iii

Thermal Environment for Electronic 

Equipment Used by First Responders 

M. K. Donnelly 

W. D. Davis 

J. R. Lawson 

M. J. Selepak 

NIST Technical Note 1474 
iv

NIST Technical Note 1474 

Thermal Environment for Electronic 

Equipment Used by First Responders

M. K. Donnelly 

W. D. Davis 

J. R. Lawson 

M. J. Selepak 

Building and Fire Research Laboratory 

January 2006

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

William Jeffrey, Acting Under Secretary for Technology

National Institute of Standards and Technology

 William Jeffrey, Director
v

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

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

identification is not intended to imply recommendation or endorsement by the 

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

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

National Institute of Standards and Technology Special Publication 1474 

Natl. Inst. Stand. Technical Note 1457, 36 pages (January 2006) 

CODEN: NSPUE2 
vi

Table of Contents 

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

List of Tables.............................................................................................v 

Abstract.....................................................................................................1 

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

Equipment Standards.....................................................................................2 

Description of the Thermal Environment..............................................................5 

Fire Equipment Evaluator (FEE).......................................................................10 

Equipment Testing.......................................................................................11 

a. PASS Devices........................................................................................11 

b. Batteries................................................................................................13 

c. Gas Detectors.........................................................................................16 

d. Other...................................................................................................16 

Recommendations........................................................................................16 

a. Thermal Classes......................................................................................16 

b. PASS Standards.......................................................................................17 

Future Investigations.....................................................................................24 

Acknowledgement........................................................................................25 

References.................................................................................................26 

Appendix A................................................................................................28 

a. Fee Instrumentation Specification..................................................................28 
vii

List of Figures 

Figure 1 Thermal protection provided by current firefighters' thermal protective clothing 

ensembles................................................................................................................................ 9 

Figure 2 Relative response for dBA, dBB, and dBC filters.......................................................... 11 

Figure 3 Ham Hock A................................................................................................................... 19 

Figure 4 Ham Hock B................................................................................................................... 20 

Figure 5 Ham Hock inside Sleeve of Turnout Gear ..................................................................... 21

Figure 6 Temperature/time for Ham Hock A..............................................................22 

Figure 7 Temperature/time for Ham Hock B................................................................................ 23 

Figure A1 Drawing of FEE tunnel................................................................................................ 29 

Figure A2 Temperature Profiles near the Back Wall at a Nominal Velocity 0.9 m/s .................. 31 

Figure A3 Temperature Profiles at the Center of the Duct at a Nominal Velocity 0.9 m/s.......... 31 

Figure A4 Temperature Profiles near the Door at a Nominal Velocity 0.9 m/s ........................... 32 

Figure A5 Temperature Profiles near the Back Wall at a Nominal Velocity 1.4 m/s .................. 32 

Figure A6 Temperature Profiles near the Center of the Duct at a Nominal Velocity 1.4 m/s......33 

Figure A7 Temperature Profiles near the Door at a Nominal Velocity 1.4 m/s ........................... 33 

Figure A8 Temperature Profiles near the Back Wall at a Nominal Velocity 1.9 m/s .................. 34 

Figure A9 Temperature Profiles near the Center of the Duct at a Nominal Velocity 1.9 m/s...... 34 

Figure A10 Temperature Profiles near the Door at a Nominal Velocity 1.9 m/s ......................... 35 

Figure A11 Photograph of FEE tunnel and operator Michael Selepak. ....................................... 36 

Figure A12 Photograph of FEE test section and data program .................................................... 36 
viii

List of Tables 

Table 1 Thermal Class I.................................................................................7 

Table 2 Thermal Class II..................................................................................7 

Table 3 Thermal Class III..............................................................................8 

Table 4 Thermal Class IV...............................................................................8 

Table 5 Observed Sound Decrease with Temperature............................................12 

Table 6 Sound Decrease of PASS II with Temperature.............................................13 

Table 7 PASS with Battery at Ambient Temperature.............................................13 

Table 8 Time Test for Battery A at Ambient Temperature.......................................14 

Table 9 First Temperature/Time Test for Battery A..............................................14 

Table 10 Second Temperature/Time Test for Battery A............................................15 

Table 11 Temperature/Time Test for Battery B....................................................15 

Table 12 Recommendations for Thermal Classes..................................................17 

Table 13 Ham Hock Dimensions......................................................................18 
1

Thermal Environment for Electronic Equipment Used by First Responders 

M. K. Donnelly, W. D. Davis, J. R. Lawson, and M. J. Selepak 

Abstract 

The increased use of electronic equipment by emergency responders where that equipment may 

be repeatedly exposed to elevated temperature, humidity and smoke conditions requires an 

examination of current manufacturing tests and standards used to certify this equipment. This 

paper provides a review of the current equipment standards for electronic equipment used by 

firefighters and other first responders. A thermal classification method is developed based on the 

protective characteristics of firefighter turn-out gear that should serve as the basis for developing 

thermal standards for electronic devices used by first responders. The Fire Equipment Evaluator 

(FEE) that was constructed to investigate the performance of electronic equipment in simulated 

fire conditions will be described. Results of testing the Personal Alert Safety System (PASS) are 

presented along with recommendations for revised standards for PASS. 

Key words: fire fighters; protective clothing; burns (injuries); heat flux; sensors; temperature 

measurements; test methods; thermal properties; PASS
2

Thermal Environment for Electronic Equipment Used by First Responders 

M. K. Donnelly, W. D. Davis, J. R. Lawson, and M. J. Selepak 

Introduction 

Firefighters and other first responders often operate in adverse building fire environments. Fire 

environments can expose the responders and their equipment to high temperatures, high 

humidity, severe thermal radiation and smoky conditions. These adverse conditions can be 

catastrophic to the normal operation of electronic equipment. The purpose of this project is to 

examine current standards applied to electronic equipment used by firefighters to ensure that 

these devices are designed to operate under emergency conditions. Where applicable, the project 

will identify the need and provide the scientific basis for the development of new equipment 

standards and test methods. Preliminary research for this project involved three tasks. The first 

task was to survey the electronic equipment used by firefighters and determine the current 

standards and guidelines being used. The second task was to research typical thermal 

environments to which the firefighters, and thus their equipment, are routinely exposed in order 

to determine conditions for designing the test apparatus. The third task was to develop the test 

apparatus that would provide a controlled environment that simulates realistic conditions during 

firefighting. 

In the subsequent sections of the paper, the current equipment standards for electronic equipment 

used by firefighters and other first responders will be described, fire conditions in which this 

equipment must operate will be defined, the test apparatus to simulate these conditions will be 

described, and initial results of equipment testing will be presented. 

Equipment Standards 

First responders use a variety of electronic equipment to assist them with their response to 

emergencies. Electronic location devices are useful in tracking the position of firefighters inside 

a burning structure. A Personal Alert Safety System (PASS) is an electronic device that emits 

audible alarms to notify others of the location of a downed firefighter. Portable radios have 

gained much popularity for use by first responders. The radios can be used as both 

communication devices and also as location devices. Communication sets for wideband ad hoc 

networks are now in the development stage, but may become important location devices for 

firefighters in the near future. Other electronic devices used by firefighters include thermal 

imaging cameras, gas detectors, thermal detectors, and voice amplifiers. 

The National Fire Protection Association (NFPA) publishes a series of National Fire Codes, 

which provide standards and recommendations for the fire service. The NFPA does not 

participate in enforcing these codes. That is left to the individual fire departments, local and 

state authorities. 

The NFPA standard regarding structural firefighter protective clothing is NFPA 1971 Standard 

on Protective Ensemble for Structural Fire Fighting [1]. This standard covers all wearable 
3

elements of the ensemble including helmets, hoods, gloves, footwear, coats and other protective 

garments. This standard is useful as a reference because it lists temperatures and heat flux levels 

the firefighter ensemble is designed to protect against. This can provide a guide to the upper 

bounds of what a firefighter may safely encounter. Chapter 6-6 in NFPA 1971 [1] outlines the 

heat resistance test for the protective garments. In these tests, the garments must withstand a 

temperature of 260 °C (500 °F) for 5 minutes without igniting, melting, dripping or separating. 

