Round Robin Study of Total Heat Flux

Gauge Calibration at Fire Laboratories

William M. Pitts

Annageri V. Murthy

John L. de Ris

Jean-Rémy Filtz

Kjell Nygård

Debbie Smith

Ingrid Wetterlund

NIST Special Publication 1031
iv

NIST Special Publication 1031

October 2004

U.S. Department of Commerce

Carlos M. Gutierrez, Secretary

Technology Administration

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology

Hratch G. Semerjian, Acting Director

Round Robin Study of Total Heat Flux

Gauge Calibration at Fire

Laboratories

William M. Pitts

Building and Fire Research Laboratory

Annageri V. Murthy

Aero-Tech, Inc.

Hampton, VA 23666

John L. de Ris

FM Global Research

Norwood, MA

Jean-Rémy Filtz

LNE: French National Metrology and Testing Laboratory

Trappes, France

Kjell Nygård

SINTEF NBL Norwegian Fire Research Laboratory

Trondheim, Norway

Debbie Smith

The Fire Division of BRE

Garston, UK

Ingrid Wetterlund

SP Swedish National Testing and Research Institute

Borås, Sweden
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 1031

Natl. Inst. Stand. Technol. Spec. Publ. 1031, 36 pages (October 2004)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2004

_________________________________________

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
iii

ABSTRACT 

Total heat flux gauges are widely employed in fire research and fire testing laboratories. Several 

fire laboratories have developed systems for calibrating these gauges. There are major 

differences between these calibration facilities, and prior to this work there had been no 

systematic check of the degree of agreement between calibrations from the different laboratories. 

This report describes the results of two round robins in which five fire laboratories performed 

independent calibrations of two sets of Gardon and Schmidt-Boelter total heat flux gauges (i.e., a 

total of four gauges). One set of the gauges was also calibrated by two additional laboratories 

whose primary focus is measurement and calibration. Even though systematic variations 

between calibrations from the different laboratories were identified, the degree of agreement is 

viewed as being satisfactory for most fire testing and research purposes. Additional 

experimental findings concerning gauge angular response and linearity are presented. The need 

to develop a better understanding of the relative response of this type of gauge to radiative and 

convective heat transfer is emphasized. 

Keywords: calibrating; convective heat transfer; Gardon gauges; heat flux; round robin; 

Schmidt-Boelter gauges; thermal radiation 
iv

TABLE OF CONTENTS 

page

Abstract.......................................................................................................................................iii

LIST OF TABLES .....................................................................................................................v 

LIST OF FIGURES ...................................................................................................................vi 

1. INTRODUCTION.................................................................................................................1

2. ROUND ROBIN DESIGN ...................................................................................................1

3. CALIBRATION FACILITIES............................................................................................2

 3.1 Fire Laboratories...........................................................................................................2

 3.1.1 BRE/FRS..................................................................................................................2 

 3.1.2 FM Global................................................................................................................3 

 3.1.3 BFRL........................................................................................................................4 

 3.1.4 SINTEF....................................................................................................................6 

 3.1.5 SP .............................................................................................................................7 

 3.1.6 Summary of Fire Laboratory Calibration Facilities.................................................9 

 3.2 Measurement Laboratory Calibration Facilities........................................................10 

 3.2.1. LNE.........................................................................................................................10 

 3.2.2. PL............................................................................................................................11 

 3.3 Heat Flux Gauge Manufacturer ...................................................................................12 

4. RESULTS ..............................................................................................................................12 

 4.1 First Round Robin .........................................................................................................12 

 4.2 Calibrations of First Round Gauges by the Manufacturer, LNE, and PL...............15 

 4.3 Second Round Robin .....................................................................................................18 

 4.4 Comparisons Based on Predicted Full Scale Gauge Responses ................................21 

5. DISCUSSION........................................................................................................................22 

6. SUMMARY ...........................................................................................................................28 

7. RECOMMENDATIONS......................................................................................................28 

8. REFERENCES......................................................................................................................28 
v

LIST OF TABLES 

page

Table 1. Radiant Source, Method for Characterizing Heat Flux Level at 

 Gauge, Source View Angle and Maximum Heat Flux at the 

 Gauge for Fire Laboratory Calibration Methods...................................................9 

Table 2. Results of Linear Least Squares Curve Fits to Calibrations for 

 the Schmidt-Boelter Gauge Used in the First Round Robin ..................................13 

Table 3. Results of Linear Least Squares Curve Fits to Repeated BFRL 

 Calibrations of the Schmidt-Boelter Gauge Used in the First 

 Round Robin...............................................................................................................14 

Table 4. Results of Linear Least Squares Curve Fits to Calibrations for 

 the Gardon Gauge Used in the First Round Robin.................................................15 

Table 5. Results of Linear Least Squares Curve Fits to Repeated BFRL 

 Calibrations of the Gardon Gauge Used in the First Round Robin ......................16 

Table 6. Calibration Results from Two Measurement Laboratories and 

 the Gauge Manufacturer for the Schmidt-Boelter Gauge Used in 

 the First Round Robin ...............................................................................................16 

Table 7. Calibration Results from Two Measurement Laboratories and the 

 Gauge Manufacturer for the Gardon Gauge Used in the First 

 Round Robin...............................................................................................................16 

Table 8. Results of Linear Least Squares Curve Fits to Calibrations for 

 the Schmidt-Boelter Gauge Used in the Second Round Robin ..............................18 

Table 9. Results of Linear Least Squares Curve Fits to Repeated BFRL 

 Calibrations of the Second Schmidt-Boelter Gauge................................................19 

Table 10. Results of Linear Least Squares Curve Fits to Calibrations for 

 the Gardon Gauge Used in the Second Round Robin ..........................................20 

Table 11. Results of Linear Least Squares Curve Fits to Repeated BFRL 

 Calibrations of the Second Gardon Gauge............................................................20 

Table 12. Manufacturer Calibrations Results for the Schmidt-Boelter and 

 Gardon Gauges Used during the Second Round Robin.......................................21 
vi

LIST OF FIGURES 

page

Figure 1. A schematic of the BRE/FRS apparatus for heat flux gauge 

 calibration is shown. It consists of a gas-fired radiant panel as 

 the heat source and specially designed equipment for holding and 

 positioning either the calibrated reference gauge or the gauge to 

 be tested.....................................................................................................................3 

Figure 2. A schematic for the FM Global calibration apparatus is shown.........................4 

Figure 3. View of the BFRL heat flux gauge calibration facility. From right 

 to left the major components are the housing for the high pressure 

 2000 W tungsten-halogen filament lamp, flux redistributor, and 

 gauge holder. ............................................................................................................5 

Figure 4. A picture from below and a schematic are shown of the spherical 

 furnace used for heat flux gauge calibration by SINTEF. ...................................6 

Figure 5. A cross-sectional view of the calibration furnace (NT FIRE 050) 

 used at SP for heat flux gauge calibration is shown. ............................................8 

Figure 6. A cross-sectional view of the LNE vacuum black body cavity 

 (VBBC) used for heat flux gauge calibration is shown.........................................10 

Figure 7. A schematic layout of the PL 25 mm Variable-Temperature 

 Blackbody is shown..................................................................................................11 

Figure 8. Calibration results from the five participating fire laboratories 

 for the Schmidt-Boelter total heat flux gauge used during the first 

 round robin are plotted as heat flux versus gauge reading..................................13 

Figure 9. Results of repeated calibrations by BFRL of the two gauges used 

 during the first round of the round robin are plotted as applied heat 

 flux versus gauge reading. Symbols correspond to: ● repeated 

 calibrations at the start of the round robin, ■ results following 

 calibrations by the fire laboratories, and ▲ results following 

 calibrations by the measurement laboratories......................................................14 

Figure 10. Calibration results from the five participating laboratories for the 

 Gardon total heat flux gauge used during the first round robin are 

 plotted as heat flux versus gauge reading............................................................15 

Figure 11. Calibration results by the gauge manufacturer, LNE, and PL for 

 the Schmidt-Boelter total heat flux gauge used during the first round 

 robin are plotted as heat flux versus gauge reading. The results for 

 the five fire laboratories shown in Fig. 8 are included as dotted lines..............17 

Figure 12. Calibration results by the gauge manufacturer, LNE, and PL for the 

 Gardon total heat flux gauge used during the first round robin are 

 plotted as heat flux versus gauge reading. The results for the five fire 

 laboratories shown in Figure 10 are included as dotted lines. ..........................17 

