Patent GB-540344-A (GB)
Title: Improvements in temperature responsive devices employing electric resistances
IPC: H01C7/04
Filed: 1940-06-29
Abstract: 540,344. Resistances. BRITISH THOMSON-HOUSTON CO., Ltd. June 29, 1940, No. 11048. Convention date, June 29, 1939. [Class 37] A temperature-responsive device incorporating a negative-temperature coefficient resistor and suitable for use as a pyrometer or thermostat, comprises outer and inner electrodes 10, 11 separated by a mass 13 of the resistance-material and formed of " Monel " metal (Registered Trade Mark) or nickel according to the operating temperature. The material 13, which should have the same thermal-expansion coefficient as the electrodes, preferably comprises a finely-divided mixture of the proportions by weight of 0.25 copper oxide (CuCo2) 2.52 magnesium oxide, and 1.47 sodium silicate, the latter comprising silicon oxide and sodium oxide in the proportions 1 : 3.22. Higher proportions up to 6 of copper oxide may be used, or it may be omitted or replaced by zinc oxide, nickel oxide, or chromium oxide, and the magnesium oxide may be replaced by zirconium oxide or aluminium oxide. The electrode-surfaces to contact with the material 13 are prepared by oxidizing their surfaces in air with a flame and, while hot, powdering them with sodium silicate. Electrode 12 is put into a mould 14, Fig. 4, and a small amount of the finely-divided material, previously freed of moisture by a 3-hour heat treatment at 400‹ F., is inserted and compressed by a plunger at about 10 tons/in. <2> pressure so as to produce a conicalsurfaced wad 17 on which the electrode 11 is centred. Further small amounts of material are successively inserted and compressed, the plunger being apertured to fit over the electrode 11. The whole is then removed from the mould 14 by a tool inserted in orifice 20 and placed in an open-ended porcelain tube in an electric furnace and during an hour brought up to 950<0> F. and kept at that temperature for one hour. The temperature is then raised during two hours to 1975 F. and held for 10 minutes, and then slowly lowered to 1100‹ F. and held for 16 hours, after which it is further lowered. During the process, the outside of the unit is an oxygen-free atmosphere, but the inside is open to the normal atmosphere to prevent reduction of the copper oxide CuO2. Further, the mass 13 is sintered into a compact whole with the prepared surfaces of the electrodes. The unit is then " aged" at 1000‹ - 1100‹ F. for 40 - 50 hours, the temperature being above the ultimate operating temperature, and a current of 20 milliamps is passed through the material during " aging" so as to reconvert any CuO that may still be present into CuO2. For furnace-temperature control, the unit is threaded and cemented into a steel tube 22 secured by a screw 30a in an asbestos cap 25 and welded to lead 29 in connection with a terminal 28. An asbestos-covered lead 23 brased to electrode 11 is connected to a further terminal, alkyd resin cement being employed to seal up the joints. In Fig. 10, a hollow inner electrode 32 is welded to a lead 36 passing through, and soldered to the outside of a bolt 37 in an asbestos block 38. The outer electrode 33 is welded to a tube 35 and a lead 40 is welded to it and to a bolt 39. For " flattening " the resistance-temperature curve, external resistance may be connected in series and/or parallel with the unit.