GB1487101A - Apparatus for measuring a variable impedance such as liquid conductivity - Google Patents

Abstract

1487101 Resistance measuring bridges A F MILLS 3 Oct 1974 [14 May 1974 2 Oct 1974] 21231/74 and 42824/74 Heading G1U For measuring a variable impedance, a transformer applies an output signal from a relaxation oscillator to the impedance, any change in the impedance causing a change in the signal which is detected. The arrangement is used to measure the concentration of a detergent solution. The four arms of a bridge are composed respectively of R6; R7; C2 in parallel with R5 ; primary winding of T 1 in series with the collector-emitter junction of Tr2. A conductivity cell 1 is connected across the secondary winding of T1. A relaxation oscillator comprised of capacitor C1 charging via resistor R1 and discharging through a programmable unijunction transistor Tr 1 has its output frequency and amplitude determined by R3 and RV1 with a diode D1 providing temperature compensation. The oscillator output pulses 5 are fed to the transistor Tr2 which thus supplies an approximate squarewave voltage to the cell 1. C 2 and R 5 stabilize the voltage at the transistor emitter and this is compared with the reference voltage at the junction of R6 and R7 by a meter M1. The arrangement is calibrated by inserting the cell in a solution of known concentration and adjusting RV1 to give centre-scale deflection (about 25ÁA) on M1. Any change in the cell 1 resistance due to change in conductivity of the detergent solution causes a change in the collector load of Tr2 and thus a change in emitter voltage. Also, the change in collector load is reflected through the transistor appearing as a change in the oscillator load, which affects the oscillator frequency and amplitude, accentuating the change in emitter voltage. The battery supply may be tested by closing PB1 when S1 is "OFF".