Figure 13. Calibration results from the five participating laboratories for the 

 Schmidt-Boelter total heat flux gauge used during the second round 

 robin are plotted as heat flux versus gauge reading. The result for 

 the gauge manufacturer is included for reference..............................................18 
vii

Figure 14. Results of repeated calibrations by BFRL of the two gauges used 

 during the second round robin are plotted as applied heat flux 

 versus gauge reading. Symbols correspond to: ● repeated 

 calibrations at the start of the round robin and ■ calibration 

 following calibrations by the fire laboratories....................................................19 

Figure 15. Calibration results from the five participating laboratories for the 

 Gardon total heat flux gauge used during the second round robin 

 are plotted as heat flux versus gauge reading. The result for the 

 gauge manufacturer is included for reference....................................................20 

Figure 16. Calibration results for the five fire laboratories and two 

 measurement laboratories are compared for the two gauges used 

 during the first round robin by plotting the predicted full scale 

 responses (i.e., the heat flux corresponding to a 10 mV gauge output) 

 versus the manufacturer’s results (line). Uncertainties based on 

 experimental results and determined as described in the text are 

 indicated for the results from the fire laboratories. ...........................................21 

Figure 17. Calibration results for the five fire laboratories are compared for 

 the two gauges used during the second round robin by plotting the 

 predicted full scale responses (i.e., the heat flux corresponding to a 

 10 mV gauge output) versus the manufacturer’s results (line). 

 Uncertainties based on experimental results and determined as 

 described in the text are indicated. ......................................................................23 

Figure 18. The angular response recorded by FM Global for the Schmidt- 

 Boelter gauge used during the first round robin is shown in the 

 top graph as normalized gauge response versus radiation angle. 

 The data are replotted in the lower graph after dividing the 

 normalized gauge response by the cosine of the incident angle (θ). 

 The solid line is the result of a fit of the experimental data to the 

 indicated form with ε = 0.018. ..............................................................................24 

Figure 19. The angular response recorded by FM Global for the Gardon 

 gauge used during the first round robin is shown in the top graph as 

normalized gauge response versus incidence angle. The data are 

 replotted in the lower graph after dividing the normalized gauge 

 response by the cosine of the incident angle (θ). The solid line is the 

 result of a fit of the experimental data to the indicated form with 

ε = 0.025. .................................................................................................................25 

Figure 20. Residuals derived from linear least squares curve fits to 

 calibration results recorded by SP are plotted as a function of the 

 gauge reading in millivolts for the A) first round Schmidt-Boelter 

 gauge, B) first round Gardon gauge, C) second round Schmidt- 

 Boelter gauge, and D) second round Gardon gauge...........................................27 
1

1. INTRODUCTION 

Many types of experimental measurements are important for fire science and engineering. In 

recent years there has been an increased emphasis on improving the quality of these 

measurements and the need to quantify their uncertainties. In 2000 the Building and Fire 

Research Laboratory (BFRL) at the National Institute of Standards and Technology (NIST) 

hosted a workshop on this topic entitled “Measurement Needs for Fire Safety: An International 

Workshop” at the request of the FORUM for International Cooperation in Fire Research. [1] 

One of the recommendations generated by the workshop participants was that a collaborative 

effort among the member laboratories should focus on characterizing the uncertainties associated 

with heat flux measurement in fire environments. 

As a result of this meeting the FORUM Heat Flux Measurement Working Group was formed in 

July, 2000. During initial discussions it was discovered that a variety of different approaches 

were being employed in the laboratories to calibrate heat flux gauges. Schmidt-Boelter and 

Gardon gauges were typically used for total heat flux measurements. To the knowledge of the 

participants, the various calibration approaches had not been directly compared previously, and it 

was decided to conduct a calibration round robin in order to assess the consistency of the 

different approaches. Ultimately two independent round robins using Schmidt-Boelter and 

Gardon gauges were completed with five fire laboratories participating. Additionally, the four 

gauges were calibrated by the manufacturer, and the two gauges used during the first round robin 

were calibrated by two additional calibration laboratories, LNE in France and the Physics 

Laboratory (PL) at NIST. 

The need for improved calibration facilities and procedures has been generally recognized in a 

variety of engineering fields requiring accurate heat flux measurement. A workshop held at 

NIST in 1999 summarized the needs of several industries, along with those of fire science. [2] 

The workshop proceedings include a summary of an effort within the International Standards 

Organization aimed at standardizing heat flux gauge calibrations for use in fire science and 

engineering studies. Much of the work supporting the development of this new standard has 

been performed as part of a European cooperative project known as HFCAL. Even though some 

laboratories have participated in both the HFCAL study and the current round robin, the 

investigations have been performed independently. 

2. ROUND ROBIN DESIGN

The fire laboratories participating in the round robin were BRE/FRS, FM Global, BFRL, 

SINTEF, and SP. The heat flux gauge calibration facilities utilized by the participating 

laboratories differ substantially from one another. An approach was adopted in which each fire 

laboratory carried out calibrations using their existing method following standard internal 

procedures. 

The pattern (nomenclature utilized here is taken from the Guide for Interlaboratory 

Comparisons [3]) was the “basic circular” in which these gauges were first calibrated by a 

“pivot” laboratory, in this case BFRL, and then sent sequentially to the other participating 
2

laboratories. At the completion of the circle the gauges were returned to BFRL for recalibration 

to check that travel and handling had not measurably altered the responses of the two gauges. 

Two commercial heat flux gauges from Medtherm Corporation1

, a 2.54 cm Gardon gauge 

(Model 64-10-18ST) and a 1.27 cm Schmidt-Boelter gauge (Model 32-10SB-18ST), procured by 

BFRL were calibrated by the five laboratories during each of two round robins. Since new 

gauges were employed for each round robin, a total of four gauges were tested. The nominal 

full-scale ranges for the gauges were 100 kW/m2

. Calibrations for each gauge were made by the 

manufacturer prior to delivery, but were not disclosed to BFRL until the completion of a round 

robin. The gauges were shipped between the laboratories in a foam-rubber-lined case. 

 

At the conclusion of the first round robin the two gauges were sent to the two calibration 

laboratories, LNE and PL, for additional calibrations. Following each of these calibrations 

BFRL again checked the gauges for calibration changes. 

Three of the laboratories, LNE, SP, and PL, that calibrated the first set of gauges used 

approaches that are mentioned as primary calibration methods in part 2 of the proposed ISO 

standard that is currently under development. [4] One of the fire laboratories, BRE/FRS, used a 

method mentioned as a secondary calibration method in part 3 of the ISO standard under 

development. [5] The approaches used for the three remaining fire laboratories are not 

mentioned in the ISO documents. 

3. CALIBRATION FACILITIES 

3.1 Fire Laboratories 

3.1.1 BRE/FRS 

The calibration of heat flux gauges for use as working standards is carried out by comparing their 

response at various levels of irradiance with the response of a secondary standard gauge at the 

same levels of irradiance. The measurements are made at multiple heat flux levels by varying 

the distance between the radiant source and the gauges. 

The radiant source is a 0.3 m by 0.3 m porous refractory burner operating on pre-mixed natural 

gas with air. It is mounted vertically and is operated in the temperature range of 800 °C to 

1000 °C. 

Both the secondary standard heat flux gauge and the instrument to be calibrated are mounted 

side-by-side on a sliding frame and moved into the measuring position, opposite the center of the 

radiant source, in turn. The output from the measuring gauge is recorded until it has stabilized. 

The average value of the output is then determined over a one minute period. The two gauges 

are moved into the measuring position alternately until two corresponding averages for the 

secondary standard gauge and the instrument to be calibrated do not differ by more than 1%. 

 

1

 Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the 

experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of 

Standards and Technology, nor does it imply the materials or equipment are necessarily the best available for the purpose. 
3

The incident irradiance on the gauges is increased by moving them closer to the radiant source. 

The upper limit of irradiance within this facility is 55 kW/m2. Beyond this limit, the gauges are 

susceptible to significant convective effects. A schematic diagram of the apparatus is shown in 

Fig. 1. Maximum heat flux values used in the current calibrations were close to the maximum 

available, ranging from 52 kW/m2

 to 56 kW/m2. 

Figure 1. A schematic of the BRE/FRS apparatus for heat flux gauge calibration is shown. 

It consists of a gas-fired radiant panel as the heat source and specially designed 

equipment for holding and positioning either the calibrated reference gauge or 

the gauge to be tested. 

The calibration apparatus is located within a small room to ensure that the environment is 

draught free. 

The secondary radiometer was calibrated by LNE. 

