Method of measuring emittance of thermal insulation materials at low temperatures

The main difficulty in measuring the emittance of thermal insulation materials at temperatures below 500 K, namely interference from background radiation, was overcome in an experimental test stand consisting of a vacuum chamber with a mechanism which rotates the test specimen inside. The specimen in the form of a flat disk is held firmly in a retainer fastened to the lower end of the vertical motor shaft. The temperature of the specimen is regulated by a cooler and a disk of conducting material with a controllable heater between them, the metal disk ensuring that the surface of the insulation material remains isothermal. Both the cooler and metal disk are eccentric relative to the axis of rotation so as not to obstruct the path for the radiant flux from the specimen surface through holes in the retainer to the radiation receiver undereath. Temperature and radiant energy are measured under a pressure not exceeding .01 Pa, whereupon the emissivity is calculated according to the Stefan-Boltzmann law. There are two main systematic errors in the emissivity readings. The source of one error is reflection of the radiant flux into the receiver by hot components of the apparatus. The source of another error is cooling of segments of the specimen passes through the cold zone between shield and radiation meter. The two errors are in opposite directions and, therefore, compensate one another to a large extent.