GB800550A - Improvements in temperature detectors - Google Patents

Abstract

800,550. Fire-detector cables. GRAVINER MANUFACTURING CO., Ltd., and WILKINSON SWORD, Ltd. April 26, 1955 [May 7, 1954], No. 13365/54. Class 36. An electrical temperature detector comprises two conductors, one a nickel-chromium alloy or a borated copper-clad nickel-iron wire 10, Fig. 1, and the other a surrounding tubular stainless steel sheath 11, the conductors being separated by a glass material having a negative coefficient of resistance and by discrete refractory particles 13, e.g. alumina, which prevent the conductors touching when the temperature rises to 1000‹ C. for example, and the glass softens. Apparatus for manufacturing such a detector comprises a stand 20, Fig. 3, which carries a lead-screw 21 on which is mounted a carrier 22 for supporting a glass tube 23, a gas-fired heater 24 for melting the end of the tube, a reel 26 and pulleys 28 from which the conductor wire 25 is drawn through the tube 23, a hopper 29 and feed tube 30 for feeding the refractory particles between the wire 25 and the glass tube 23, and a vibrator 32 to pack the particles. In operation, the end of the glass tube 23 is fed into the heater 24 and the wire 25 pulled down to draw the fused glass on to the wire. Draw-off rollers 33 driven by a motor 34 are then used to continue the coated wire pulling. As glass is drawn off, the tube 23 is moved down into the fusion source 24 by the screw 21 driven through a gear-box 35 from a motor 34, the speed of movement of the tube 23 being much less than the speed of the coated wire moved by the rollers 33 due to the thinness of the glass coating, the relative speeds being adjusted by means of the gear-box 35. An optical device 36 may be provided to allow visual monitoring of the coating. A further fusion source 37 may be provided to re-fuse the glass coating to ensure adhesion of the glass and wire. The coated wire is then directed by a suitable support 38 on to a cutting bench 39. The wire, whose coating may not yet be solidified throughout, is then sheathed by threading it into the sheath 11 which is then reduced by a die 14 (Fig. 2, not shown), or otherwise reduced by swaging or pressing. The detector may again be heated during or after application of the outer sheath in order to give better contact between conductor and coating. In a modification both conductors may be in the form of wires which after coating with glass having the refractory particles distributed therein may subsequently be introduced into an outer non- conductive sheath. In this construction it may be desirable to pack further material into the sheath. In the Provisional Specification, silica may be used for the refractory particles, and the outer sheath may be formed from spirallywound and seam-welded metal strip. In another method the inner conductor is coated with a mixture of silica powder, glass and a suitable binder.