MEASURING CELLS FOR MEASURING ELECTRICAL CONDUCTIVITY OF LIQUIDS

摘要

1517697 Conductivity measurement GEORGE KENT Ltd 22 July 1975 [2 Aug 1974] 34210/74 Heading G1N Liquid conductivity is measured in a cell comprising co-axial annular electrodes by determining the alternating current between two electrodes 1, 4 necessary to maintain constant the voltage between electrodes 2, 3, a fifth electrode 5 being located on the side of electrode 1 remote from the voltage electrodes 2, 3 and connected to electrode 1 by an amplifier which presents a low impedance to electrode 5 and a high impedance to electrode 1. Leakage from the fifth electrode thus connected does not affect the measurement. Preferably a sixth electrode 6, placed on the side of electrode 5 remote from the other electrodes and connected to electrode 4 and earth is also provided as an earth return path for leakage from electrode 5. As shown, the voltage between electrodes 2 and 3 is compared at 26 with a reference 28 and the difference drives voltage controlled generator 16 to alter the current supplied between cell electrodes 1, 4 by secondary 12. The current passes through resistors 18, 20 and the voltage across these, which is a measure of the liquid conductivity is amplified at 36, 38, supplied to phase sensitive rectifier 40 which uses the reference voltage from amplifier 24 as a reference and the output is smoothed at 42. Temperature compensation. Thermistor 34, in parallel with resistor 20, is arranged in the liquid. Alternatively a temperature sensor in the liquid may control the gain of amplifier 36. Range limitation. A portion of the reference voltage may be tapped off from potentiometer 48 and applied to amplifier 38 which then functions as a difference amplifier. This permits improved sensitivity over a small range. Bell construction. The electrode surface is preferably flush with the insulating material of the cell wall. The axial separation between first and fourth, first and fifth, and, fifth and sixth electrodes is preferably equal. The second and third electrodes are preferably symmetrically located between the first and fourth electrodes and mutually spaced by half the first and fourth electrode separation.