3.1.2 FM Global 

For the past thirty years, FM Global has calibrated its gauges by placing them at several different 

distances in front of a hot furnace orifice that emits a known level of radiance. It is a primary 

method of calibration based on first principles (e.g. the Stefan-Boltzmann constant) rather than 

being based on a transfer calibration from some other device. 

Figure 2 illustrates the method of calibration. The facility includes a furnace (F) with a conical 

blackbody target (T) emitting a known quantity of blackbody radiant heat flux. The furnace is 

designed to maximize the effective emissivity of the target. Radiation exits the furnace through a 
4

Figure 2. A schematic for the FM Global calibration apparatus is shown. 

water-cooled gold-plated orifice (IS) followed by a much larger water-cooled outer shield (OS). 

The target temperature is measured by the optical pyrometer (OP). Measurement precision is 

greatly enhanced through the use of a computer-controlled water-cooled shutter (S) and black 

shielding surrounding the furnace orifice. The shutter periodically blocks radiation from the 

orifice to avoid the effect of stray radiation not coming from the orifice. The gauge (G), being 

calibrated, is mounted on a rotational stage (R) and optical bench (OB). The carriage is precisely 

positioned at various distances from the orifice using Invar rods. The gauge, shutter, and 

shielding are all water cooled at the temperature of the ambient air to minimize effects of stray 

radiation, convective heat transport and air drafts. Calibrations are generally reproducible to 

± 0.5 % and accurate to ± 1 %. 

This facility measures the gauge sensitivity to both normally incident flux and to flux incident at 

oblique angles. This is a significant advantage. Gauges often have unexpected angular 

dependencies that one must take into consideration. A drawback of calibrating gauges outside 

the furnace is that heat fluxes incident on the gauge are two orders of magnitude less than the 

100 kW/m2

 fluxes that can be measured in fires. One must rely on the gauges having a linear 

response. Maximum applied heat fluxes for the gauges used in the current round robins were 

close to or just less than 1.0 kW/m2

. 

3.1.3 BFRL 

The BFRL heat flux gauge calibration system was assembled in the mid 1970s. It has been 

described in a recent NIST Report of Test. [6]. It is based on comparing the response of the 

gauge to be calibrated with that for a calibrated secondary standard gauge for a range of applied 

heat fluxes. 

F Furnace

T Target – silicon carbide (Hexoloy) 

S Shutter – water-cooled and computer controlled

OS Outer water-cooled shield

I S Inner water-cooled shield with gold-plated orifice plate

T

G Gage – water cooled 

R Rotational stage

O B Optical bench

O P Optical Pyrometer

O B 

F 

O S 

S G O P 

R

T 

I S 
5

The heat source consists of a commercial radiant heater incorporating a 2000 W tungstenhalogen filament lamp. The lamp is placed at one of the foci of a large ellipsoidal reflector, with 

the entrance to a kaleidoscope flux redistributor being located at the other. The distance between 

the foci is 30.5 cm. A metal housing surrounds the half of the ellipsoidal mirror containing the 

lamp. The mirror passes through the wall of the housing and extends 10 cm before being cut off 

short of the second focus of the ellipse. The lamp housing with extended mirror can be seen in 

the photograph shown in Fig. 3. 

Figure 3. View of the BFRL heat flux gauge calibration facility. From right to left the 

major components are the housing for the high pressure 2000 W tungstenhalogen filament lamp, flux redistributor, and gauge holder. 

A blackened radiation shield with a 6.4 cm square cut out centered on the mirror axis is 

positioned 2.5 cm from the lamp. A 7.6 cm long box, open at both ends, is attached over the 

radiation shield cut out on the opposite side of the lamp. (see Fig. 3) The interior of the box 

provides internal reflections of light that redistribute the energy flux by forming and 

superimposing multiple images such that the flux distribution becomes more uniform at the exit, 

similar to the multiple images formed by a kaleidoscope. The gauge to be characterized is 

positioned at a reproducible location on the far side of the kaleidoscope flux redistributor using a 

specially designed mount. 

Heat flux calibrations are performed using a secondary-standard heat flux gauge that was 

calibrated by the Radiometric Physics Division of the National Bureau of Standards (now NIST). 

An arbitrary heat flux level is set by adjusting the lamp power supply voltage and current, and 

the lamp is allowed to stabilize (20 min). After stabilization the output (mV) of the secondarystandard heat flux gauge is recorded. The secondary standard is then replaced with the gauge to 

be tested, and the output is again recorded. Once a calibration cycle is complete, the lamp output 

is adjusted to the next flux level, and the measurement procedure is repeated. Generally, four 
6

flux levels over a 0 kw/m2

 to 15 kW/m2 range are chosen. For the current round robins, 

maximum applied heat fluxes for a given calibration ranged from 15.8 kW/m2

 to 16.6 kW/m2. 

Following the completion of a calibration, the results are plotted as incident heat flux (kW/m2) 

versus the corresponding gauge output readings in mV. The experimental data are fit using a 

linear least squares procedure, and the gauge sensitivity is reported in terms of (kW/m2)/mV. 

3.1.4 SINTEF 

Calibrations of heat flux gauges are performed at SINTEF using a MIKRON300 spherical 

furnace (Fig. 4) with a small opening at the bottom. The furnace is considered to be a primary 

black body radiation standard. The radiative flux impinging on the sphere’s inner opening is 

obtained from the interior furnace surface temperature and the Stefan-Boltzman equation. 

Temperature variations along the cavity surface are estimated to be less than 1 °C. The radiative 

flux is considered uniform for the full 180° view available at the inside of the opening. 

Radiative heat flux levels can be varied from 0 kW/m2 to 200 kW/m2 by adjusting the 

temperature up to 1100 °C. Maximum heat fluxes applied for the current calibrations were 98 

kW/m2 and 59 kW/m2 for the first- and second-round gauges, respectively. 

 

530 mm

165 mm

153 mm

MIKRON M300 

SPHERICAL FURNACE

49 mm

200 mm

Reference 

thermocouple

Heat flux meter

 

Figure 4. A picture from below and a schematic are shown of the spherical furnace used 

for heat flux gauge calibration by SINTEF.

The interior surface temperature of the furnace is recorded using a calibrated thermocouple with 

an output traceable to ITS-90 [7]. Typical uncertainties are ± 2 °C. Various types of uncertainty 

are considered for the temperature measurement including thermocouple aging, various 

measurement uncertainties, effects of possible cold air draughts on the thermocouple, and view 
7

effects associated with the spherical cavity opening. The uncertainty in temperature 

measurement is estimated to result in an uncertainty of less than 1.7 % in the opening radiative 

heat flux over the range 50 kW/m2 to 100 kW/m2. 

Differences in temperature between the heated air inside the furnace and the room-temperature 

air outside can lead to the formation of air currents in the opening. As a result of these air flows, 

there is the potential for convective heat transfer to the gauge. Since Schmidt-Boelter and 

Gardon gauges are total heat flux gauges, they respond to convective heat transfer at their 

measuring surfaces. Additional uncertainties in the calibration are introduced due to convection. 

The convective contribution is dependent on the type of heat flux gauge, furnace temperature and 

the properties of the gas inside the furnace. Since the radiative heat flux increases faster than 

convection with temperature, the relative convective contribution decreases at higher flux levels. 

This means that calibrations at high heat flux levels have less relative uncertainty than at low 

levels. 

If a reference total heat flux gauge with a well characterized response (i.e., low uncertainty) is 

available, the convective heat transfer component can be obtained by inserting the gauge flush 

with the furnace inner opening and measuring its response for each furnace temperature and 

using its known sensitivity to determine the total heat flux, Etotal . The convective heat flux 

component, Econvective, is calculated as Econvective = Etotal – Eradiative, where Etotal is the measured heat 

flux and Eradiative is determined from the known black body temperature. The response of the 

heat flux gauge to be calibrated is also recorded for the same conditions, and its output is 

compared to the reference. Provided that the two heat flux meters have similar convective 

characteristics, the convective contribution to the heat flux can be determined for the gauge 

being calibrated. The uncertainty in the calibration of the reference heat flux gauge is quantified 

and added to the total uncertainty. 

If a reference flux meter with sufficiently low uncertainty is not available, it is necessary to 

estimate/calculate the convective component and add this to the radiative heat flux in order to 

calculate Etotal for each furnace setting. The response of the heat flux gauge to be calibrated is 

then measured at each level, and the appropriate amount of convective heat transfer is added to 

the known radiative component. The uncertainty of the convective contribution has to be 

quantified and added to the total uncertainty. 