Chapter 6-9 [1] covers the radiant heat resistance test for footwear. In this test the footwear is 

exposed to10 kW/m2

 for 1 minute. The thermal exposure for the other protective garments is 

much higher, as described in the Thermal Protective Performance Test, Chapter 6-10 [1]. The 

gloves, hoods, wristlets and multilayer protective garment composites are exposed to a total heat 

flux of 83 kW/m2

 for 30 s, which represents a flashover fire environment. 

NFPA 1500, Standard on Fire Department Occupational Safety and Health Program, Chapter 7-

13 [2], requires firefighters to activate Personal Alert Safety Systems (PASS) “in all emergency 

situations that could jeopardize that person’s safety due to atmospheres that could be IDHL1, 

incidents that could result in entrapment, structural collapse of any type, or as directed by the 

incident commander or incident safety officer.” NFPA 1500, Chapter 7.13 [2] also states that the 

PASS devices comply with NFPA 1982 Standard on Personal Alert Safety Systems (PASS) [3] 

and that these devices be tested weekly and maintained as instructed by the manufacturer. In 

accordance with the NFPA standard, most fire departments have regulations requiring 

firefighters to wear PASS devices when using Self-Contained Breathing Apparatus (SCBA) and 

in other emergency situations. 

The PASS devices are the only electronic equipment with a specific and detailed NFPA standard 

regarding testing and performance of the apparatus. NFPA 1982 [3] is a comprehensive 

document concerning standards and performance requirements for the PASS devices including 

an entire chapter on test requirements. Chapter 6-11 of NFPA 1982 details the heat resistance 

test for the PASS devices. It requires the PASS devices to function properly after exposure for 5 

min at a temperature of 260 °C (500 °F). This is the same temperature requirement given in 

NFPA 1971 [1] for protective clothing ensemble. A heat and flame test for PASS devices is 

described in Chapter 6-12 of NFPA 1982 [3]. 

Some PASS devices are equipped with thermal detectors which can cause an alarm when a 

firefighter experiences an extreme temperature, or has spent a long time at a lower temperature 

threshold. The thermal performance of these thermal sensors is being investigated by NIST 

researchers [Bryner et al. 2002] [4]. When thermal detectors operate as part of a PASS device, 

they are subject to NFPA 1982 [3]. 

The only other electronic equipment groups addressed by NFPA standards are two-way mobile 

communication equipment and two-way portable communication equipment. The devices are 

included in the NFPA 1221 standard for Installation, Maintenance and Use of Emergency 

Services Communications Systems [5]. Sections 8.3.5 and 8.3.6 summarize general equipment 

usage. The only requirements pertaining to the fire environment are clause 8.3.5.4, which states 

 

1

 Immediately Dangerous to Health or Life (IDHL). Any condition that would do one of the following: (a) Pose an 

immediate or delayed threat to life; (b) Cause irreversible adverse health effects; (c) Interfere with an individual’s 

ability to escape unaided from a hazardous environment. (NFPA 1670: 1.3) 
4

“Mobile radios and associated equipment shall be manufactured for the environment in which 

they are to be used”, and clause 8.3.6.2 which states “Portable radios shall be manufactured for 

the environment in which they are to be used and shall be of a size and construction to allow 

their operation with the use of one hand.” No specifications for the “environment” are given, 

and no test procedures or performance criteria are outlined. 

The NFPA standards for portable radios are not specific, but many of the manufacturers listed 

guidelines or specifications to which their equipment adhered. For instance, some manufactures 

have radios that are used as accessories to Self-Contained Breathing Apparatus (SCBA), and as 

such these conformed to NFPA 1981 Standard regarding open-circuit SCBA [6]. Some of the 

manufacturers used the military specification MIL-STDF 810 testing procedure [7]. This 

specification provides a procedure for testing at a temperature and moisture level; however, these 

levels are selected by the manufacturer. The temperatures selected for the testing were not near 

that of a fire environment and were as low as 60 °C (140 °F). 

The emerging technology of the wireless, wideband “ad hoc” networks hopes to provide critical 

information regarding the location of firefighters inside buildings. Because this technology is 

still developing, no specific standards exist governing this equipment. Many of the developers 

indicated that their devices would eventually be embedded in current firefighting equipment, 

such as SCBA or PASS, and would therefore conform to any NFPA standards for that 

equipment. The ad hoc networks will be dependent on signal routers strategically placed inside 

the buildings in order to relay signals and determine a firefighter’s position in the building. The 

routers are to be left unattended, either as permanent fixtures in the building or as temporary 

“bread crumbs” left behind by the responders moving through the building. The routers will 

have to be built to very rugged standards since they could be unattended in the path of a fire. 

None of the manufacturers of the wireless ad hoc network equipment had any thermal 

environment performance standards in place. All of the equipment is still in the prototype stage 

and has not been tested for functionality in the thermal environment. None of the manufactures 

were designing for the NFPA 1981 SCBA [6] or 1982 PASS [3] requirements at this time. 

The thermal imaging camera is a relatively new technology that has become very widespread in 

the past few years. There are no NFPA standards or other independent standards governing 

thermal imaging cameras. The performance guidelines for thermal imaging cameras are chosen 

by the individual manufacturers and are product specific. Individual manufacturers set their own 

operational heat ratings which ranged from a low of 70 °C (158 °F) to a high of 450 °C (842 °F). 

One manufacturer conformed to the NFPA 1982 PASS standard for heat and flame exposure [3]. 

Occasionally, electronic gas detectors are used by firefighters to determine the presence of 

hazardous or life threatening gases in an area. As with the cameras, there are no standards 

regarding the operation of these devices. The specifications for the gas detectors are selected by 

the manufacturer. Most conform to the UL 913 Intrinsic Safe standard [8]. All of the detectors 

investigated claimed to operate in environments up to 50 °C (122 °F). Some had relative 

humidity ratings between 90P% and 95 %, while others were simply deemed “moisture 

resistant”. 
5

Description of the Thermal Environment 

The advent of the new equipment and technology has greatly improved the personal protective 

gear worn by firefighters. The disadvantage to this is that firefighters are now able to enter more 

dangerous conditions than they previously did and this can mean more risks and dangers to the 

firefighters. Firefighters may be relying more heavily on their technology to warn them of 

dangers they cannot “feel” and to safeguard them in the event of an emergency. The equipment 

may be subjected to more dangerous conditions than in the past and a heavier reliance on the 

equipment calls for a better understanding of its limits. To prepare a test protocol for the 

equipment, information regarding the thermal environment encountered by firefighters was 

needed. Much of the information available is based on data collected during firefighter training 

scenarios and full-scale fire experiments. During actual emergencies, the focus is on saving lives 

and suppressing fires, not collecting data. 

Actual fires produce different thermal environments based on various factors including the initial 

environment, the ignition source, the combination and arrangement of fuel, ventilation, etc. 

Many have used a simplified way to classify the thermal environment based on the resulting 

temperature and heat flux. Several researchers have taken a similar route of designating thermal 

classes, using temperature and flux ranges to describe the thermal environment within each class. 

Tables 1 - 4 summarize the thermal environments that researchers and groups have developed. 

As shown in the tables, the researchers designated either three or four distinct thermal classes, 

each with a different description and length of time during which a firefighter was expected to 

function within that class. 

Among the first to define thermal classes based on temperature and heat flux was Ordinanz [9] 

who defined three thermal classes. This analysis was applied to firefighter clothing by Abbott 

and Schulman [10] and Coletta [11]. The International Association of Firefighters (IAFF) 

prepared a set of four thermal classes based on Abeles work [12] as part of their Project FIRES 

report [13]. In 1992, the Federal Emergency Management Agency (FEMA) published a report 

that included their recommendations for thermal classes as a function of temperature and heat 

flux [14]. Studies conducted by Foster and Roberts in 1995 (Fire Research and Development 

Group, FRDG) represent relatively recent work associated with the evaluation of firefighting 

environments [15]. This study also divided firefighting environments into four conditions: 

routine, hazardous, extreme, and critical. The results were based on data collected by firefighters 

outfitted with sensors during fire training scenarios. 

