Volume 1, Issue 7 (July 2004)
An Explanation and Comparison of Sweating Hot Plate Standards
There are several standards that require the use of a hot plate apparatus for measuring the dry thermal resistance (insulation) and the evaporative resistance of fabrics: ASTM D 1518, ASTM F 1868, and ISO 11092. In addition, several National Fire Protection Association standards for protective clothing (NFPA 1951, 1971, 1977, and 1999) specify the use of this method. All of these methods measure the total resistance (i.e., the resistance of the fabric and the resistance of the surface air layer). The resistance of the air layer alone is determined by conducting a “bare plate” test. This air layer resistance is often subtracted from the total resistance to determine the fabric resistance values. Different standards use different terms for these resistances (Ret or Ref), and they report the resistances in different units (m2 · kPa/W or m2 · Pa/W). In addition, some methods call for guarded plates, whereas others use smaller plates, for which the experimenter must adjust the data for thick specimens to compensate for the lack of a guard. Some methods control the air velocity over the specimen, while others allow any air speed, as long as the calibration requirements on standard fabrics are met. This paper discusses the differences in test instruments, conditions, measured parameters, and units so that data from different labs can be compared and understood. In addition, data from an ASTM interlaboratory study is presented so that the repeatability and reproducibility of the different test protocols on a variety of materials can be discussed.