The temperature of cooling water representing the temperature of the heat flux gauge body is 

also measured in order to allow the net heat flux to the gauge to be determined. 

3.1.5 SP 

The calibrations were performed according to NT FIRE 050 [8, 9, 10], which is a primary 

calibration method. The method has been mainly used for calibration of total heat flux gauges of 

the Gardon and Schmidt-Boelter types. The system enables calibration from 2 kW/m2

 to 

85 kW/m2. For the current calibrations maximum applied heat fluxes were 85 kW/m2 and 

80 kW/m2 for the Schmidt-Boelter and Gardon gauges used in the first round, respectively, and 

76 kW/m2

, and 69 kW/m2 for the gauges used in the second round. Heat flux gauges with a 
8

housing diameter of up to 50 mm and a sensitive area diameter of up to 10 mm can be 

accommodated. 

The method consists of a blackbody radiation source designed as a well insulated, electrically 

heated spherical furnace chamber with an aperture at the bottom as shown in Fig. 5. The 

temperature of the furnace is accurately controlled and is quite uniform on the inside cavity 

surface, assuring a high precision for the radiation level. The method has an uncertainty equal to 

or less than ± 3.0 % with a 95 % confidence interval. 

Key 

1 Electric heater 5 Thermocouple 

2 Ceramic clay 6 Fixed part of cooler 

3 Low density ceramic insulation 7 Movable part of cooler 

4 High density ceramic insulation 8 Heat flux gauge 

Figure 5. A cross-sectional view of the calibration furnace (NT FIRE 050) used at SP for 

heat flux gauge calibration is shown.

A cooling device housing the heat flux gauge is inserted in the opening at the bottom of the 

furnace. The inside diameter of the furnace is larger than 4.5 times the restricting aperture of the 

fixed cooler at the bottom of the furnace. The aperture in the cooling device defines the view 

factor between the furnace radiating surface and the heat flux gauge. 

Heat flux gauges to be calibrated are inserted through the aperture with the sensing surface 

oriented horizontally. The cooler has the same temperature as the heat flux gauge. The 

influence of convection is thus highly reduced. The cooler insert has a number of shields, which 

protect the gauge from receiving radiation reflected from the walls of the cooler. The shields 

help to maintain the stratification of air so that convective airflow is minimized. The heat flux 

gauge views only the controlled environment of the blackbody emitter and the much cooler 

2

3

1

4

6

7 8

5
9

surfaces of the cooler insert. The radiation level of this blackbody emitter depends solely on the 

measured temperature, making it traceable to international temperature calibration standards. 

The calibration is performed at 10 flux levels, evenly distributed over a temperature range from 

400 °C to a temperature corresponding to the maximum flux level of the gauge. The size of the 

sensing element for the gauge is required to calculate the view factor for the gauge. The 

manufacturer supplies this information. The distance between the top of the movable cooler and 

the receiving sensor of the heat flux gauge is also measured for this purpose. When the 

temperature of the black body has stabilized to within ±1 °C/min of the set level, records of the 

water and the furnace temperature together with the output signal from the gauge are taken over 

a 1 minute period (approximately 120 records). [10] 

3.1.6 Summary of Fire Laboratory Calibration Facilities 

Table 1 summarizes major characteristics for the calibration systems used by the participating 

fire laboratories. The type of heat source is indicated along with a general description of the 

approach used for estimating the heat fluxes impinging on the gauge to be calibrated and whether 

the gauge view of the heat source is limited to restrained angles near the gauge normal (narrow 

angle) or fills the entire field-of-view of the gauge (hemispherical). The last column lists 

nominal values of maximum heat flux that the various systems can impose on a heat flux gauge. 

Table 1. Radiant Source, Method for Characterizing Heat Flux Level at Gauge, Source 

View Angle and Maximum Heat Flux at the Gauge for Fire Laboratory 

Calibration Methods 

Fire 

Laboratory Source Characterization Angle Maximum Applied HF 

BRE/FRS Porous Burner Secondary 

Standard Narrow Angle 55 kW/m2

FM Global 

Target in 

Cylindrical 

Black Body 

Optical 

Pyrometry Narrow Angle 1 kW/m2

BFRL 

Tungsten Lamp 

in Ellipsoidal 

Furnace 

Secondary 

Standard Narrow Angle 15 kW/m2

SINTEF Spherical Black 

Body 

Calibrated 

Thermocouple Hemispherical 200 kW/m2

SP Spherical Black 

Body 

Calibrated 

Thermocouple Narrow Angle 85 kW/m2
10

3.2 Measurement Laboratory Calibration Facilities 

3.2.1. LNE 

The LNE facility utilizes a traceable black body source, operated under vacuum in order to 

minimize convection, to irradiate the gauge to be calibrated. The Vacuum Black Body Cavity 

(VBBC) can reach irradiance levels up to 100 kW/m2

. However, due to the limited resistance of 

heat flux gauges in hot and low-pressure environments, the approach enables heat flux gauge 

calibration up to about 60 kW/m2

. The method consists of measuring the output voltage of the 

heat flux gauge for known flux levels. The absolute pressure in the blackbody cavity is 

maintained below 5 Pa. In that case, the convective heat transfer is consequently reduced and 

accounted for with lower uncertainty. Radiation levels and residual convection are determined 

through modelling. 

The apparatus (Fig. 6) consists of two essential parts: the gun, which is a cylindrical tube fixed 

on a carriage including the black body cavity, the heat flux gauge and its cooling pipes and the 

furnace, containing a set of heating resistors. Four regulators, whose thermocouples are 

localized close to each heater, control the heating of the cavity. The temperature range of the 

black body cavity is 23 °C to 900 °C. 

Thermal insulation 

Blackbody cavity 

=FURNACE

Water-cooling pipes 

(holder + fluxmeter)

Output voltage Pumping Cooled holder 

=GUN

Fluxmeter

Thermocouples

To cavity 

control 

Heater 

Diaphragm 

690 mm 

480 mm 

180 mm

Figure 6. A cross-sectional view of the LNE vacuum black body cavity (VBBC) used for 

heat flux gauge calibration is shown.

Pressure measurement within the cavity is performed by means of a Pirani gauge. A set of 

thermocouples is mounted partly along and partly in the rear section of the black body cavity, 

allowing a determination of the temperature field. 
11

The calibration procedure consists of the following steps: 1) stabilizing the cavity at the 

requested temperature, 2) placing the heat flux gauge into the cavity so that its sensitive aperture 

is in the correct plane, 3) after 1 hour, pumping the cavity down to 1 Pa, and 4) recording the 

heat flux gauge voltage for 300 s (30 values). The recorded quantities for each heat flux level 

are the computed total flux and the heat flux gauge output voltage, both with their standard 

deviations. The VBBC records also include the actual pressure, the residual relative convective 

flux and the overall uncertainty. 

3.2.2. PL 

The PL technique uses the principle of electrical substitution radiometry to calibrate heat flux 

gauges up to 50 kW/m2

. The reference standard is a room-temperature electrical substitution 

radiometer whose calibration is traceable to the High Accuracy Cryogenic Radiometer (HACR), 

the United States primary standard for optical radiation power, through a chain of calibrations. 

The calibration uses the 25 mm diameter dual cavity variable temperature blackbody (VTBB) as 

a broadband radiant source. Figure 7 shows a schematic layout of the VTBB apparatus and the 

calibration scheme. The direct resistance heating of the graphite tube element using large AC 

currents at low voltages allows quick heating and cooling of the cavity. An optical pyrometer 

measures the VTBB temperature by sensing radiation from one end of the cavity. A PID 

controller regulates the power supply to maintain the cavity temperature to within ± 0.1 K of the 

set value. The maximum operating temperature is 2973 K. 

Figure 7. A schematic layout of the PL 25 mm Variable-Temperature Blackbody is shown.

The reference standard radiometer is water cooled and is suitable for continuous operation. The 

test sensor and the reference radiometer are located outside the blackbody exit in a test plane at a 

fixed distance from the blackbody aperture. The variation in the incident heat flux level at the 

sensor location is obtained by varying the VTBB temperature. The distance between the 

blackbody exit and the test plane is about 12.5 mm for calibrations up to 50 kW/m2

. For 
12

calibrations in the lower ranges of 25 kW/m2

 and 10 kW/m2, the corresponding distances are 

about 62 mm and 140 mm, respectively. 

