A Mixed Flow Thermal Transmittance Tester for Textiles

A laboratory instrument is described that measures thermal transmittance of textiles under a mixed flow state of free convection. The apparatus uses solid state heat flow transducers situated on large flat plate heat sinks that are critically spaced, guarded, and temperature-regulated. A distinctive state of heat transfer is produced for which the ratio of the total transmittance to the conductive heat flow is constant. This controlled, natural convection feature makes it possible to standardize readily thermal transmittance results under defined free convective conditions. This mixed flow state is assumed to resemble actual convection of textiles encountered in use. The textile is placed between the hot and cold sinks, and heat and moisture transport, relative humidity, and air temperature are displayed on a real time basis using a microprocessor which converts millivolt output into the parameters mentioned. Thermal resistance versus fabric thickness relationships obtained with various in strument configurations are compared under different indoor environmental con ditions.