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28 APPENDIX B MEASUREMENT OF TEMPERATURE DISTRIBUTION ON HOT PLATE A suitable piece of apparatus for measuring the tem- perature distribution across the hot plate is illustrated in Figure B-l. The measuring element is to consist of a find thermocouple with the junction flattened and brazed to a disc of copper or brass foil 5 mm in diameter. This is to be placed at a measuring point and covered with a piece of insulating material (asbestos millboard) 5 mm in thickness and l0 to l5 mm in diameter and held by a vertical glass rod which moves freely in a tubular guide and to which a fixed load is applied. Temperature measurements are to be made along two dia- meters at right angles and at points 20 mm apart and record- ed as in Figure B-2. The thermocouple must be allowed to reach a steady temperature at each point. The measured surface temperature usually will be lower than the set point temperature of the plate depending on the detailed construction of the thermocouple. This is im- material and can be ignored. The essential requirement is an accurate measurement of temperature differences rather than of absolute values.
29 in to w Q O -P Q) M Cn 3 fi -1-1 â¢rH «J â¢a ^ M 0) o a u e o <u (0 -p (1) 0) M O d) in a, 3 o -P 4* tn fi C Q) â¢H C M Q) ^^ rH W O n3 S Q) en l= PQ
30 Plate Temperature: 350° Scale 1:1 FIGURE B-2 Fluctuations of surface temperature over entire plate: 5 °C (Maximum difference from desired temperature: 8 °C)
