Measurement of the thermal resistance and thermal transmission

The measurement of the thermal resistance with the sweating guarded hot plate according to ISO 11092 assesses the intrinsic resistance of the specimen plus a transition resistance from fabric to air. This transition resistance is dependent on the convective and the radiant heat loss from the surface of the fabric to the atmosphere. If a fabric is worn as an under-garment, there will usually be no convection and only limited radiation between the layers. Spencer-Smith (1977a) showed that internal convection between layers can be neglected if the air layer is smaller than 8 mm. Furthermore, as long as the fibre content in a fabric is higher than 9%, only thermal conduction needs to be considered in the fabric (Woo et al. 1994). It is therefore important only to assess the thermal conductivity of fabrics. ISO 5085-1 (1989) and ISO 5085-2 (1989) provide a method to determine the thermal resistance of fabrics. The specimen is placed onto a heating plate and covered by a cold plate with a defined pressure. The principle is that the temperature drop across the heating plate of known thermal resistance is assessed as well as the temperature on the surface of the cold plate. The thermal resistance of the specimen can then be calculated from these figures. The SI unit of the thermal resistance is m2K/W, but a widely used unit is the 'tog' (1 tog = 0.1 m2K/W). Another unit used to quantify the overall thermal insulation of garments is the 'clo' defined by Gagge et al. (1941). One 'clo' is equivalent to the insulation required to keep a seated subject comfortable at an air temperature of 210C with an air movement of 0.1 m/s, which corresponds to the insulation provided by an ordinary dress suit.

The new standard (ASTM D 7024 2004) describes a method to determine the overall (dynamic) thermal transmission coefficient of textile fabrics and measure the amount of latent energy in textiles. This method was developed to assess the thermal efficiency of phase change materials (PCM), which provide a temperature-regulating function by absorbing or releasing energy through aggregate state changes. It allows the measurement of the steady-state thermal resistance of a fabric as well as the determination of a 'temperature-regulating factor (TRF)'. The TRF is used to compare fabrics that store or release energy.

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