ASTM E3057-19

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 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, 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, 1 h) 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 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 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 10 min [Sultan, M., 2006 and 2008 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 5.1.10 Reports by Sultan, M., (2006 and 2008) 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.

5.2.1 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 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).