Patent GB-751398-A (GB)
Title: Improvements in or relating to electric resistance thermometer units and the manufacture thereof
IPC: G01K7/18
Filed: 1954-03-26
Abstract: 751,398. Resistance thermometers. BAILEY METERS & CONTROLS, Ltd. March 26, 1954 [March 26, 1953], No. 8883/54. Class 40(1) An electric resistance thermometer includes an elongated, corrosion and heatresistant metal sheath, a temperature-sensitive resistance coaxial with the sheath and formed of wire insulated therefrom by an electrically insulating heat - conducting filling of granular refractory material and leads extending from the ends of the resistance wire within the sheath through opposite ends of the sheath via ceramic insulators which seat the filling within the sheath. A metal body, e.g. of solder is bonded to each lead and its adjacent insulator. The resistance wire 40 is of Mb for use up to 1500‹C. or Ni for lower temperatures and is fixed into leads made of stainless steel capillary tubing the connection being completed by flattening the tube and spot welding. Alternatively (as in Fig. 2) solid leads 20, 21 of pure Ni, the ends of which are flattened and rolled into tube form, are used. The sheath 41, which may be of stainless steel annealed in hydrogen is suspended with its axis vertical with the wire and lead assembly positioned in it and a vulcanized fibre plug threaded over the lower lead and pressed into the sheath, which is then gradually filled with granular refractory material, e.g. Al 2 O 3 , MgO, the latter, which is hard, being ground to at least 200 mesh to prevent damage to the wire during compacting. The material is compaced by reciprocating a piston tube between sheath and wire, and a further fibre plug inserted in the upper end of the sheath which is spun over it. The sheath is then swaged throughout its length to reduce the internal cross-section by 11 per cent and compact the material, cut back beyond the plugs and ceramic insulators 43 are threaded on the leads and pressed into the ends of the sheath which are then spun over to retain them. The holes through which the leads leave are sealed against leakage of granules. by application of solder which bonds to the lead and the ceramic insulator. The sheath is bent into the final U-form shown in Fig. 2 after swaging and the ends 39<SP>1</SP>, 39<SP>11</SP> are press-fitted and then welded into head 39, which is externally threaded to screw into, e.g. an apertured furnace wall and the head' assembly 39, 139 is screwed into terminal box 30. The leads 31, 32 connecting leads 20, 21 to terminals 33, 34 fixed in the box are formed with bends to allow differential expansion between themselves and head 39. In another embodiment (Fig. 3) the resistance wire runs along the axis of straight sheath 41<SP>1</SP>, the lead 21 from the outer end of the sheath being returned to head 39 through a bent tube 24, welded into the remote end of the sheath, from which it is insulated by bead insulators 25 threaded on it. The resistor may be connected in one arm of an A.C. bridge, the unbalance voltage of which is amplified and used to operate a capacitor-run motor which re-balances the bridge and operates a recording pen on a chart.