The lower thermal classes, Tables 1 and 2, represent the lowest risks for the firefighters but still 

constitute hazardous working conditions. Firefighters may be exposed to these conditions for 

extended periods of time, which increases the chance of injury, especially as the protective gear 

is heated. The presence of moisture makes these conditions worse for firefighters [16]. 

Electronic equipment carried by firefighters is likely to be exposed to these conditions for 

extended lengths of time on repeated fire calls. Repeated exposure at seemingly low-impact 

conditions may cause the electronics to deteriorate overtime. 

Thermal Class III shown in Table 3 describes dangerous conditions that a firefighter may 

encounter fighting a large fire. The firefighter would possibly be exposed to these conditions 
6

after long time durations in the lower classes. Electronic equipment would be expected to work 

in this environment after being preheated in the less intense environments. This class represents 

the upper limit for normal operation by a firefighter.

Thermal Class IV in Table 4 represents the most severe conditions and is characterized by 

elevated temperatures and fluxes typical of what would be expected in flashover. These are 

emergency conditions that would not normally be encountered by a firefighter, but may be 

experienced if the firefighter is caught in a rapidly deteriorating situation. They could only be 

tolerated for seconds before the firefighter would need to leave the area, or succumb to the 

intense heat. It is likely that most electronic equipment would fail if under these conditions for 

any length of time. 

 

The thermal classes are useful for categorizing the increasing risk to firefighters as a function of 

time, temperature, and heat flux. They provide guidelines for the typical times firefighters may 

be subjected to these conditions. But the thermal classes illustrate discrepancies between experts 

regarding the criteria for the different levels, and designations such as “ordinary” and 

“hazardous” leave room for interpretation. The researchers did not always specify what data 

were used to make their determinations. 

The most severe fire conditions encountered in residential and commercial fires not involving 

explosions are conditions where a room or rooms are in flashover. The heat release rate required 

for flashover of the standard international room (ISO 9705) is about 1700 kW [17] and the flow 

of hot gases from the doorway or window may exceed 400 °C (752 °F) [18]. NIST experiments 

show that in the flashover environment, gas temperatures in the room may reach 1000 °C 

(1832 °F) and heat flux at the floor can measure170 kW/m2

. 

Significant temperatures and fluxes can occur even for rooms not at flashover. Quintiere 

investigated heat fluxes several meters from the doorway of a room containing a fire not at 

flashover [19]. Fluxes as high as 4.5 kW/m2 were measured. In tests conducted on firefighters 

by Foster and Roberts, they found that a flux level of 10 kW/m2

 could only be tolerated by fully 

equipped firefighters for one minute [15]. They also found that there was damage to the 

equipment at this flux level. 

Experiments by Gross and Fang showed that severe conditions may exist even for low intensity 

fires, where flame temperatures are about 700 °C (1292 °F) [20]. At the edge of a burning 

wastebasket they measured heat fluxes of 10 kW/m2

 to 40 kW/m2 and air temperatures of 100 °C 

(212 °F) to 400 °C (752 °F) at the ceiling above the wastebasket fire. 

In addition to the elevated temperatures and heat flux present in a thermal environment, there is 

often a large amount of humidity, especially if firefighters are using water to suppress the fire. 

Thermal radiation from the fire changes water flowing from a hose line to water vapor, 

increasing the humidity. High humidity is of concern for the electronic equipment. Water vapor 

could enter the instruments and condense inside the electronic elements, adversely impacting 

their operation. 
7

 Table 1 Thermal Class I 

Group, 

Year 

Designation Description Ranges 

USFA FEMA, 

1992 

None 

Abeles 

Project Fires 

1980 

Class 1 Up to 30 min Temperature to 40 °C (104 °F)

Flux to 0.5 kW/m2

IAFF 

(Based on 

Abeles), 1985 

Class1 Up to 30 min Temperature to 40 °C (104 °F)

Flux to 0.5 kW/m2

Foster and 

Roberts, 

FRDG, 1995 

Routine Elevated temperature, no 

direct thermal radiation, 

25 min 

Temperature to 100 °C (212 °F)

Flux to 1.0 kW/m2

Coletta, 

1976 

None 

Abbott, 

1976 

None 

Table 2 Thermal Class II 

Group, 

Year 

Designation Description Ranges 

USFA FEMA, 

1992 

Routine One or two objects burning Temperature 20 °C (68 °F) to 

60 °C (140 °F)

Flux 1.0 kW/m2

 to 2.1 kW/m2

Abeles 

Project Fires 

1980 

Class 2 Small fire in a room, 

15 min 

Temperature 40 °C (104 °F) to 

95 °C (203 °F)

Flux 0.5 kW/m2

 to 1.0 kW/m2

IAFF 

(Based on 

Abeles), 1985 

Class 2 Small fire in a room, 

15 min 

Temperature 40 °C (100 °F) to 

93 °C (200 °F)

Flux 0.5 kW/m2

 to 1.0 kW/m2

Foster and 

Roberts, 

FRDG, 1995 

Hazardous Elevated temperature and 

direct thermal radiation, 

10 min 

Temperature 100 °C (212 °F) to 

160 °C (320 °F)

Flux 1.0 kW/m2

 to 4.0 kW/m2

Coletta, 

1976 

Routine Fighting fires from a 

distance 

Temperature to 60 °C (140 °F)

Flux 0.4 kW/m2

 to 1.25 kW/m2

Abbott, 

1976 

Routine Fighting fires from a 

distance 

Temperature to 70 °C (158 °F)

Flux 0.5 kW/m2

 to 1.7 kW/m2
8

Table 3 Thermal Class III 

Group, 

Year 

Designation Description Ranges 

USFA FEMA, 

1992 

Ordinary Serious fire, next to a room in 

flashover 

(10 to 20) min maximum 

Temperature 60 °C (140 °F) to 

300 °C (572 °F)

Flux 2.1 kW/m2

 to 25 kW/m2

Abeles 

Project Fires 

1980 

Class 3 Totally involved fire, 

5 min 

Temperature 95 °C (203 °F) to 

250 °C (482 °F)

Flux 1.0 kW/m2

 to 1.75 kW/m2

IAFF 

(Based on 

Abeles), 1985 

Class 3 Totally involved fire, 

5 min 

Temperature 93 °C (200 °F) to 

260 °C (500 °F)

Flux 1.0 kW/m2

 to 1.75 kW/m2

Foster and 

Roberts, 

FRDG, 1995 

Extreme Rescue, retreat from flashover 

or backdraft 

Temperature 160 °C (320 °F) to 

235 °C (455 °F)

Flux 4.0 kW/m2

 to 10.0 kW/m2

Coletta, 

1976 

Hazardous Outside burning room or 

small building 

Temperature 60 °C (140 °F) to 

300 °C (572 °F)

Flux 1.25 kW/m2

 to 8.3 kW/m2

Abbott, 

1976 

Ordinary Outside burning room or 

small building 

Temperature 70 °C (158 °F) to 

300 °C (572 °F)

Flux 1.7 kW/m2

 to 12.5 kW/m2

Table 4 Thermal Class IV 

Group, 

Year 

Designation Description Ranges 

USFA FEMA, 

1992 

Emergency Severe and unusual, 

15 to 30 s for escape 

Temperature 300 °C (572 °F) to 

1000 °C (1832 °F)

Flux 25 kW/m2

 to 125 kW/m2

Abeles 

Project Fires 

1980 

Class 4 Flashover or backdraft, up 

to 10 s 

Temperature 250 °C (482 °F) to 

815 °C (1500 °F)

Flux 1.75 kW/m2

 to 42 kW/m2

IAFF 

(Based on 

Abeles), 1985 

Class 4 Flashover or backdraft, up 

to 10 s 

Temperature 260 °C (500 °F) to 

815 °C (1500 °F)

Flux 1.75 kW/m2

 to 42 kW/m2

Foster and 

Roberts, 

FRDG, 1995 

Critical Could be encountered 

briefly 

Temperature 235 °C (455 °F) to 

1000 °C (1832 °F)

Flux 10 kW/m2

 to 100 kW/m2

Coletta, 

1976 

Emergency Not normally encountered, 

may be during flashover 

Temperature 300 °C (572 °F) to 

1000 °C (1832 °F)

Flux 8.3 kW/m2

 to 105 kW/m2

Abbott, 

1976 

Emergency Not normally encountered, 

may be during flashover 

Temperature 300 °C (572 °F) to 

1100 °C (2012 °F)

Flux 12.5 kW/m2

 to 208 kW/m2
9

Note: 

In Table 4, it is observed that the values from Abeles Project Fires study have been slightly 

altered by the IAFF classification system. This was apparently done in order to round off values 

on the Fahrenheit (°F) temperature scale. All numbers in the IAFF classification system were 

rounded to the nearest 100 °F value. 

