The thermal resistance of a test specimen is measured according to ASTM F1939 Standard Test Method for Radiant Heat Resistance of Flame Resistant Clothing Materials with Continuous Heating.
A bank of infrared, tubular, translucent quartz lamps, provides a radiant heat exposure of 2.0 cal/cm2 sec (8.4 watts/cm2) lasting 60 seconds or less. The test fabric or composite is mounted in a sample holder positioned parallel with the heat source and protected using a metal shutter before and after the test run. The heat transferred through the test specimen is measured by a sensor and recorded by an especially designed data acquisition software program. The temperature vs. time trace in conjunction with sensor constants is used to compute heat flux. A square wave exposure sequence is used so that results can be related to values obtained by Stoll. Overlay of human tissue tolerance obtained through integration of the Stoll curve with respect to time determines tolerance time to second degree burns. The RPP value is the product of the incident heat flux and the recorded tolerance time to second degree burn (cal/cm2).