After stabilization of the temperature, the blackbody unit is positioned in front of the radiometer 

and the test sensor, sequentially. The output signals are recorded for the test duration in 

approximately 0.4 s intervals. The sensor responsivity is calculated by a linear regression fit to 

the sensor signal (mV) data for different values of incident flux (kW/m2

) measured by the 

radiometer. The measured responsivity of the sensor is expressed in mV/(kW/m2). The 

regression curve fit for the data is generally linear with regression factors close to unity. The 

relative expanded total uncertainty (U) expressed for a coverage factor of k =2, is about 2 %. 

3.3 Heat Flux Gauge Manufacturer 

Medtherm Incorporated normally supplies a calibration curve for each total heat flux gauge they 

deliver. For this study the calibrations were carried out prior to shipping of the gauges, but the 

results were not provided to NIST until a given round robin was completed. 

4. RESULTS 

4.1 First Round Robin 

Each fire laboratory reported the results for the calibrations of the two heat flux gauges in terms 

of the coefficients determined from linear least squares curve fits of the data when plotted as 

kW/m2 versus the response of the gauge in mV. In some cases the y-intercept was forced to pass 

through the plot origin, while in others it was allowed to “float,” and a y-intercept value from the 

fit is reported. Normal laboratory protocols were followed. 

There are many different approaches that might be used to compare the results from different 

laboratories. Two are used here. The first is to graphically compare the calculated lines for heat 

flux versus gauge reading. Since the y-intercepts are close to zero for those fits that were 

allowed to float, this is nearly the same as comparing the slopes for the lines. The second 

method was to compare the predicted heat flux readings corresponding to the nominal full-scale 

voltage outputs (10 mV) for the gauges. 

Table 2 lists first round-robin results for the Schmidt-Boelter gauge as intercepts (calculated 

value or zero if forced through origin) and slopes of linear least squares curve fits of imposed 

heat flux versus gauge reading in mV. Indicated uncertainties are values determined from linear 

regression analysis of fits to the experimental data and represent two standard deviations (i.e., 

coverage factor of k = 2). Note that these experimentally derived uncertainties should not be 

interpreted as total uncertainties for the calibrations. Figure 8 shows the results for the fire 

laboratories included in Table 2. There is some scatter in the data from the different laboratories, 

but in general the curves fall close together. 

In order to assess the repeatability of the BFRL calibrations, the two gauges were initially 

calibrated three times on different days. After calibration by the other four fire laboratories, the 

gauges were returned to BFRL and were recalibrated. The three initial calibrations along with 
13

Table 2. Results of Linear Least Squares Curve Fits to Calibrations for the SchmidtBoelter Gauge Used in the First Round Robin 

 y-intercept Slope 

Fire Laboratory Value Uncertainty Value Uncertainty 

SP 0.375 0.009 9.055 0.002 

BRE/FRS -0.40 0.20 8.51 0.05 

FM Global 0.0 -- 8.99 0.06 

SINTEF 0.0 0.04 9.42 0.11 

BFRL -0.03 0.04 9.24 0.04 

Figure 8. Calibration results from the five participating fire laboratories for the SchmidtBoelter total heat flux gauge used during the first round robin are plotted as 

heat flux versus gauge reading.

post-round robin results for the Schmidt-Boelter gauge are shown in Fig. 9. Results for a 

calibration made following the return of the gauge from LNE and PL are also included in the 

figure and will be discussed below. All of the calibration curves fall close together for the 

Schmidt-Boelter gauge (Table 3). The small uncertainties based on the experimental data for the 

intercepts and slopes reflect the high quality of the linear fits. Correlation coefficients, R, for the 

fits were higher than 0.9999. Even though small, differences between the various calibrations 

are somewhat larger than can be explained based simply on the statistics of curve fitting. The 

scatter in the repeated calibrations is much smaller than the differences between laboratories 

evident in Fig. 8 and summarized in Table 2. This indicates that the variations observed between 

calibrations at the different laboratory are not due to changes in the gauge during the round 

robin. 

The Gardon gauge used in the first round robin was calibrated in the same way as described 

above for the Schmidt-Boelter gauge. Figure 10 compares the results from the participating fire 

Gauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

SP

BRE/FRS

FM Global

SINTEF

BFRL
14

Figure 9. Results of repeated calibrations by BFRL of the two gauges used during the first 

round of the round robin are plotted as applied heat flux versus gauge reading. 

Symbols correspond to: ● repeated calibrations at the start of the round robin, 

■ results following calibrations by the fire laboratories, and ▲ results 

following calibrations by the measurement laboratories. 

Table 3. Results of Linear Least Squares Curve Fits to Repeated BFRL Calibrations of the 

Schmidt-Boelter Gauge Used in the First Round Robin 

laboratories. Table 4 lists the y-intercepts (if reported) and slopes of the calibrations along with 

uncertainties determined from the fits. 

For a given heat flux, the Gardon gauge generates roughly 25 % less output voltage than the 

Schmidt-Boelter gauge, even though both gauges have the same nominal full-scale ranges. The 

calibration curves for both gauges have very similar behaviors. For both gauges the calibration 

lines determined by the various laboratories fall relatively close together. However, the 

differences between laboratories are apparently not random. The relative ordering of the lines is 

nearly the same for both gauges, with SINTEF measuring lower responses and BRE/FRS higher. 

The results for the three remaining laboratories are in between these limits and lie close together. 

Gauge Reading (mV)

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Applied Heat Flux (kW/m2

)

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

Gardon

Schmidt-Boelter

 y-intercept Slope 

Calibration Value Uncertainty Value Uncertainty 

1 -0.001 0.057 9.186 0.052 

2 -0.035 0.036 9.250 0.033 

3 -0.055 0.088 9.279 0.081 

Post Round 

Robin -0.002 0.026 9.236 0.091 

Post Calibration 

Labs -0.001 0.027 9.082 0.023 
15

Figure 10. Calibration results from the five participating laboratories for the Gardon total 

heat flux gauge used during the first round robin are plotted as heat flux versus 

gauge reading. 

Table 4. Results of Linear Least Squares Curve Fits to Calibrations for the Gardon Gauge 

Used in the First Round Robin 

 y-intercept Slope 

Fire Laboratory Value Uncertainty Value Uncertainty 

SP 1.26 0.04 11.912 0.009 

BRE/FRS -0.14 0.08 11.27 0.03 

FM Global 0.0 -- 12.24 0.06 

SINTEF 0.0 0.04 12.76 0.08 

BFRL 0.05 0.08 12.28 0.10 

Repeatability measurements for the Gardon gauge performed at NIST are plotted in Fig. 9 and 

are summarized in Table 5. With one exception, correlation coefficients, R, for the fits were 

higher than 0.9999. As found for the Schmidt-Boelter gauge, the measurements made at the start 

and end of the round robin agree well. Variations were considerably smaller than those found 

for calibrations performed at the different laboratories, confirming that differences between the 

various laboratories are not due to changes in the response of the gauge. 

4.2 Calibrations of First Round Gauges by the Manufacturer, LNE, and PL 

Following the completion of the first round robin, both gauges were sent to LNE and PL for 

independent calibration. During testing at LNE water flow to the Gardon gauge was 

inadvertently interrupted, and the gauge overheated. As a result, the surface was damaged. It 

was subsequently cleaned and repainted. Following their return to BFRL, the Schmidt-Boelter 

gauge and the refurbished Gardon gauge were recalibrated. Results for the two gauges are 

included in Fig. 9 and are summarized in Table 3 and Table 5. The calibration for the SchmidtGauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

120

140

SP

BRE/FRS

FM Global

SINTEF

BFRL
16

Table 5. Results of Linear Least Squares Curve Fits to Repeated BFRL Calibrations of the 

Gardon Gauge Used in the First Round Robin 

Boelter gauge agrees well with earlier calibrations, while the repaired Gardon gauge appears to 

be slightly less sensitive (≈ 4 %) than before it was damaged. Calibration results for the Gardon 

gauge reported for LNE are based on measurements recorded prior to the damage, while the PL 

Gardon gauge calibration was made using the refurbished device. 

The results for the two measurement laboratories, along with the Medtherm calibration, are 

summarized in Table 6 and Table 7 for the Schmidt-Boelter and Gardon gauges, respectively. 

Uncertainty values provided by the laboratories are included and represent the total uncertainties 

with a coverage factor of 2 (95 %). The three calibrations are compared graphically in Fig. 11 

and Fig. 12 for the Schmidt-Boelter and Gardon gauges. The results for the five fire laboratories 

are shown as dotted lines for comparison. 