An example of changing thermal exposures associated with a transition to flashover conditions is 

shown in Figure 1 [21]. Since firefighters are normally wearing thermal protective clothing 

while firefighting, and firefighting tactics are designed to help protect the firefighter from 

dangerous thermal exposures, the human skin does not immediately experience high 

temperatures when their protective ensembles are challenged by sudden changes in fire 

dynamics. The data plots in Figure 1 provide some insight into the degree of thermal protection 

provided by current day firefighters’ thermal protective clothing. These data were generated at 

NIST using a gas-fired radiant panel that was producing a radiant heat flux of 2.5 kW/m2

measured at the front surface of an assembly of fabrics and moisture barrier materials that 

represent the construction of a typical firefighters’ protective garment system. At approximately 

160 s, a burner producing a sheet of flames was applied to the garment assembly’s surface. This 

was done to approximate an initiation of a flashover condition. The garment system was allowed 

to heat with a combined total heat flux of 60 kW/m2

 so that temperature conditions on the inside 

garment layers could be studied. At the end of the exposure the flames were removed and turned 

off and a thermal shield was placed between the garment assembly and the radiant panel 

returning the radiant heat flux level to a nominal level of zero. 

Response of Dry Garment

to 2.5 kW/m2 & 60 kW/m2 With Flames

0

100

200

300

400

500

0 60 120 180 240 300 360 420 480

Time (s)

Te m pe ra ture (C)

Garment Shell Exterior Inside Garment Beside Trim

Inside Garment Behind Trim

2.5 kW/m2

60 kW/m2

with Flame

Figure 1 Thermal protection provided by current firefighters' thermal protective clothing 

ensembles. 

Data plots show that after the initial thermal exposure begins the garment shell temperature 

gradually increases and remains higher than the temperature inside the garment next to the 

human skin. This temperature difference varies as the garment system heats and begins to level 
10

out with a temperature difference of approximately 100 °C (180 °F). When the flames are added 

to replicate a flashover condition this temperature difference increases to well over 150 °C 

(270 °F). At a point between 250 °C (482 °F) and 300 °C (572 °F) temperature conditions on 

the inside layer of the garment system begin to approach conditions where human skin may feel 

pain and/or burn injuries may occur. This demonstrates that electronic equipment used by 

emergency responders and worn on the exterior of firefighters’ garments must be able to 

withstand thermal conditions significantly higher than temperatures that cause human skin burn 

injuries. 

Shell fabrics used in the construction of NFPA 1971 compliant thermal protective ensembles 

typically do not exhibit thermal damage until they reach temperatures above 300 °C (572 °F). 

NFPA 1971 [1] calls for firefighters’ protective ensembles to be tested at a thermal exposure of 

260 °C (500 °F) for 5 min. This thermal requirement was primarily selected to help eliminate 

garment materials that would melt and drip or stick to human skin during challenging thermal 

exposures while firefighting operations are underway, and it was designed to insure a minimum 

level of performance so that garment systems would maintain their protective properties under 

this type of exposure. 

Since firefighters’ personal protective electronic equipment (example: Personal Alert Safety 

System PASS) and emergency responder communications equipment (example: radio) perform 

similar life safety functions, it is reasonable to consider using the NFPA 1971 thermal exposure 

criteria for testing electronic devices that are used in the same firefighting environments [1]. It is 

equally as important that these electronic devices function at a time when the life of an 

emergency responder is at great risk. 

Fire Equipment Evaluator (FEE) 

A Fire Equipment Evaluator (FEE) was designed and constructed to simulate expected 

conditions encountered by firefighters up to Thermal Class III fires conditions. Target 

conditions included being able to reach a temperature of 300 °C (572 °F) and a total thermal flux 

of 20 kW/m2

. The 20 kW/m2 value was chosen rather then the 25 kW/m2 recommended by 

USFA based on the fact that 20 kW/m2 is considered the radiation flux at the time of flashover, 

which is a Thermal Class IV condition. The resulting device is shown in Figure A1. It consists 

of a stainless steel closed circuit, fan driven, air flow loop 220 cm long, 174 cm high, and 38 cm 

wide. The current test chamber is 0.91 m (36 in) long with a 0.38 m (15 in) square cross section. 

The test chamber can be expanded to fit larger equipment as needed. Current operating 

conditions within the test chamber include: 

• Flow rates from 0.5 m/s to 2.0 m/s. 

• Temperature up to 300 °C (572 °F). 

• Convective Heat Flux up to 16 kW/m2. 

• Radiant Flux up to 4 kW/m2

. 

• Gas injection to test gas sensors. 

Controlled exposure of the electronics to water and smoke will be added once the initial thermal 

testing of the firefighter devices is completed. 
11

Instrumentation for the FEE includes: 

• Thermocouples for temperature measurement 

• Bi-directional probe for velocity measurement 

• Flux gauges oriented to measure both convective and radiant flux 

• Sound meter to measure sound intensities 

• Real-time spectrum analyzer to measure radio performance 

Temperature measurements were made in the test section using an array of five type K 

(Chromel-Alumel) thermocouples, each with a bead diameter of 1.0 ± 0.2mm. Temperature 

measurements were conducted both horizontally and vertically with thermocouple arrays that 

define the temperature distribution in the test chamber. Figures A2 – A10 in Appendix A show 

the temperature variation in the test chamber as a function of flow velocity. Higher flows 

yielded a more uniform test environment across the chamber. Temperature measurements from 

thermocouples are estimated to be within ±1.0 °C (±2 °F) of the correct temperature. 

Uncertainties in the flow velocity using the bi-directional probe are estimated to be ±0.2 m/s. 

Heat flux measurement uncertainties are estimated to be ±0.2 kW/m2 

Equipment Testing 

a. PASS Devices 

NFPA 1982, the current standard for PASS devices, requires that they provide an alarm at 

95 dBA for a spherical radial distance of 3 m (9.9 ft) [3]. A pre-alarm signal, which sounds for 

an interval of not more than 13 s, must reach a level of 100 dBA and remain at or above this 

level for 3 to 5 seconds. A low battery signal that operates for one hour at a sound level between 

70 dBA and 100 dBA is also required. The dBA rating is a sound measurement using a filter 

that corresponds to the response of the ear for modest sound levels where the ear is fairly 

insensitive to low frequencies. Figure 2 provides the response curves for dBA, dBB, and dBC 

filters 

Figure 2 Relative response for dBA, dBB, and dBC filters 
12

The two thermal tests that these devices are required by NFPA 1982 [3] to pass are: 

a. Being heated in an oven for 5 min at a temperature of 260 °C (500 °F) and then being 

removed from the oven and tested for the above operations. 

b. Being heated in an oven for 15 min at a temperature of 95 °C (203 °F) and then 

exposed to flames for 10 s at a peak temperature of between 815 °C (1500 °F) and 

1150 °C (2102 °F) with a mean of 950 °C (1742 °F). 

The purpose of testing the PASS in the FEE was not to test the capability of these devices to 

conform to the NFPA tests, but to examine conditions that first responders would encounter 

where a properly functioning PASS device would mean the difference between life and death. 

Since Thermal Class III conditions represent the most extreme conditions that firefighters would 

experience for extended lengths of time, it was decided to test the devices under these conditions. 

Tests were conducted with maximum temperatures of 300 °C (572 °F) or less for time periods of 

five or more minutes. A fresh battery was used in the PASS device at the beginning of each test. 

