GB1155481A - A Supervisory System including a Measuring Bridge - Google Patents

Abstract

1,155,481. Measuring bridge circuits; methanometers. CHARBONNAGES DE FRANCE. June 3, 1966 [June 4, 1965], No.24916/66. Headings G1N and G1U. [Also in Division H3] A measuring bridge incorporating an impedance controlled by a parameter to be supervised is energized intermittently from a D.C. supply, and the variation in the output signal from the bridge when the supply is interrupted is transformer coupled to a tripping device which will initiate a tripping action for operating an alarm (or protection device) for all values of the parameter above a given threshold. The tripping device is unresponsive to any output signal occurring during a time immediately following connection of the bridge, so that initial reverse voltage spikes do not operate the alarm. In a bridge of the type containing a hot filament used in determining the content of methane in an atmosphere by catalytic combustion, the output increases in one direction as the methane content increases and then, above a certain content, sharply inverts to provide an output of the opposite polarity. To use the bridge to detect methane contents above a certain (low) level, outputs in the one direction above a certain threshold and all outputs in the other direction must operate an alarm. The bridge 1 containing the filament 13 and forming part of a supervisory fire damp meter is pulsed periodically by a D.C. supply Va switched on by a system 2 incorporating a multi-vibrator (T1, T2, Fig. 5) whose output is amplified and passed via one of a set of capacitors (C1 . . . . C N ) to a further capacitor (C4) which charges over a number of cycles, until a trigger circuit (T 5 , T 6 ) is operated. The further capacitor then discharges (via a transistor T4) to pulse on a transistor (T7) in the bridge circuit supply. The bridge is thus energized by way of a regulating circuit 3 (T8) for the period set by the trigger circuit. Thus the bridge is interrogated periodically at times determined by the chosen capacitor (C1 . . . . C N ). The change in output from the bridge at the end of an interrogation is fed via a transformer 4 to a trigger circuit 5 D (T9, T11), which triggers if the signal is above a positive threshold set from a potentiometer 5 ID (P1), and via an inverter (T13) to a trigger circuit 5 I (T10, T11), which triggers if the signal is negative. The trigger circuit is prevented from operating for a time period after the bridge supply is switched on by the arrangement 52 (C5-R6-D4), which provides a negative voltage (to T11 base) for a time less than that for which the bridge is switched on. When the trigger circuit operates, a relay 7 (Re) is energized.