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Platinum Metals Rev., 1962, 6, (4), 149

A Thermocouple for Transient Temperatures

Reliability of Spring-Loaded Open-Contact Probe

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In the investigation of materials for use in the construction of missiles and spacecraft, accurate methods of measuring high transient surface temperatures are essential. At the Lockheed Missiles – Space Company a spring-loaded open-contact thermocouple probe has been developed and its design and performance are described by P. M. Hahn in a recent paper (Materials Research and Standards, 1962,2, (5), 403-404).

Initial tests were made with a spring-loaded tantalum-sheathed platinum: 13 per cent rhodium-platinum thermocouple and a hammer-welded platinum: 13 per cent rhodium-platinum thermocouple in the measurement of surface temperatures of molybdenum and copper discs ⅛ inch thick. Results obtained showed that the response of this spring-loaded thermocouple was too slow for accurate measurement of rapidly increasing temperatures. A new type of open-contact probe for the spring-loaded thermocouple was then designed. In this, the thermocouple wires, 0.010 inch diameter, are insulated by an 0.071 inch diameter ceramic tube which is itself sheathed with stainless steel 0.012 inch thick. An air gap separates the spherical tips of the thermocouple wires, the material under examination serving as a common conductor. Tests using this open-contact thermocouple with spring loads of 2 to 4 pounds on heated copper and molybdenum discs gave results in excellent agreement with those obtained with the hammer-welded thermocouple. Accuracy was maintained with temperature increases of up to 800°C per second and with temperatures in excess of 1200°C.

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