PASS devices from two manufacturers (referred in the paper as I and II) were tested in the FEE. 

Initially, the PASS device was tested by heating the test area in the FEE to 260 °C (500 °F) and 

then inserting the PASS for five minutes. The PASS device was removed and the alarm sound 

was measured within a minute of removal. It was noted that the PASS sounded at lower dBA 

levels when hot but recovered rapidly as it cooled. Devices from manufacturer I tended to have 

more problems with melting and deformation due to heating to 260 °C (500 °F) than from 

manufacturer II. The devices from Manufacturer I also had a tendency to stop alarming and shut 

off during the cooling period. 

Examples of sound changes experienced by both devices are presented in Table 5. The change 

in sound intensity is equal to the difference in dBA between the measurements of sound emitted 

by the device in the FEE when the PASS is at 23 °C (73 °F) and at an elevated temperature after 

the specified period of heating. The sound meter was located 7.62 cm (3 in) from the glass door. 

Table 5 Observed Sound Decrease with Temperature

PASS Device Temperature 

 (°C)/ (°F) 

Time 

 (s) 

Sound Difference 

(dB) 

I 260/(500) 53 13 

I 260/(500) 213 14 

II 260/(500) 300 17 

II 200/(392) 306 17 

II 150/(302) 302 0 

A PASS Device from manufacturer II was subjected to a continuous heating regime up until 

failure. The results of this test are shown in Table 6. During the initial 5 minutes, as the unit 

was at a temperature of 100 °C (212 °F), there was no loss to the sound measurement. But as the 

temperature was increased, the sound decreased until the device failed. This PASS device 

recovered after cool down. The scenario simulated here might be a more realistic scenario for a 
13

firefighter who spends some time in regions of elevated temperature and then encounters a 

Thermal Class III event. 

Table 6 Sound Decrease of a PASS Device from Manufacturer II with Temperature

Temperature (°C)/(°F) Time (s) Sound Difference (dB) 

23/73 0 0 

100/212 302 0 

150/302 885 6 

200/392 1377 14 

300/572 (not reached) 1440 Failure 

In order to determine if the heating was causing the battery to fail, tests were conducted with the 

battery powering the PASS device remaining at ambient temperature. The current drawn from 

the battery to the PASS device was monitored during the heating period. Table 7 summarizes 

the results of the tests. 

Table 7 PASS Device with Battery at Ambient Temperature

PASS 

Device 

Temperature 

(°C) /(°F) 

at 300 s 

Lights 

Flashing 

(mA) 

First 

Alarm 

(mA) 

Second 

Alarm 

(mA) 

Full 

Alarm 

(mA) 

Sound 

Diff 

(dBA) 

II 23/73 2 16 - 18 20 - 46 140 0 

II 260/500 2 16 - 18 40 130-144 16 

The sound diminished by 16 dBA during the heating test even though the battery remained at 

ambient temperature. 

This PASS device from manufacturer I failed to alarm during this test. The current draw from 

the flashing lights on the PASS remained a constant 6 mA to 7 mA until the unit failed 423 s into 

the test. 

b. Batteries 

The battery types used in PASS devices were tested to see how they would respond to Thermal 

Class III fire conditions. It should be noted that the manufacturer’s maximum operating 

temperature for these batteries is 54 °C (129 °F), far lower than the conditions at which PASS 

devices are required to operate. Presumably, the PASS outer case and battery compartment is 

designed to provide thermal protection for the battery. Two types of batteries, A and B, were 

investigated. To test the batteries for high temperature performance, they were placed in the duct 

and connected to a 1000 Ω resistor located outside the duct. The current flow and voltage across 

the resistor were monitored as a function of time and temperature. The capability of battery A 

was tested under a 1053 Ω load at room temperature, with no heating as shown in Table 8. The 
14

battery demonstrated a capability to supply a current of approximately 9 mA for 5400 s under 

this load. 

The batteries were then tested at elevated temperatures. Tables 9 and 10 provide the data 

collected during two tests of type A batteries. The batteries were placed in the tunnel and the air 

temperature was increased to nominally 260 °C (500 °F) and then held at that temperature. In 

both tests, the battery was able to sustain a fairly consistent current level for roughly 600 s at 

260 °C (500 °F) but would deform and explode within the next 300 s. Table 11 shows the data 

for the test of battery type B. The current level for this battery began dropping after less than 

500 s of heating at approximately 260 °C (500 °F) and the voltage and current continued to drop 

more sharply than the type A batteries as exposure to this elevated temperature continued. This 

battery was not heated to explosion. 

Table 8 Time Test for Battery A at Ambient Temperature 

Time (s) Voltage (V) 

0 9.56 

390 9.49 

900 9.43 

1200 9.40 

1500 9.37 

1800 9.34 

2320 9.30 

4200 9.16 

4500 9.15 

4800 9.13 

5400 9.10 

Table 9 First Temperature/Time Test for Battery A 

Time (s) Voltage (V) Current (mA) Temperature 

(°C)/(°F) 

0 9.45 9.00 23/73 

160 9.40 8.96 27/81 

300 9.39 8.95 110/230 

480 9.38 8.94 217/423 

660 9.35 8.90 254/489 

780 9.36 8.92 261/502 

960 9.35 8.91 261/502 

1080 9.33 8.89 262/504 

1140 9.31 8.86 262/504 

1200 9.23 8.82 263/505 

1300 9.17 8.70 264/507 

1460 8.99 8.57 263/505 

1660 8.87 8.45 261/502 

1780 8.82 8.40 261/502 

1810 Battery Exploded 
15

Table 10 Second Temperature/Time Test for Battery A 

Time (s) Voltage (V) Current (mA) Temperature 

(°C)/(°F) 

0 9.36 8.86 23/73 

160 9.33 8.84 27/81 

300 9.32 8.83 112/234 

480 9.30 8.81 218/424 

660 9.29 8.79 253/487 

780 9.30 8.80 256/493 

960 9.30 8.81 260/500 

1080 9.27 8.78 262/504 

1140 9.23 8.74 262/504 

1200 9.20 8.70 262/504 

1320 9.07 8.59 261/502 

1380 8.98 8.50 261/502 

1440 Bulging at battery 

bottom 

 

1453 Battery Exploded 

Table 11 Temperature/Time Test for Battery B 

Time (s) Voltage (V) Current (mA) Temperature 

(°C)/(°F) 

110 9.23 8.80 23/73 

180 9.22 8.79 38/100 

300 9.21 8.77 111/232 

480 9.19 8.75 218/424 

660 9.17 8.73 253/487 

780 9.17 8.74 256/493 

960 9.20 8.76 260/500 

1080 9.18 8.75 262/504 

1140 8.74 8.23 262/504 

1200 7.70 7.31 263/505 

1260 7.58 7.21 263/505 

1320 7.47 7.10 257/495 

1440 7.29 6.94 223/433 

1740 6.50 6.30 165/329 

3160 4.90 4.67 23/73 
16

c. Gas Detectors 

The temperature dependence of a hand-held oxygen detector was examined in order to develop 

some experience with these types of detectors. Since this detector alarms when the oxygen 

concentration drops below a specified level, a nitrogen purge was set up with a flow meter and 

solenoid valve that provided a flow of 140 cc/min of N2 to the oxygen sensor that was sufficient 

to set off the alarm. The detector was placed in the tunnel and gradually heated. The 

manufacturer of the detector states that the high temperature operating limit is 50 °C (122 °F). 

The measurements in the FEE suggest that the unit performs reliably to 102 °C (216 °F), which 

is when the visual display begins to fade. The low oxygen alarm continued to sound at this 

temperature. In all tests, as the temperature of the unit dropped back to 80 °C (176 °F), the 

display recovered and the unit appeared to function normally. 

d. Other 

Testing of hand-held radios used by first responders has begun. At this point, there is not enough 

data to establish the thermal capabilities of these devices. Testing of infrared cameras has not 

started but will begin in the next phase of the project. It is also planned to test PASS devices that 

are integrated into Self-Contained Breathing Apparatus, as the thermal response of these devices 

will be different compared with a stand-alone PASS device. 

