Patent GB-692116-A (GB)
Title: Improvements in compensated thermopiles
IPC: G01J5/12
Filed: 1951-01-08
Abstract: 692,116. Thermocouples. LEES & NORTH. RUP CO. Jan. 8, 1951 [Jan. 18, 1950], No. 536/51. Class 53. [Also in Group XL (b)] A thermocouple or thermopile for receiving and measuring radiant energy is provided with a thermal shunt for causing the E.M.F. to be independent of the ambient temperature. The shunt is in thermal contact with but electrically insulated from the hot and cold junctions. Fig. 4 shows a number of thermocouples 19 mounted on a metal ring 20 forming a part of a radiation pyrometer in which the radiation is caused by an optical system to fall on a target 25 consisting of a platinum disc covered by a disc of mica to which the hot junctions of the thermocouples are attached by a ceramic glaze. The glaze may be black to form a radiation-absorbing surface. The cold junctions 18 of the couples are connected thermally to but electrically insulated from the ring 20 by a ceramic cement. When the hot and cold junctions are at the same temperature no E.M.F. is produced at the terminals 26, 27 and this condition should be obtained when no radiation falls on the target 25. Changes in the ambient temperature may however produce an E.M.F. and affect the value of the E.M.F. produced when the thermopile is in operation. It is shown in the Specification that if the elements of the thermocouples become better conductors of heat with rise of temperature so that the rate of flow of heat from the target 25 to the ring 20 increases with temperature compensation for changes in the ambient temperature can be obtained by providing a thermal shunt 28 which becomes a poorer conductor of heat as the temperature rises and which connects the target with the ring 20. If the couples consist of chromel and constantan elements the shunt 28 may consist of a nickel wire. Calculations are included in the Specification giving optimum dimensions for the wires and corresponding shunt. Fig. 9 shows a modification in which the couples consist of constantan wires 50 secured to a target 25 and ring 20 by ceramic frits. Chromel and copper wires 51, 55 are connected alternately to the wires 50, the chromel wires constituting the second element of each chromelconstantan couple, the copper wires providing heat-conducting paths to maintain the output of the thermopile independent of the ambient temperature. Fig. 10 shows a further modification in which thermocouple elements 61 are joined to diagonally disposed elements 62, the junctions being secured by ceramic frits to targets 63, 64. Output leads 65, 66 are connected to the end elements 61 and acquire a difference of potential when the targets are at different temperatures. The targets are connected by thermal shunts 67, 68 which render the output voltage independent of the ambient temperature. Fig. 5 shows the ring 20, the target 25 and the elements 19 of the thermopile shown in Fig. 4 arranged in a casing 21 which is a part of a radiation pyrometer. The ring 20 is supported by three legs 20a secured to the ring 20 and abutting at one end against the end of the casing 21 and at the other against a concave mirror 13 kept in position by a spring 21e. Radiation is focussed on the target through an opening 14a.