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27 APPENDIX A CONSTRUCTION OF HEATER SURFACE Provided the requirements presented in Chapter 2, sec- tion B4a(~), page 9 are satisfied, the detailed construction of the heated surface is not critical. It should consist of a circular plate of stainless steel provided with a "skirt" (Figure I) and may be mounted on any suitable electrically heated ''hot plate" commercially available. aluminum and ordinary steel are not recommended for the heated surface because of the potential corrosion problems and the possi- biJity that an aluminum surface couth be destroyed when metal powders were being tested. There are two ways of achieving a sufficiently uniform temperature distribution across the heated plate, the choice of which depends primarily on the heating device available. If the heater consists, for example, of exposed coiled fila- ments intended to run at red heat, there should be an air gap of about 10 mm between the heater and the plate so that heat transfer occurs by radiation and convection. If, how- ever, the heater is designed for direct contact and heat transfer occurs mainly by conduction, the plate needs to be much thicker if hot spots are to be avoided. A thickness of not l ess than 20 mm is recor~unended. The genera l arrangement shown in Figure ~ is se If- exp] anatory. It is preferable to insert indicating and control ~ ing thermocouples in holes drilled radially from the edge of the plate and parallel to the surface at a depth of l run from the s ur face . The base of the hot plate should be provided with f eet in order to clear the support for a thermocouple stretched horizontal ly across the surface e The thermocouple is mounted between spring-1 oaded carriers on threaded vertical rods. The he ight of the the rmoc oup ~ e can be ad j u s ted by me ens 0 f wing nuts. ~~