

Encyclopedia, Vol. MMX 6th ed., Stamford, CT, Omega Engineering, 2007.

[6] McCaffrey, B. J., and Heskestand, G., A Robust Bidirectional Low-velocity Probe for Flame 

and Fire Application, Combustion and Flame, 26:125-127,1976.

[7] Donnelly M. K., Davis W. D., Lawson, J. R., and Selepak, M. S., “Thermal Environment for

Electronic Equipment used by First Responders,” National Institute of Standards and

Technology, Gaithersburg, MD, NIST Technical Note 1474, January 2006.

[8] Bird, R.B., Stewart, W.E., and Lightfoot, E.N., Transport Phenomena, John-Wiley & Sons, 

New York, 1960.

[9] Modell, M. and Reid, R.C., Thermodynamics and Its Applications, Prentice-Hall, New 

Jersey, 1974.

Acknowledgements 

The authors would like to thank Jiann C. Yang for his assistance with performing calculations 

and developing the model used in this paper. The authors would also like to thank Daniel 

Madrzykowski for his guidance and assistance with this investigation.
23

Appendix

A simplified model was developed to describe the thermal exposure process for the SCBA 

cylinder during thermal exposure in the flow loop. Figure A1 shows an idealized representation 

of the SCBA cylinder exposed to the elevated temperature airflow. The breathing air in the 

cylinder is considered to be a system (System 1). As a first approximation, the short periodic 

disruptions of the airflow during the inhaling and exhaling cycles of the breathing simulator are

ignored, and the model assumes a continuous airflow out of the SCBA cylinder. The temporal 

temperature variation at the exit of the cylinder is assumed to represent the temperature within 

the SCBA facepiece and is used to compare with the experimental measurements.

Figure A1. Simplified model for thermal exposure of SCBA cylinder.

T1

N1

P1

U1

h1

T1,l

u1,l

P1,l

h1,l

u1

System 1

Cylinder

v1,l

n1,l
24

Table A1. Table of Terms for Model

Description Units

T∞ Temperature of air surrounding the cylinder K

U1 Total internal energy of the air cylinder J

P1 Pressure in cylinder Pa

T1 Temperature of air in the cylinder K

N1 Total number of moles in the system mol

h1 Molar enthalpy of system J/mol

u1 Molar internal energy of system J/mol

P1,l Pressure at the cylinder outlet Pa

T1,l Temperature of air at the cylinder outlet K

h1,l Molar enthalpy at the cylinder outlet J/mol

v1,l Molar volume at the cylinder outlet m3/mol

u1,l Molar internal energy at the cylinder outlet J/mol

n1,l Molar output of the system mol

t Time s

For this simple approximation, the gas flow rate out of the cylinder is assumed to be constant. 

The system under consideration, System 1, is the breathing air in the cylinder. From the First 

Law of Thermodynamics for a simple open system (air cylinder) [9],

l l dU Q W h d n 1 1 1 − 1, 1, = δ − δ (1)

where U1 is the total internal energy of the air cylinder (System 1), Q1 is the heat added to the 

cylinder from the surroundings, W1 is the work done by the system, l h1, is the molar enthalpy at 

the cylinder outlet, and n1,l is the molar output from the system.

With δW1 = 0, Eq. (1) can be written as
25

l l d U Q h d n 1 1 − 1, 1, = δ (2)

Taking the time derivative of Eq. (2),

dt

dn

h

dt

Q

dt

dU l

l

1,

1,

1 1 − δ = (3)

Eq. (3) can be expressed in terms of molar internal energy, u1, of System 1 and the total number 

of moles in System 1, N1.

( )

dt

dn

h

dt

Q

dt

dN

u

dt

du N

dt

d N u l

l

1,

1,

1 1

1

1

1

1 1 − δ = + = (4)

A mole balance of the system results in

dt

dn

dt

dN1 1,l − = (5)

If a constant molar flow rate out of the cylinder, K1, is assumed, then

1

1 1, K

dt

dn

dt

dN l − = = (6)

Integrating Eq. (6),
26

N N K t 1 = 1,i − 1 (7)

where N1,i is the initial total number of moles of air in the cylinder.

Substituting Eq. (6) and Eq. (7) into Eq. (4),

( ) 1 1 1 1, 1

1

1, 1 u K Q h K

dt

du N K t i − + = − l  (8)

with 1

1 Q dt

Q  = δ

as the heat transfer rate to the system from the surroundings.

Substituting for the molar enthalpy, l h1, , gives

( ) 1 1 1 1, 1, 1, 1

1

1, 1 u K Q (u P v ) K

dt

du N K t i − + = − l + l l  (9)

where u1,l, P1,l, and v1,l are the molar internal energy, pressure, and molar volume of the air at the 

cylinder outlet, respectively.