Table 6. Calibration Results from Two Measurement Laboratories and the Gauge 

Manufacturer for the Schmidt-Boelter Gauge Used in the First Round Robin 

Laboratory y-intercept Slope Combined Uncertainty 

LNE 0.37 8.995 < 3 % 

PL -- 8.640 2.1 % 

Medtherm -- 8.826 3 % 

Table 7. Calibration Results from Two Measurement Laboratories and the Gauge 

Manufacturer for the Gardon Gauge Used in the First Round Robin 

Laboratory y-intercept Slope Combined Uncertainty 

LNE 0.62 12.083 < 3 % 

PL -- 11.73 2.1 % 

Medtherm -- 11.481 3% 

For both gauges the calibrations by the measurement laboratories and the manufacturer fall near 

the mid-ranges of those recorded by the fire laboratories and are also seen to fall relatively close 

together. However, as observed for the fire laboratory calibrations, there does seem to be a 

pattern in the results. For both gauges, LNE reported that the heat flux gauges were slightly less 

sensitive than indicated by Medtherm and PL. PL measured a slightly higher sensitivity for the 

Schmidt-Boelter gauge than reported by Medtherm, while the opposite was true for the Gardon 

gauge. If the 4 % increase in the slope of the calibration line for the refurbished Gardon gauge 

 y-intercept Slope 

Calibration Value Uncertainty Value Uncertainty 

1 0.07 0.11 12.28 0.14 

2 0.04 0.16 12.35 0.19 

3 0.04 0.09 12.21 0.10 

Post Round 

Robin 0.07 0.02 12.45 0.03 

Post Calibration 

Laboratories 0.05 0.07 12.86 0.08 
17

Figure 11. Calibration results by the gauge manufacturer, LNE, and PL for the SchmidtBoelter total heat flux gauge used during the first round robin are plotted as 

heat flux versus gauge reading. The results for the five fire laboratories shown 

in Fig. 8 are included as dotted lines. 

Figure 12. Calibration results by the gauge manufacturer, LNE, and PL for the Gardon 

total heat flux gauge used during the first round robin are plotted as heat flux 

versus gauge reading. The results for the five fire laboratories shown in Figure 

10 are included as dotted lines. 

determined by BFRL is taken into account, the PL calibration curve for this gauge will fall below 

that reported by Medtherm, consistent with the Schmidt-Boelter results. This suggests that there 

Gauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

Medtherm

LNE

PL

Gauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

120

140

Medtherm

LNE

PL
18

are small systematic differences between calibrations performed by the fire laboratories, 

measurement laboratories, and manufacturer. 

4.3 Second Round Robin 

As an additional check of the results, the five fire laboratories conducted a second round robin 

utilizing new Schmidt-Boelter and Gardon gauges and the same protocol as for the first round 

robin. 

Table 8 lists results for the Schmidt-Boelter gauge for the second round robin, and Fig. 13 shows 

plots of the data. Repeatability tests made by BFRL are summarized in Table 9 for the three 

initial calibrations and for a fourth calibration following the return of the gauge to BFRL after 

calibration by the other fire laboratories. The data exhibit a high degree of linearity with 

correlation coefficients, R, of 0.9999 or better. The results are shown graphically in Fig. 14 and 

indicate the Schmidt-Boelter gauge was stable during the course of the round robin tests. 

Table 8. Results of Linear Least Squares Curve Fits to Calibrations for the SchmidtBoelter Gauge Used in the Second Round Robin 

Figure 13. Calibration results from the five participating laboratories for the SchmidtBoelter total heat flux gauge used during the second round robin are plotted as 

heat flux versus gauge reading. The result for the gauge manufacturer is 

included for reference. 

 y-intercept Slope 

Laboratory Value Uncertainty Value Uncertainty 

SP 0.21 0.15 12.54 0.04 

BRE/FRS 0.8 0.4 11.31 0.13 

FM Global 0.0 -- 12.52 0.08 

SINTEF 0.0 0.04 13.8 0.3 

BFRL -0.04 0.04 12.79 0.06 

Gauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

120

140

SP

BRE/FRS

FM Global

SINTEF

BFRL

Medtherm
19

Table 9. Results of Linear Least Squares Curve Fits to Repeated BFRL Calibrations of the 

Second Schmidt-Boelter Gauge 

Figure 14. Results of repeated calibrations by BFRL of the two gauges used during the 

second round robin are plotted as applied heat flux versus gauge reading. 

Symbols correspond to: ● repeated calibrations at the start of the round robin 

and ■ calibration following calibrations by the fire laboratories.

Table 10 and Fig. 15 provide the corresponding results for the Gardon gauge. The results of 

repeated calibrations at BFRL for this gauge are summarized in Table 11 and plotted in Fig. 14. 

The reproducibility was extremely good and the linearity is high as indicated by R values for the 

fits, which were higher than 0.9999. The close agreement of the calibrations provides additional 

support for the conclusion that the change in response for the Gardon gauge used in the first 

round robin was associated with its overheating and repainting. 

 

As for the first round robin, Medtherm provided calibrations to BFRL for the two gauges after 

the completion of the round robin. Calibration results and reported total uncertainties are listed 

in Table 12. Results for the two gauges have been included in Fig. 13 and Fig. 15 for 

comparison purposes. 

These calibrations indicate that the Gardon gauge used in the second round robin was slightly 

more sensitive than the Schmidt-Boelter gauge. As expected, calibration curves for all of the 

 y-intercept Slope 

Calibration Value Uncertainty Value Uncertainty 

1 -0.03 0.07 12.77 0.10 

2 -0.04 0.11 12.81 0.14 

3 -0.06 0.08 12.78 0.10 

Post Round 

Robin -0.05 0.07 12.90 0.09 

Gauge Reading (mV)

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Applied Heat Flux (kW/m2

)

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

Gardon

Schmidt-Boelter
20

Table 10. Results of Linear Least Squares Curve Fits to Calibrations for the Gardon 

Gauge Used in the Second Round Robin 

Figure 15. Calibration results from the five participating laboratories for the Gardon total 

heat flux gauge used during the second round robin are plotted as heat flux 

versus gauge reading. The result for the gauge manufacturer is included for 

reference. 

Table 11. Results of Linear Least Squares Curve Fits to Repeated BFRL Calibrations of 

the Second Gardon Gauge 

 y-intercept Slope 

Calibration Value Uncertainty Value Uncertainty 

1 -0.02 0.03 12.29 0.04 

2 0.01 0.05 12.24 0.06 

3 0.01 0.07 12.26 0.08 

Post Round 

Robin 0.01 0.02 12.26 0.03 

gauges used in both round robins have similar behaviors. As observed during the first round 

robin, results from SINTEF lie somewhat above those for the other fire laboratories when plotted 

as applied heat flux versus gauge reading, three of the laboratories report similar intermediate 

values, and the BRE/FRS curves are the lowest. For both gauges the Medtherm calibrations fall 

 y-intercept Slope 

Laboratory Value Uncertainty Value Uncertainty 

SP 0.9 0.2 11.68 0.06 

BRE/FRS 1.11 0.51 10.96 0.18 

FM Global 0.0 -- 12.22 0.07 

SINTEF 0.00 0.04 13.1 0.4 

BFRL 0.00 0.03 12.26 0.03 

Gauge Reading (mV)

0 2 4 6 8 10

Heat Flux (kW/m2) from Calibration

0

20

40

60

80

100

120

140

SP

BRE/FRS

FM Global

SINTEF

BFRL

Medtherm
21

Table 12. Manufacturer Calibrations Results for the Schmidt-Boelter and Gardon Gauges 

Used during the Second Round Robin 

Gauge Slope Combined Uncertainty 

Schmidt-Boelter 12.285 3 % 

Gardon 11.455 3 % 

above the BRE/FRS result, but lie below the curves reported by SP, FM Global, BFRL, and 

SINTEF. 

4.4 Comparisons Based on Predicted Full Scale Gauge Responses 

Predicted heat flux values corresponding to full-scale responses are used to provide quantitative 

assessment of the agreement of calibrations from the different laboratories. Figure 16 shows the 

predicted heat fluxes corresponding to a full-scale voltage output, 10 mV, for the gauges used in 

the first round robin. Predictions based on calibrations for the fire and measurement laboratories 

are plotted as functions of the manufacturer’s calibration. Error bars represent 95 % confidence 

intervals for the predicted values based on linear regression analysis of the calibration 

measurements. 

Figure 16. Calibration results for the five fire laboratories and two measurement 

laboratories are compared for the two gauges used during the first round robin 

by plotting the predicted full scale responses (i.e., the heat flux corresponding 

to a 10 mV gauge output) versus the manufacturer’s results (line). 