Recommendations 

a. Thermal Classes 

The concept of Thermal Classes is valuable as it provides temperature – time targets for 

equipment manufacturers. The issue that needs to be resolved is to define the classes in a 

manner that is realistic for conditions experienced by firefighters and is also not so broad as to 

lose its usefulness for equipment manufacturers. The USFA and FEMA defined a three class 

system while a number of other researchers proposed a four class system. There appears to be an 

advantage to defining a four class system in that life safety conditions are better defined and 

breaking the ordinary category defined by USFA and FEMA into two categories provides better 

definition for the operational capabilities of different electronic equipment. 

The proposed four Thermal Class model is presented in Table 12. The rationale for Thermal 

Class I is based on the analysis of Foster and Roberts (FRDG) [15] for firefighters. It also is 

appropriate for an equipment standard as it was noted in tests using the FEE that the display for 

the oxygen detector tended to fail at temperatures just over 100 °C (212 °F) making devices 

using these types of displays Thermal Class I devices. 

The FRDG recommendation was also used to define Thermal Class II conditions. From an 

equipment standpoint, this may be the Thermal Class for unprotected emergency radios but 

additional testing is required in order to understand the failure modes for this type of equipment. 

The recommendation for Thermal Class III was based on a number of issues. Because Thermal 

Class III is a region where firefighters work for a short time (5 min) and the heat flux must be 
17

less than 20 kW/m2

, which is considered the onset of flashover, the 10 kW/m2 value for heat flux 

was chosen. Work done at NIST has shown that over short exposure times exterior surface 

temperatures of garments above 100 oC resulting from 250 oC air temperatures caused interior 

garment temperatures of 55 °C (131 °F) to 60 oC (140 °F) when human skin inside the garment 

may feel pain and/or burn. The 260 °C (500 °F) criteria was selected based on recommendations 

by FEMA [14], USFA, and IAFF [13] suggesting that firefighters could work for 5 min at this 

temperature. This temperature also fits nicely with the NFPA 1982 standard for PASS devices 

[3] and the NFPA 1971 standard for protective ensembles [1]. 

Table 12 Recommendations for Thermal Classes 

Thermal Class Maximum 

Time (min) 

Maximum 

Temperature 

 (°C)/(°F) 

Maximum 

Flux (kW/m2

) 

I 25 100/212 1 

II 15 160/320 2 

III 5 260/500 10 

IV <1 >260/500 >10 

Thermal burns to the respiratory tract must be considered when applying the concept of Thermal 

Class. The moisture content of the heated air is important as dry air must be at least 120 °C 

(248 °F) to reach the threshold for burns to the larynx. For humid air, steam, or air containing 

smoke the threshold for burns to the larynx may drop to 100 °C (212 °F) [22]. This analysis 

suggests that firefighters exposed to Thermal Class II or higher conditions must have the 

appropriate thermal protection to reduce the exposure to that of Thermal Class I conditions. 

b. PASS Standards

Since PASS must work as long as there is potential for rescue, the requirements which PASS 

must meet will depend on the capability of the firefighter’s protective clothing to sustain life in 

adverse conditions. Since firefighters would not be able to perform search and rescue operations 

in Thermal Class IV conditions, the PASS standard should be based on Thermal Class III 

conditions. At issue is the need to determine the length of time that a trapped firefighter could 

survive in Thermal Class III. Survival in Thermal Class III conditions will depend on the 

thermal protection of the firefighter’s protective clothing, the air capacity of the breathing 

apparatus, and the extent of any injuries suffered during entrapment or incapacitation. 

Experiments were conducted to determine the temperature/time profile that would be 

experienced by firefighters protected by turnout gear and experiencing Thermal Class III 

conditions. Since human testing is not acceptable for this Thermal Class, an appropriate 

simulant was used. The simulant used was a ham hock. The choice was based on research 

performed by Moritz and Henriques [23] who showed that pig skin was an acceptable simulant 

for human skin. 
18

Each ham hock was instrumented with two thermocouples attached by thread to the outside 

surface of the skin and two attached at the bottom surface of the skin in approximately the same 

location as the ones affixed to the skin surface. A fifth thermocouple, sewn to the inside surface 

of the turnout sleeve monitored the temperature of this surface and a sixth thermocouple sewn to 

the outside surface monitored the outside sleeve temperature. The seventh thermocouple 

monitored the temperature of the airflow upstream from the end of the sleeve. The sleeve was 

mounted inside the FEE with the end inside the FEE closed such that no hot air could enter. The 

sleeve was fed through a hole in the bottom of the FEE with the thermocouple wires exiting 

through the hole. The physical dimensions of the two ham hocks are given in Table 13. 

Table 13 The Physical Dimensions of the Two Ham Hock Tested 

Ham Hock Mass 

 (g) 

Large Diameter 

(cm) 

Small Diameter 

(cm) 

Length 

(cm) 

A 672.6 14.6 x 7.6 8.6 x 5.7 13.6/7.9 

B 599.0 15.2 x 7.6 10.2 x 5.1 10.2/7.6 

Ham hocks have large and small ends which are irregular in diameter. The diameter 

measurements are in perpendicular directions with the largest diameter given first. Opposite 

lengths are also given as the distance from end to end varies around the diameter. Pictures of 

ham hock A and B instrumented with thermocouples are shown in Figures 3 and 4 
19

Figure 3 Ham Hock A
20

Figure 4 Ham Hock B 

The average skin thickness for Ham Hock B at the end of the test was 2.5 ± 0.4 mm. This is 

substantially larger than the 0.08 mm skin depth that was used for developing the 44 °C (111 °F) 

target temperature for the onset of skin damage based on work by Moritz and Henriques[23]. 

Therefore, only surface temperatures will be used in the analysis. Figure 5 shows the orientation 

of the sleeve in the FEE. 
21

Figure 5 Ham Hock inside Sleeve of Turnout Gear 

Figures 6 and 7 present the temperature time history for the two experiments. It took 

approximately 150 seconds to start the heating from the beginning of the data collection at time 

zero and 660 seconds to 750 seconds for the flow to reach a steady 260 °C (500 °F). 
22

Ham Hock A

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Time s

Temperature oC

TC1

TC2

TC3

TC4

TC5

TC6

TC7

Figure 6 Temperature/time for Ham Hock A
23

Ham Hock 3

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

0 500 1000 1500 2000 2500

Time s

Temperature o

C

TC1

TC2

TC3

TC4

TC5

TC6

TC7

Figure 7 Temperature/time for Ham Hock B 

The thermocouple locations were: 

• TC1 – upstream air temperature 

• TC2 – under skin right of center 

• TC3 – on skin right of center anchored by thread 

• TC4 – under skin left of center 

• TC5 – on skin left of center anchored by thread 

• TC6 – outside sleeve 

• TC7 – inside sleeve sewn to fabric 

Moritz and Henriques[23] found that when the basal layer of the skin, 80 μm below the skin 

surface, reached 44 °C (111 °F) the rate of injurious change exceeds that of recovery. The 

temperature at the surface of the skin will be slightly higher and depends on the intensity of the 

heat flux [Stoll and Chianta] [24]. For the purposes of this analysis, it will be assumed that when 

the surface temperature of the simulant reaches 44 °C (111 °F), damage to the skin will occur. 

When the skin temperature reaches a value of 57 °C (135 °F) to 63 °C (145 °F), third degree 

burns may occur within one second of exposure [Dussan, B. I., and Weiner, R. I.] [25]. A 60 °C 

(140 °F) skin temperature will represent the baseline for determining how long a PASS device 

needs to operate. For the two ham hocks tested, the minimum time for one of the thermocouples 

monitoring skin temperature to reach 44 °C (111 °F) was 8.5 min and 11.0 min and to reach 60 

°C (140 °F) was 18 min and 25.7 min for ham hock A and B respectively. It took 11 min and 

12.5 min to reach the operating temperature of 260 °C (500 °F) for the ham hock A and B 
24

experiments respectively. The 44 °C (111 °F) times support the Thermal Class III 

recommendation that the firefighter should only spend 5 min in this zone. 

