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27 APPENDIX A CONSTRUCTION OF HEATER SURFACE Provided the requirements presented in Chapter 2, sec- tion B4a(l), 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 l) 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- bility that an aluminum surface could 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 l0 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 less than 20 mm is recommended. The general arrangement shown in Figure l is self- explanatory. It is preferable to insert indicating and controlling thermocouples in holes drilled radially from the edge of the plate and parallel to the surface at a depth of l mm from the surface. The base of the hot plate should be provided with feet in order to clear the support for a thermocouple stretched horizontally across the surface. The thermocouple is mounted between spring-loaded carriers on threaded vertical rods. The height of the thermocouple can be adjusted by means of wing nuts.
26 ll. H.G. Dorsett, Jr., M. Jacobson, J. Nagy, and R.P. Williams, Laboratory Equipment and Test Procedures for Evaluating Explosibility of Dusts, Bureau of Mines Report of Inves- tigations 5624, l960. l2. Proposal not yet published; communication from Interna- tional Electrotechnical Commission working group. l3. International Electrotechnical Commission, ~Krr Apparatus and Method for Determining the Self-Ignition Temperature of a Dust Layer, IEC/3lH/WG2, February l975.