Uncertainties based on experimental results and determined as described in the 

text are indicated for the results from the fire laboratories. 

The averages and scatter (2σ) for the results from the five fire laboratories are 90.4 kW/m2

 ± 7.2 

kW/m2

 and 121.2 kW/m2 ± 11.0 kW/m2 for the Schmidt-Boelter and Gardon gauges, 

respectively. The variations correspond to about ± 8.0 % and ± 9.1 % of the two averaged 

values, respectively. The averages can be compared to the corresponding results based on the 

Manufacturer's Calibration (kW/m2)

85 90 95 100 105 110 115 120

Laboratory Calibration (kW/m2

)

80

90

100

110

120

130

140

SP

BRE

FMG

SINTEF

BFRL

LNE

PL

Col 1 vs Col 1 

Plot 2 Regr

Schmidt-Boelter

Gardon
22

manufacturer’s calibrations of 88.3 kW/m2

 and 114.8 kW/m2. The averaged values from the fire 

laboratory calibrations are 2.4 % and 5.5 % higher than those based on the manufacturer’s 

calibration for the Schmidt-Boelter and Gardon gauges, respectively. 

The LNE and PL predicted full-range values are 90.3 kW/m2

 and 86.3 kW/m2 for the SchmidtBoelter gauge, differing from the fire laboratory averages by - 0.1 % and - 4.5 %, respectively. 

For the Gardon gauge the LNE and PL results are 121.4 kW/m2 and 117.6 kW/m2, which differ 

from the fire laboratory averages by + 0.1 % and - 3.1 %. It should be kept in mind that the 

response of the Gardon gauge was changed slightly between the times when LNE and PL made 

their calibrations. 

Due the limited number of measurements and the damage to the Gardon gauge, it is difficult to 

compare the results from LNE and PL. Even so, it appears that there are small systematic 

differences between the calibration results for the two laboratories. These differences just fall 

within the combined 2σ uncertainties reported by these laboratories. 

The close agreement between calibrations recorded by the fire laboratories and the two 

measurement laboratories shows that the fire laboratories’ calibrations agree well with each other 

and provide results that fall within a few percent of the actual heat flux. 

Figure 17 compares predicted full-scale heat fluxes based on the fire laboratory calibrations for 

the gauges used in the second round robin. Averages and scatter (2σ) for the five fire 

laboratories are 126.2 kW/m2

 ± 17.3 kW/m2 and 124.7 kW/m2 ± 21.2 kW/m2 for the SchmidtBoelter and Gardon gauges, respectively. The variations correspond to 13.7 % and 17.0 %, 

respectively, which are somewhat larger than observed during the first round robin. 

5. DISCUSSION 

Given the significant differences between the calibration methods used in the various fire 

laboratories, the agreement for the calibrations, as reflected by the variations from the average 

values for the nominal full-scale readings of the four gauges, is viewed as good. However, there 

are systematic variations in calibration results. SINTEF reported the lowest sensitivity for the 

two gauges tested during the first round robin, while BRE/FRS reported the highest. When the 

results from these two fire laboratories are omitted, average and scatter (2σ) full-scale heat flux 

values for the three remaining laboratories become 91.1 kW/m2

 ± 2.5 kW/m2 (2.7 %) and 121.9 

kW/m2

 ± 2.7 kW/m2 (2.2 %). The variations are reduced by factors of 3 and 4 from those when 

results for all five laboratories are included. 

The same relative ordering of predicted full-scale heat flux readings among the fire laboratories 

was found for the two gauges used in the second round robin. Averages and variations for the 

gauge responses change to 126.2 kW/m2

 ± 2.8 kW/m2 (2.2 %) for the Schmidt-Boelter gauge and 

124.9 kW/m2

 ± 12.9 kW/m2 (9.9 %) for the Gardon gauge when the high and low results are 

omitted. For the second round robin Schmidt-Boelter gauge, the reduction in the scatter gained 

by omitting the high and low calibration values is comparable to the first round result. However, 

for the Gardon gauge the reduction is less than a factor of one, compared to a factor of four for 

the first round results. This is due to a larger scatter between measurements from the three 
23

Figure 17. Calibration results for the five fire laboratories are compared for the two 

gauges used during the second round robin by plotting the predicted full scale 

responses (i.e., the heat flux corresponding to a 10 mV gauge output) versus the 

manufacturer’s results (line). Uncertainties based on experimental results and 

determined as described in the text are indicated. 

remaining fire laboratories for this Gardon gauge than observed for the other three sets of 

calibrations. 

There is a plausible explanation that partially explains the lower responses recorded by the 

SINTEF calibration system as compared to the other fire laboratories. As noted earlier, the 

SINTEF calibration method uses a full-hemisphere radiant source, while the systems used by the 

other fire laboratories constrain the thermal radiation striking the gauges to narrower angles. FM 

Global measured the angular sensitivities of the two gauges used for the first round robin. The 

results are shown in Fig. 18 and Fig. 19. In the top panels, the responses of the gauges have been 

normalized by the responses recorded for normal incidence (θ = 0) and plotted against incident 

angle (θ) for the thermal radiation. In the lower panel the normalized response is divided by 

cos(θ) and again plotted against incident angle. If the gauge had a perfect angular response, the 

result would be a constant value of 1 for all incident angles for the lower plot. 

It is clear from the results in the lower panels of Fig. 18 and Fig. 19 that the responses of the two 

gauges fall below the idealized cosine response for large incidence angles. The angular 

dependencies were fit using an equation of the form, 

, 2

2

2 π θ εθ

π θ

− −

− (1) 

where ε is a parameter to be determined. The results were ε = 0.018 and ε = 0.025 for the 

Schmidt-Boelter and Gardon gauges, respectively. The resulting curves are included as solid 

Manufacturer's Calibration (kW/m2)

114 116 118 120 122 124

Laboratory Calibration (kW/m2

) 110

120

130

140 SP

BRE/FRS

FMG

SINTEF

BFRL

Schmidt-Boelter

Gardon
24

Figure 18. The angular response recorded by FM Global for the Schmidt-Boelter gauge 

used during the first round robin is shown in the top graph as normalized 

gauge response versus radiation angle. The data are replotted in the lower 

graph after dividing the normalized gauge response by the cosine of the 

incident angle (θ). The solid line is the result of a fit of the experimental data to 

the indicated form with ε = 0.018. 

lines in the lower panels of Fig. 18 and Fig. 19. By integration, the areas under the curves were 

determined, and it was found that that the ratios of absorbed heat flux for the devices were 

reduced by 0.97 and 0.96 for the Schmidt-Boelter and Gardon gauges, respectively, when 

compared to those assuming a perfect cosine angular dependence. Since most of the fall off in 

sensitivity occurs for large θ, these ratios should provide good approximations for the expected 

reductions in sensitivity when narrow-angle and hemispherical calibrations are compared. The 

angular effects have the correct sign to explain the lower sensitivities reported by SINTEF for 

the four gauges. The magnitudes are consistent with observed differences between the average 

Schmidt-Boelter Gauge

Angle (degrees)

-80 -60 -40 -20 0 20 40 60 80

Normalized Heat Flux Gauge Reading

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normalized Gauge Readings

Cosine Curve

Angle (degrees)

-80 -60 -40 -20 0 20 40 60 80

Normalized Heat Flux Gauge Reading/cos(θ) 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normalized Gauge Readings/cos(θ)

(π/2−θ)/(π/2−θ−εθ^2) 
25

Figure 19. The angular response recorded by FM Global for the Gardon gauge used 

during the first round robin is shown in the top graph as normalized gauge 

response versus incidence angle. The data are replotted in the lower graph 

after dividing the normalized gauge response by the cosine of the incident angle 

(θ). The solid line is the result of a fit of the experimental data to the indicated 

form with ε = 0.025. 

values for the fire labs and the SINTEF measurement values for the first round robin, but only 

partially compensate for those reported by SINTEF for the second round robin. 

The variations in calibrated heat flux gauge response between the fire laboratories are larger than 

the calibration repeatability for individual laboratories as shown by the consistent ordering of 

results for the four gauges and the relatively small uncertainties in data fits from individual 

laboratories. These observations indicate that there are systematic variations between results 

from the fire laboratories. In addition to angular sensitivity, there are a number of additional 

gauge response and calibration system parameters that might result in such systematic variations. 

