Patent GB-1204604-A (GB)
Title: Improvements in or relating to detection apparatus
Inventor: GOULD PERCY
IPC: G08B17/06
Filed: 1968-03-29
Abstract: 1,204,604. Temperature alarm system. CLIFFORD EDWARDS Ltd. 8 Nov., 1968 [29 March, 1968], No. 15163/68. Heading G4N. [Also in Divisions H1 and H3] A temperature sensor for use in a fire or hot gas spillage alarm system, e.g. in the gas turbine engine of an aircraft, comprises two thermocouple hot junctions 9, 10, Fig. 3, one of which 10 is embedded in a body 3 of thermally insulating material while the other 9 is exposed. The two cold junctions, which in the embodiment are between wires 6, 8 and stems 5 of respective terminals 4, are sufficiently close together to be at the same temperature under operative conditions. As shown two of the thermocouple wires 6, 8 are of Chromel, the third one 7 being of " Alunel " (Registered Trade Mark), and the insulating body 3 is of alumina. A glass plug 3A seals the cold junctions. A number of such sensors may be arranged in a series chain, e.g. evenly distributed along an electrically conductive tube with the exposed hot junctions 9 projecting through holes in the tube wall. A cable connecting the sensors in series extends along the base of the tube and is connected through a resistor to the tube adjacent to, and on each side of, each detector unit. If the cable breaks the circuit for the detector chain is completed through part of the tube and two sensors. Fig. 4 shows schematically a system using a series chain 22 of such sensors. In the event of a fire or similar temperature increase a potential from the chain 22 is applied to a chopper 12, controlled by a square wave oscillator 23, to chop the output from the chain 22. This chopped signal is then amplified in an A.C. amplifier 13 and the amplified signal is applied to a level detector 14 which causes an alarm 15 to be energized when the amplified signal exceeds a predetermined value. Three test facilities are provided. Firstly a button 18, when depressed, applies to the chopper 12 through the sensor chain 22 a D.C. potential of the same sense as that produced by the chain 22 in the event of fire &c., and of sufficient magnitude to cause the alarm 15 to be energized. Secondly a test unit 19 is provided, including a rectifier which produces a D.C. voltage from the output of the A.C. oscillator in the chopper 12, which D.C. voltage is applied to the chopper 12 through the chain 22. This voltage is also large enough to cause the alarm 15 to be energized. Thirdly a twoway calibration switch 21 is provided. In one position this switch applies directly to the chopper 12 a D.C. voltage from the test unit 19 which is large enough to energize the alarm 15, while in its other position the switch 21 applies   directly to the chopper 12 a D.C. voltage which is insufficient in magnitude to energize the alarm 15. This last facility permits correct adjustment of the sensitivity of the system. The sensitivities of individual sensors may be adjusted by the application of suitable shunts across the sensor terminals. In a modification two alarm devices (15, 25), Fig. 6 (not shown), are provided, with separate level detectors (14, 24) responsive to different levels so that one alarm operates in the case of relatively low temperature increases while the other only operates when the higher temperatures associated with fire occur. A standby detector chain (26) may also be provided. This is connected so as to be normally inoperative but when the primary chain (22) breaks the standby chain (26) is brought into operation. A signalling system involving the use of an additional level indicator (28) indicates when the primary or secondary detector chains are broken.