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Evaluation/calculation and assessment of thermal resistance, evaporative resistance, and THL of fabrics
In the late 1990s, the ISO 5085-1 and BS 4745:2005 standards (guarded hot plate and Togmeter method) were widely used to evaluate the thermal resistance of fabrics, and the CGSB 1977 standard (control dish method) was highly acceptable for evaluating fabrics’ evaporative resistance. In order to evaluate both thermal and evaporative resistance, the ISO 11092 standard (Measurement of Thermal and Water Vapor Resistance under Steady-State Condition using Sweating Guarded Hot Plate) was developed in 1993 by the scientists at the Hohenstein Institute in Germany. Then, the NFPA developed a method to determine THL through thermal protective fabrics or fabric systems using the ISO 11092 standard. Later, this THL evaluation method was added to several NFPA standards (NFPA 1971, NFPA 1977, NFPA 1951, NFPA 1999) . Next, the members of the ASTM F23 committee decided to compile the evaluation procedures of thermal resistance, evaporative resistance, and THL in one document, which resulted in the ASTM F 1868 standard: Thermal and Evaporative Resistance of Clothing Materials Using a Sweating Hot Plate [379-381,385]. This ASTM F 1868 standard is widely used to evaluate/calculate thermal resistance, evaporative resistance, and THL.
The ASTM F 1868 standard covers the evaluation/calculation of thermal resistance, evaporative resistance, and THL of fabrics, films, coatings, foams, leathers, and multilayer fabric assemblies used in clothing, under steady-state conditions . The “sweating guarded hotplate” that acts as a human body is used to evaluate the thermal and evaporative resistances of a fabric or a multilayered fabric system (Fig. 5.23) and is housed in a closed chamber so that the experimental parameters (hot plate temperature, ambient air temperature, ambient air relative humidity, ambient air velocity) can be controlled manually and effectively. This guarded plate is composed of a test plate, guard section, and bottom plate; each can be electrically controlled at a constant temperature in the range of human skin temperature (33-36°C), and the temperature control may be achieved by independent adjustment to the voltage, current, or both, supplied to the guarded plate using solid-state power supplies, solid-state relays (proportional time on), adjustable transformers, variable impedances, or intermittent heating cycles. Here, the guard section is designed to prevent the lateral loss of heat from the test plate; and the bottom plate is maintained to prevent the downward loss of heat from the test plate and guard section. The guard section and bottom plate force all the heat generated in the test plate to flow in an upward direction . The temperature of the guarded plate and its ambient air can be measured with an accuracy of ±0.1 °C using temperature sensors (thermistors, thermocouples, resistance temperature devices (RTDs), or equivalent sensors). The relative humidity can be measured with an overall accuracy of ±4% using either a wet-and-dry bulb psychrometer, a dew point hygrometer, or any other electronic humidity measuring device. And, the air velocity can be measured with an accuracy of ±0.1 m/s using a hot wire anemometer attached at 15 mm from the plate surface [385,386].
The following sections discuss individually and in detail the procedures of thermal resistance evaluation, evaporative resistance evaluation, and THL using the ASTM F 1868 standard. Although the ASTM F 1868 standard is the best way to isolate and evaluate thermal resistance and evaporative resistance individually, some researchers feel that it is not realistic, as dry and evaporative heat losses occur simultaneously from a human body to the ambient environment, and there may be interactions between the dry and evaporative heat flows which this test does not capture . Additionally, the devices used in ASTM F 1868 for determining clothing properties ignore the physiological state of the wearer and are inadequate to evaluate the transient thermal properties of clothing. Keeping this point in mind, Psikuta, Wang, and Rossi  developed a thermo-physiological human simulator that can provide more realistic measurement of comfort properties of clothing. By using this human simulator, ISO is currently developing a standard (ISO CD 18640) that can evaluate the physiological comfort properties of clothing.
Fig. 5.23 Thermal and evaporative resistance evaluation tester.
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