Gardon Gauge

Angle (degrees)

-80 -60 -40 -20 0 20 40 60 80

Normalized Heat Flux Gauge Reading

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normalized Gauge Readings

Cosine Curve

Angle (degrees)

-80 -60 -40 -20 0 20 40 60 80

Normalized Heat Flux Gauge Reading/cos(θ) 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normalized Gauge Readings/cos(θ)

(π/2−θ)/(π/2−θ−εθ^2) 
26

These include wavelength sensitivity, relative responses to convective and radiative heat transfer, 

and non-linear gauge response. A variety of calibration system parameters can also introduce 

differences. Systems that utilize a secondary standard are linearly dependent on its calibration. 

Any variations between the calibration standards between laboratories should appear as 

differences in the results. Secondary standards have all of the potential sensitivities listed above 

that can impact the calibrations. Some of the laboratories’ calibration systems require a number 

of additional measurements such as black-body temperature, distance from the source, and the 

angle of the radiative source observed. Any uncertainties in these measurements propagate into 

the final calibration result. 

The results allow some analysis of the relative effects of potential error sources. Four of the fire 

laboratories found that their calibration data plotted as applied heat flux versus gauge response 

could be fit to straight lines with high precision, as reflected by correlation coefficients that were 

generally higher than 0.9999. In contrast, SINTEF found indications of nonlinear gauge 

responses that appeared to increase with applied heat flux. Based on their results, these 

researchers have concluded that a better characterization of the linearity of gauge response 

should be available before calibrations are routinely extended beyond the heat flux range over 

which they are calibrated. 

Additional insight into the possible role of nonlinearity of gauge response on the calibrations is 

obtained by considering the extrapolated full-range readings reported for the four gauges that 

were calibrated. Due to the wide variation in the maximum heat flux applied to the gauges (see 

the values included for the individual laboratories in Section 3.2) for the calibrations, the amount 

of extrapolation required to estimate a full-scale heat flux reading varied substantially among the 

fire laboratories. Since any nonlinearity in the response will affect the accuracy of such 

extrapolations, the absence of large variations in the extrapolated full-range values suggests that 

nonlinearity in gauge response is not introducing significant differences. 

Researchers at SP used the results of their calibrations to perform a more detailed analysis 

concerning the linearity of the calibrations. Figure 20 shows plots of residuals from the linear 

least squares curve fits versus the gauge reading for each of the four gauges calibrated during the 

two round robins. As expected, the residuals are small for both types of gauges. Interestingly, 

the magnitudes of the residuals for the Gardon gauges are somewhat larger and, while the 

residuals for the Schmidt-Boelter gauge appear to be randomly distributed, systematic variations 

with gauge reading are observed. Such variations indicate that the responses of the Gardon 

gauges are slightly nonlinear. In fact, a second order curve was found to fit the results for the 

Gardon gauges more accurately than linear fits. This observation does not mean that the gauge 

response is second order, but provides further evidence that Gardon gauge response is more 

nonlinear than for a Schmidt-Boelter gauge. 

Due to the wide variety of heat flux sources (see Table 1) and the different calibration heat flux 

levels used, the source temperatures varied over an extensive range both during a given 

calibration as well as between laboratories. As a result, the wavelength distributions for the 

sources varied substantially as well. The linear response of the gauges, as well as the ability to 

substantially extrapolate from the measured results with good agreement between laboratories, 
27

Figure 20. Residuals derived from linear least squares curve fits to calibration results 

recorded by SP are plotted as a function of the gauge reading in millivolts for 

the A) first round Schmidt-Boelter gauge, B) first round Gardon gauge, C) 

second round Schmidt-Boelter gauge, and D) second round Gardon gauge. 

indicates that the high absorbance coatings used on the gauges have nearly flat wavelength 

responses over the relevant wavelength range. 

Heat flux gauges are often calibrated assuming that radiative heating is the only heat transfer 

mode. However, both Gardon and Schmidt-Boelter gauges are sensitive to the convective heat 

transfer that occurs when the gas temperature at the gauge surface is different than the gauge 

surface temperature. Due to differences between the calibration facilities, there is the potential 

for significant variations in the relative contributions of convective heat transfer to gauge 

response. The relatively close agreement of the calibration results between fire laboratories 

suggests that variations in the relative contributions of convective heat transfer to the total heat 

transfer to the gauge have only small effects on the calibrations. It should be noted that this does 

not imply that convective heat transfer processes are absent in a given calibration system, but 

only that their effects are cancelled out in some manner. For instance, in experiments utilizing 

secondary-standard gauges, the convective component will have no effect on the calibration if it 

makes the same contribution as for the gauge being calibrated. 

A.

Gauge Reading (mV)

012345678

Residuals

-0.3

0.0

0.3

C.

Gauge Reading (mV)

012345678

Residuals

-0.3

0.0

0.3

B.

Gauge Reading (mV)

012345678

Residuals

-0.8

0.0

0.8

D.

Gauge Reading (mV)

012345678

Residuals

-0.5

0.0

0.5
28

It is worthwhile to emphasize again that the degree of agreement for calibrations performed in 

the different laboratories is viewed as satisfactory for fire test needs. It does appear that there is 

room for improvement in calibration consistency based on more carefully controlled experiments 

and analysis. Before undertaking such an effort, it is important to consider whether the potential 

improved agreement between fire laboratories justifies the associated costs and the need for 

modifying current calibration practices. 

6. SUMMARY 

The results of the two round robins of heat flux gauge calibrations have demonstrated that the 

agreement between the five fire laboratories is satisfactory for most fire research purposes and 

standard fire testing. Absolute values and variations in results between these laboratories are 

comparable to those reported by measurement laboratories. Systematic variations observed in 

the results suggest it should be possible to further improve agreement between calibration results 

from the fire laboratories. 

It should be recognized that the data presented in this publication are representative of the 

calibration procedures at the fire laboratories at a particular moment in time. There are a number 

of ongoing activities in the area of heat flux gauge calibrations at the fire laboratories and 

elsewhere with the aim of improving the understanding of heat flux gauge calibration 

methodologies and improving the agreement between laboratories. 

7. RECOMMENDATIONS 

The role of convective heat transfer in the response of gauges sensitive to both radiation and 

convection is not very well understood and needs to be elucidated, both for calibration purposes 

and for their use in fire tests and other applications. The absorptivity, which is a property of the 

surface coating, also requires additional study because it affects the relative amounts of radiative 

and convective heat transfer to the gauge surface. 

When the calibration is performed as a transfer calibration the uncertainty analysis should also 

consider the influence of the reference gauge being of a different type than the one being 

calibrated. Gardon and Schmidt-Boelter gauges are not equally sensitive to convection, and the 

error due to convection can therefore be greater if the reference and calibration gauge are of 

different types. 

Finally the FORUM Heat Flux Measurement Working Group strongly encourages the formation 

of commercial calibration services for routine calibration of heat flux gauges traceable to a 

primary method. 

8. REFERENCES

[1] T. Ohlemiller, E. L. Johnsson, and R. G. Gann, Editors, Measurement Needs for Fire Safety: 

Proceedings of an International Workshop. National Institute of Standards and Technology 

Internal Report NISTIR 6527, Gaithersburg, MD, 270 pp, June, 2000. 
29

[2] W. L. Grosshandler, Heat Flux Transducer Calibration: Summary of the 2nd Workshop. 

National Institute of Standards and Technology Internal Report 6424, Gaithersburg, MD, 27 

pp, November, 1999. 

[3] Guide for Interlaboratory Comparisons, Recommended Practice RP-15, National 

Conference of Standards Laboratories, March, 1999.

[4] ISO/DIS 14934-2, Fire tests — Calibration and use of heat flux meters: Part 2: Primary 

calibration methods, under DIS ballot. 

[5] ISO/CD 14934-3, Fire tests — Calibration and use of heat flux meters: Part 3: Secondary 

calibration methods, under development 

[6] W. M. Pitts, J. R. Lawson, and J. R. Shields, NIST/BFRL Calibration System for Heat-Flux 

Gages, Report of Test FR 4014, National Institute of Standards and Technology, 

Gaithersburg, MD, August, 2001. 

[7] H. Preston-Thomas, “The International Temperature Scale of 1990 (ITS-90),” Metrologia

27, 3-10, 1990. 

[8] Nordtest method NT FIRE 050, Heat flux meters: Calibration, NORDTEST, Helsinki, 1995. 

[9] S. Olsson, Calibration of Radiant Heat Flux Meters – The Development of a Water Cooled 

Aperture for Use with Black Body Cavities, SP REPORT 1991:58, Borås, 1991. 

[10] P. Andersson and I. Wetterlund, Uncertainty in Heat Flux Calibrations Performed 

According to NT FIRE 050, SP REPORT 2001:34, Borås, 2001.