If the gas is treated as ideal, then the molar internal energy of an ideal gas can be expressed as

( ) 1 0 T1 T0 u u c = + v − and ( ) 1, 0 T1, T0 u u c l = + v l − (10)
27

where u0 is the molar internal energy at some reference temperature T0 and cv is the molar 

specific heat at constant volume, assuming cv does not change significantly with temperature 

over the temperature range of interest. Eq. (9) can be rewritten as an ideal gas,

v i v vT l K RT l c K T Q K c

dt

dT

c N K t 1 1 1 1 1, 1 1,

1

1, 1 ( − ) − = − −  (11)

where R (= 8.314 J/mole K) is the universal gas constant.

Treating the pressure regulator of the cylinder as a throttling device (a constant enthalpy 

process), then

h1 = h1,l

( ) ( ) 1 1 1 1,l 1,l 1,l u + Pv = u + P v

v v T l T RT l u c T T RT u c 0 1 0 1 0 1, 0 1, + ( − ) + = + ( − ) +

T1 = T1,l

Equation (11) becomes





 



 − = ∞ − 1 − 1

1

1, 1,

1

1

1, 1 ( ) ( ) T

c

R T T

K c

U A

K

dt

dT N K t

v v

HT t

i (12)

The Q1

 term in Eq. (11) can be conveniently expressed in terms of an overall heat transfer 

coefficient, U1,HT, the heat transfer area of the cylinder, A1,t, and the temperature difference 

between the surrounding temperature, T∞, and the cylinder temperature, T1. The use of a 

constant overall heat transfer coefficient simplifies the detailed analysis of heat transfer from the 

surroundings to the cylinder surface, and from the cylinder to the interior through the composite 

cylinder wall. Equation (12) can be rewritten as
28

[ ] 1 1 1 1

1

1, 1 ( ) K St T (St 1)T

dt

dT N K t i − = ∞ − + γ − (13)

where p v v v γ = c / c = (c + R)/ c and p c is the molar specific heat at constant pressure, and 

v

HT t

K c

U A

1

1, 1, St1 ≡

, which can be considered a form of the dimensionless heat transfer Stanton 

number [8]. St1 = 0 corresponds to adiabatic condition, and St1 → ∞ corresponds to infinitely 

fast heat transfer. Integrating Eq. (13) with the initial condition at t = 0, T1 = T1,i (the initial 

temperature of the air in cylinder) obtains

(St 1)

1,

1, 1

1 1 1,

1 1 1

1

St (St 1)

St (St 1) + −

∞

∞















 − = − + −

− + − γ

γ

γ

i

i

i N

N K t

T T

T T (14)

[ ] 





























 − − − + − + − =

+ −

∞ ∞

(St 1)

1,

1, 1

1 1 1 1,

1

1

1

St St (St 1) (St 1)

1

γ

γ

γ i

i

i N

N K t

T T T T (15)

Equation (15) represents the temporal air cylinder temperature during each breathing cycle. Note 

that in Eq. (15) when St1 → ∞, T1 →T∞ . That is, for infinitely fast heat transfer, the cylinder

temperature would attain thermal equilibrium with the surroundings. 

The calculations were performed using the values listed in Table A1. Figures A2, A3, and A4 

show the comparisons of the calculations with the experimental measurements, using different 

values of St1. The graphs show that using St1 values of 3 or 4, with the simplified model, closely 

predicts the experimental measurements for the three experimental conditions.
29

Table A2. Numerical Values for Model Computations

N1,i Initial total number of moles in cylinder 117.3 mol

T1,i Initial temperature of air in cyliner 23 oC (296 K)

T∞

Temperature of air surrounding the cylinder 100 oC (373 K)

150 oC (423 K)

200 oC (473 K)

K1 Molar flow rate out of cylinder 2 2.98 10− × mol/s

γ Ratio of molar specific heat at constant pressure 

to molar specific heat at constant volume 1.4

R Universal gas constant 8.314 J/mol K

Time (s)

0 200 400 600 800 1000 1200

Temperature (K)

290

300

310

320

330

340

350

St1 = 2

St1 = 3

St1 = 4

St1 = 5

Measurements

Tunnel air @ 100 oC

Figure A2. Comparison with measurements using different St1 at 100 oC.

Airflow
30

Time (s)

0 200 400 600 800 1000 1200

Temperature (K)

280

300

320

340

360

380

400

St1 = 1

St1 = 2

St1 = 3

St1 = 4

St1 = 5

Measurements

Tunnel air @ 150 oC

Figure A3. Comparison with experimental measurements using different St1 at 150 oC.

Tunnel air @ 200 oC

Time (s)

0 200 400 600 800 1000 1200

Temperature (K)

250

300

350

400

450

St = 1

St = 2

St = 3

St = 4

St = 5

Measurements

Figure A4. Comparison with experimental measurements using different St1 at 200 oC.

Airflow

Airflow