5.1 Need for Heat Flux Measurements: 

5.1.1 Independent measurements of temperature and heat flux support the development and validation of engineering models of fires and other high environments, such as furnaces. For tests of fire protection materials and structural assemblies, temperature and heat flux are necessary to fully specify the boundary conditions, also known as the thermal exposure. Temperature measurements alone cannot provide a complete set of boundary conditions.

5.1.2 Temperature is a scalar variable and a primary variable. Heat Flux is a vector quantity and it is a derived variable. As a result, they should be measured separately just as current and voltage are in electrical systems. For steady-state or quasi-steady state conditions, analysis basically uses a thermal analog of Ohm's Law. The thermal circuit uses the temperature difference instead of voltage drop, the heat flux in place of the current and thermal resistance in place of electrical resistance. As with electrical systems, the thermal performance is not fully specified without knowing at least two of these three parameters (temperature drop, heat flux, or thermal resistance). For dynamic thermal experiments like fires or fire safety tests, the electrical capacitance is replaced by the volumetric heat capacity.

5.1.3 The net heat flux, which is measured by a DFT, is likely different than the heat flux into the test item of interest because of different surface temperatures. An alternative measurement is the total cold wall heat flux which is measured by water-cooled Gardon or S-B gauges. The incident radiative flux can be estimated from either measurement by use of an energy balance [Keltner, 2007 and 2008 (16, 17)]. The convective flux can be estimated from gas temperatures and the convective heat transfer coefficient, h [Janssens, 2007 (18)]. Assuming the sensor is physically close to the test item of interest; one can use the incident radiative and convective fluxes from the sensor as boundary conditions into the test item of interest.

5.1.4 In standardized fire resistance tests such as Test Methods E119 and E1529, or ISO 834 or IMO A754, the furnace temperature is controlled to a standard time-temperature curve. In all but Test Methods E1529, implicit assumptions have been made that the thermal exposure can be described solely by the measured furnace temperature history and that it will be repeatable from time to time and place to place. However, these tests provide very different thermal exposures due to the use of temperature sensors with very different designs for furnace control. As a result, these different thermal exposure histories produce different fire ratings for the same item. Historical variations of up to 50% or more in the qualitative fire protection ratings (for example, one hour) between different furnaces or laboratories indicate that the assumptions for time-temperature control are not well founded. Also, due to different sensors, thermal exposure in a vertical furnace is generally higher than in a horizontal furnace and thermal exposure on the floor of a horizontal furnace is generally higher than on the ceiling. These reasons provide support for why both temperature and heat flux measurements are needed to provide consistent test results.

5.1.5 In the mid-90's, the U. S. Coast Guard authorized a study of the problems in marine fire resistance tests, such as large variations in the ratings obtained in different furnaces. One important conclusion was that the thermal exposure in furnaces could not be predicted solely from furnace temperature measurements without large static and dynamic uncertainties (Wittasek, N. A., 1996 (19)).

5.1.6 One of the recommendations that resulted from NIST's investigation of the World Trade Center disaster was the need to move towards performance based codes and standards. A report developed for The Fire Protection Research Foundation expanded on this recommendation [Beyler, C., et al., 2008 (20)]. Part of this effort involves making a more comprehensive set of measurements in fire resistance tests including quantitative heat flux measurements. It also involves the development and use of "design fires" and defining their relationship with standardized test methods.

5.1.7 Work at Sandia National Laboratories on transportation accidents involving hazardous materials compares the Prescriptive and Performance based approaches [Tieszen, et al., 2010 (21)].

5.1.8 Work by the National Research Council of Canada used four (4) different temperature sensors to control a horizontal furnace. Differences in the thermal exposure (see definition in 3.2.5) were as high as 100% during the first ten minutes [Sultan, M., 2006 and 2008 (22, 23)]. Assuming the temperature measurements from the different sensors or different installations of the same sensor are actually the furnace temperature, one can predict very different thermal exposures depending on which temperature measurement method is used.

5.1.9 In another series of horizontal furnace tests, the National Research Council of Canada (NRCC) studied the effect of six (6) different temperature sensor designs on fire resistance tests in a large, horizontal furnace [Sultan, 2008 (23)]. NRCC used six different temperature sensors for furnace control: Test Methods E119 Shielded Thermocouple, ISO 834 Plate Thermometer, 6 mm MIMS TC from Test Methods E1529, Directional Flame Thermometers, and 1.6 mm MIMS TCs with grounded and ungrounded junctions. Total heat flux at the ceiling was measured using a Gardon gauge. Results showed that very different thermal exposures are possible depending on the measurement method used. During the first ten minutes of a fire resistance test, the integrated heat flux varies by a factor of two.

5.1.10 Reports by Sultan, M., (2006 and 2008) (22, 23) and Janssens, M., (2008) (18) have shown it is difficult to measure one parameter in a fire resistance test (such as the furnace temperature) and calculate the other (heat flux or thermal exposure).

5.1.11 From the discussions in 5.1, it is highly recommended that both temperature and heat flux be measured independently in fire tests.

5.2 Use for DFTs: 

5.2.1 Although both cooled and non-cooled sensors can be used to measure heat flux, the results are generally quite different. Water-cooled sensors are the direct reading Schmidt-Boelter or Gardon gauge designs that are used in some Committee E5 Methods [Test Methods E2683 and E511, respectively, have been developed for these sensors by Subcommittee E21.08 ].

5.2.2 There are three types of passive or un-cooled sensors that can be used to measure net heat flux. One is the hybrid sensor (so-called High Temperature Heat Flux Sensor, HTHFS) developed by Diller, et al., at Virginia Tech. It is designed to measure heat transfer to a surface without water cooling [Gifford, A., Hubble, D., Pullins, C., and Diller, T., 2010 (4)]. The HTHFS requires a calibration factor that is a function of sensor temperature [Pullins and Diller, 2010 (24)]. Another is the so-called "direct write heat flux sensor" which can be used at temperatures from 25 to 860°C [Trelewicz, Longtin, Hubble and Greenlaw, 2015 (25)]; this gauge requires a calibration coefficient. The third is the Directional Flame Thermometer (DFT), which was developed at Sandia National Laboratories (based on work in the UK) and elsewhere for measuring heat transfer in large sooty pool fires. DFTs do not require a calibration factor, which may be viewed as a mixed benefit. The passive sensors typically have higher temperature capability, based mainly on the Type K or N TC limit of about 1250°C. Even though they are water cooled, quite often Gardon and Schmidt-Boelter gauges do not survive in temperatures due to fouling of the sensing surface, and other effects. DFTs usually survive up to about 1100°C. They are very rugged, require no cooling, and are not susceptible to fouling of the sensing surface. These characteristics simplify installation in a wide range of fire and other applications. This standard will only address DFTs. See 10.2.2 for a more thorough discussion of heat flux gauge calibrations.

5.2.3 Early work on DFTs (and the data analysis techniques for them) focused on acquiring quantitative heat flux data to help define the thermal conditions in large, liquid hydrocarbon pool or spill fires. Large pool fires can reach quasi-steady conditions in times as short as a minute. As a result, Pool Fire DFTs were designed with 1.6 mm thick plates to provide rapid equilibration with the fire (the maximum heating rate in these fires was approximately 30°C/s).