Recommendations for standards for PASS device exposures would include: 

• Operation with 95 dBA alarm at a spherical radial distance of 3 m (9.9 ft) for an 

operation time of 20 minutes at 260 °C (500 °F) for Thermal Class III operation. 

• Operation with 95 dBA alarm at a spherical radial distance of 3 m (9.9 ft) for an 

operation time of 30 minutes at 160 °C (320 °F) for Thermal Class II operation. 

• Battery container strong enough to contain fragments produced by a battery explosion. 

The recommended operation time for Thermal Class III is based on the ham hock experiments 

that suggest an onset of third degree burns as early as 18 min after exposure to elevated 

temperatures that reach 260 °C (500 °F). The Thermal Class II recommendation is based on an 

extrapolation of the ham hock experiments to smaller temperature differences by making the 

assumption that the energy flux is mostly convective and is proportional to the first power of the 

temperature difference. The operation times depend on a number of factors, including survival at 

these temperatures, which currently are not well understood. 

Issues for PASS devices that still need resolving include: 

• What background sound intensity is encountered on the fire scene and is a 95 dBA at a 

spherical radial distance of 3 m (9.9 ft) alarm sufficient to locate a downed firefighter? 

• Is the current variation in alarm pitch at the 95 dBA level the best choice for locating a 

downed firefighter? 

• How susceptible is a PASS device to muffling problems? 

• How does repeated use of a PASS device affect its operation in critical temperature 

regions? 

• Are operation times for PASS devices sufficient to facilitate rescue at these conditions? 

• What number of repeated exposures to Thermal Class I, II, or III conditions can a PASS 

device withstand and still be regarded as operational? 

Future Investigations 

The next phase of this project will focus on defining the thermal classes of operation for 

emergency radios, and infrared cameras. The impact of repeated exposures of Thermal Class I, 

II, and III conditions for PASS devices will be documented. The impact of exposure to strong 

radiant fluxes at elevated temperatures for short time periods will also be investigated. In this 

last instance, what is being simulated is a situation where a firefighter has spent substantial time 

in Thermal Class II or III conditions and then is subjected to flashover radiation for a short 

period of time. This condition would occur when a firefighter is involved with a rescue, or a 

retreat from a Thermal Class IV condition. Experiments are also being planned to better define 

the required operational times for Thermal Class I, II, and III. 
25

Acknowledgement 

This work was sponsored the Department of Homeland Security (DHS) via the NIST Office of 

Law Enforcement Standards (OLES) to advance the development of Standards for Electronic 

Equipment used by Emergency Responders. 
26

References 

1. National Fire Protection Association, NFPA 1971 Standard on Protective Ensemble for 

Structural Fire Fighting, 2000 Edition, Volume 11. National Fire Protection Association, 

1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101. 

2. National Fire Protection Association, NFPA 1500 Standard on Fire Department Occupational 

Safety and Health Program, 2002 Edition, Volume 10. National Fire Protection Association, 

1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101. 

3. National Fire Protection Association, NFPA 1982 Standard on Personal Alert Safety Systems 

(PASS), 1998 Edition, Volume 12. National Fire Protection Association, 1 Batterymarch 

Park, P.O. Box 9101, Quincy, MA 02269-9101. 

4. Bryner, N. P., Madrzykowski, D., Stroup, D. W., “Performance of Thermal Sensors in 

Personal Alert Safety System (PASS) Devices” National Institute of Standards and 

Technology, Gaithersburg, Internal report in review. 

5. National Fire Protection Association, NFPA 1221 Standard on Personal Alert Safety Systems 

(PASS), 1998 Edition, Volume 12. National Fire Protection Association, 1 Batterymarch 

Park, P.O. Box 9101, Quincy, MA 02269-9101. 

6. National Fire Protection Association, NFPA 1981 Standard on Open-Circuit Self-Contained 

Breathing Apparatus for the Fire Service, 2002 Edition, Volume 12. National Fire Protection 

Association, 1 Batterymarch Park, P.O. Box 9101, Quincy, MA 02269-9101. 

7. MIL-STD-810F, Department of Defense, United States of America, 2000. 

8. Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook, IL 60062. 

9. Ordinanz, W., “Work in Hot Environments and Protection against Heat”, The Iron and Steel 

Institute, London. 1970. 

10. Abbott, N. J. and Schulman, S., “Protection from Fire: Nonflammable Fabrics and Coatings,” 

Proceedings of 1976 International Symposium on Flammability and Fire Retardants. Ontario, 

Canada, May 6-7, 1976. 

11. Colletta, G. C., Arons, I. J., Ashley, L. E., and Drennan, A. P., “The Development of Criteria 

for Firefighters’ Gloves Volume I: Glove Requirements. U. S. Department of Health, 

Education, and Welfare, Public Health Service, Center for Disease Control, National Institute 

for Occupational Safety and Health, Division of Physical Sciences and Engineering, 

Cincinnati, Ohio February, 1976. 

12. Abeles, Fred J., “Protective Ensemble Performance Standards, Project Fires” Phase 1B Final 

Report, Volume 2, Grumman Aerospace Corporation, Bethpage, New York, May 1980. 

13. Duffy, R. M., Sawicki, J. C., Beer, A. R., “Project Fires: Firefighters Integrated Response 

Equipment System – The Final Report,” International Association of Firefighters, 

Department of Occupational Health and Safety, Washington D.C., 1985. 

14. Federal Emergency Management Agency/United States Fire Administration “Minimum 

Standards on Structural Fire Fighting Protective Clothing and Equipment: A Guide for Fire 

Service Education and Procurement,” FA-137, United States Fire Administration, 16825 

South Seton Avenue, Emmitsburg, MD 21727. December 1992. 

15. Foster, J. A. and Roberts, G.V., “Measurements of the Firefighting Environment – Summary 

Report,” Central Fire Brigades Advisory Council Research Report number 61, 1994, Home 

Office Fire Research and Development Group, Fire Engineers Journal, United Kingdom, 

September 1995. 
27

16. Lawson, J. R., “Thermal Performance and Limitations of Bunker Gear,” Fire Engineering, 

Vol. 151 No. 8, Penn Well Publishing Company, Saddle Brook, NJ, August 1998. 

17. Babrauskas, V.; Peacock, R. D.; Reneke, P. A., “Defining Flashover for Fire Hazard 

Calculations. Part 2.,” Fire Safety Journal, Vol. 38, No. 7, 613-622, November 2003. 

18. Lawson, J. R., “Firefighter's Protective Clothing and Thermal Environments of Structural 

Fire Fighting,” National Institute of Standards and Technology, Gaithersburg, MD, U.S. Fire 

Administration, Emmitsburg, MD, NISTIR 5804; August 1996. 

19. Quintiere, J., “The Spread of Fire From a Compartment – A Review,” National Bureau of 

Standards (currently NIST), prepared for an American Society for Testing and Materials, 

ASTM Symposium, June 1978. 

20. Gross, D. and Fang, J. B., “The Definition of a Low Intensity Fire,” National Bureau of 

Standards (currently NIST), Special Publication 361, Volume 1: Performance Concept in 

Buildings; Proceedings of the Joint RILEM-ASTM-CIB Symposium, May 1972, 1972. 

21. Lawson, J. R.l, and Twilley, W. H., “Development of an Apparatus for Measuring the 

Thermal Performance of Firefighters’ Protective Clothing,” National Institute of Standards 

and Technology, Gaithersburg, MD, NISTIR 6400; October 1999. 

22. Purser, David A, “Toxicity Assessment of Combustion Products,” The SFPE Handbook of 

Fire Protection Engineering Third Edition, National Fire Protection Association, One 

Batterymarch Park, Quincy, Ma. 02269-9101 (2002) 

23. Moritz, M. D. and Henriques, F. C., “Studies of Thermal Injury II. The Relative Importance 

of time and Surface Temperature in the Causation of Cutaneous Burns” American Journal of 

Pathology, 23: 695-720 (1947) 

24. Stoll, A. M. and Chianta, M. A., “Heat Transfer Through Fabrics as Related to Thermal 

Injury,” Transactions of the New York Academy of Sciences, 33, 648-670: (1971) 

25. Dussan, B. I. and Weiner, R. I., “Study of Burn Hazard in Human Tissue and Its Implication 

on Consumer Product Design,” The American Society of Mechanical Engineers Winter 

Meeting, Washington, D. C., November (1971). 

26. McCaffrey, B. J., and Heskestad, G., “A Robust Bidirectional Low-velocity Probe for Flame 

and Fire Application”, Combustion and Flame, 26:125-127 (1976). 
28

Appendix A 

Fire Equipment Evaluator Instrumentation Specifications 

A 25 kW air duct heater (Omega Model No. ADH-025/480V/3P)∗

 is positioned in the vertical 

section of the tunnel to provide heat to the air flow. The heater is comprised of a bank of tubular 

elements through which the airflow must pass. The elements are sheathed in Incoloy, a nickeliron-chromium alloy, for protection against corrosion and oxidation. The heater is housed in a 

38 cm (15 in) by 61 cm (24 in) cross sectional area that was sized to the heater. The expansion 

area of the airflow approaching the heater expands at an approximately 30 degree angle, the 

maximum expansion angle allowable for the heater. A diagram and pictures of the FEE are 

presented in Figure A1, A11, and A12. 

Aluminum honeycomb, 10.2 cm (4.0 in) thick with a 0.3175 cm (1/8 in) cell size is positioned at 

the entrance to the heater expansion area and 5.1 cm (2.0 in) thick, 0.95 cm (3/8 in) cell stainless 

steel honeycomb is positioned 61.3 cm (24 1/8 in) from the center of the door in the 

measurement chamber. The purpose of the honeycomb is to create a uniform flow in the 

measurement chamber. 

The heater is controlled by a SCR controller (Omega Model No. SCR 73P-480-060-59) 

connected to a ramp and soak temperature controller (Omega Model No. CN8201-F1). 

A thermocouple placed in the airflow directly beyond the heater provides feedback to the 

temperature controller. The temperature controller sends a signal to the SCR, which controls the 

heater until the desired air temperature is reached. 

Temperature measurements were made using type K thermocouples. Bare-bead thermocouples 

were used to measure downstream airflows. A thermocouple equipped with a radiation shield 

was used to measure the temperature near the heater. 

Measurements of the air flow velocity in the tunnel use a bidirectional probe connected to a 

differential pressure transducer (Setra, Model 264), with an accompanying thermocouple. 

Bidirectional probes have the advantages of being robust, adjustable to changing temperatures, 

and fairly insensitive to the mounting angle. The bidirectional probe measures the velocity, v, as 

a function of the pressure drop,Δp , across the probe, and the fluid temperature, T, at the probe 

location according to the equation: 

ρ

1 (2 p)

K

v

Δ = , where RT

MW Pabsolute ( ) ρ = . 

The density, ρ, is computed as a function of temperature using the ideal gas law. The air is 

assumed to be pure, with a molecular weight (MW) of 0.029 kg/mol. Here Pabsolute is the absolute 

barometric pressure and R is the universal gas constant 8.314 J/mol K. The K is a constant, 1.08, 

as reported by McCaffrey and Heskestad [26]. 

 

∗

 The identification of any commercial product or trade name does not imply endorsement or recommendation by 

the National Institute of Standards and Technology. 
29

To measure the pressure differential, the bidirectional probe is connected to a very low 

differential pressure transducer (Setra, Model 264). The transducer has bidirectional operation 

over the range of ± 12 Pa (0.05 in H2O), with an accuracy of ± 1% of full scale. 

Figure A1 Drawing of FEE tunnel 

Two water-cooled Medtherm Schmidt Boelter total heat flux gages, model number 64-2-18, are 

used to measure the heat flux. The gages were calibrated over the range of 0 kW/m2

 to15kW/m2. 

15.2 cm 

(6 in) 

38.1 cm 

(15 in) 

174.0 cm

(68.5 in)

30.5 cm 

(12 in) 

38.1 cm 

(15 in) 91.4 cm 

(36 in) 

33.0 cm 

(13 in) 

38.1 cm 

(15 in)

61 cm 

(24 in) 

15.2 cm (6 

20.3 cm (8 

40.6 cm 

(16 in) 

44.4 cm 

(17.5 in) 

219.5 cm 

(86.4 in) 

15.2 cm (6 in)

Door with 

Window 

Measurement 

Chamber 

Heater

Pressure 

Blower 

Radiant Panel 

Honeycomb 

38.1 cm 

(15 in)

Honeycomb
30

Radiant heating is supplied using an electric infrared heater (radiant panel) composed of metal 

heating elements secured to a ceramic face. The heater is a Thermaking model, manufactured by 

Thermal-tech Corporation and has a short heat up time of 10 s. The surface area of the heater is 

25 cm by 35 cm and is capable of a maximum flux of 23.25 kW/m2

. A digital SCR controller is 

used to regulate the heater operation. A built-in thermocouple monitors the element temperature, 

providing feedback to the SCR in order to control the heater. 

The airflow is supplied by a high temperature pressure blower (Cincinnati Fan Model PBS-12A) 

with a maximum air supply of 293 L/s (620 cfm). The blower is powered by an externally 

mounted 3⁄4 horsepower motor (Baldor custom inverter duty motor). 

Data from the measurement instruments is collected at a rate of 1 Hz. This rate is sufficient to 

characterize the changes in the thermal environment experienced by the test equipment. 

The temperature profile across the top of the tunnel at the door is given in Figures A2 – A10 for 

three different flow velocities. The higher flow speeds produce a more uniform temperature 

distribution across the tunnel. A thermocouple tree, oriented vertically in the tunnel and located 

on the door centerline, was used to measure the temperature The thermocouples were spaced at 

3.8 cm, 7.6 cm, 11.4 cm, 19.0 cm, and 22.9 cm (1.5 in, 3 in, 4.5 in, 7.5 in, and 9 in) {TC1, TC3, 

TC4, TC5 and TC6} from the ceiling. The tree was located at one of three positions: near the 

back wall 7.6 cm (3 in) from the back wall, centered 19.0 cm (7.5 in) from back wall, and near 

the door 7.6 cm (3 in) from the door. Thermocouple TC1 was fixed centered in the tunnel and 

upstream 10.2 cm (4 in) from the door centerline. The bidirectional probe was located in this 

position to monitor the flow velocity. 
31

20

40

60

80

100

120

140

160

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000 1200

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Back Wall 

Figure A2 Temperature Profiles near the Back Wall at a Nominal Velocity 0.9 m/s 

20

40

60

80

100

120

140

160

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000 1200

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Center of Duct

Figure A3 Temperature Profiles at the Center of the Duct at a Nominal Velocity 0.9 m/s 
32

20

40

60

80

100

120

140

160

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000 1200

TC1

TC2

TC3

TC4

TC5

TC6

Velocity

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Door

Figure A4 Temperature Profiles near the Door at a Nominal Velocity 0.9 m/s 

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Back Wall 

Figure A5 Temperature Profiles near the Back Wall at a Nominal Velocity 1.4 m/s 
33

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Center of Duct

Figure A6 Temperature Profiles near the Center of the Duct at a Nominal Velocity 1.4 m/s 

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Door

Figure A7 Temperature Profiles near the Door at a Nominal Velocity 1.4 m/s 
34

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

2.5

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Back Wall 

Figure A8 Temperature Profiles near the Back Wall at a Nominal Velocity 1.9 m/s 

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

2.5

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Center of Duct

Figure A9 Temperature Profiles near the Center of the Duct at a Nominal Velocity 1.9 m/s 
35

20

40

60

80

100

120

140

160

0

0.5

1

1.5

2

2.5

0 200 400 600 800 1000

TC1

TC2

TC3

TC4

TC5

TC6

Velocity 

Temperature ( oC )

Velocity (m/s)

Time (s)

Thermocouple Tree Near Door

Figure A10 Temperature Profiles near the near the Door at a Nominal Velocity 1.9 m/s 
36

Photographs of the FEE 

Figure A11 Photograph of FEE tunnel and operator Michael Selepak.

Figure A12 Photograph of FEE test section and data program 
37