GB814888A - Improvements in or relating to the measuring of variables - Google Patents

Abstract

814,888. Electric tests. KENT Ltd., G. Sept. 30, 1955 [Oct. 4, 1954], No. 28566/54. Class 37. The potential difference between the electrodes of a pH cell is measured by means of a condenser which is connected alternately to one electrode and one side of the input to the measuring network and to the other electrode and the other side of the measuring network. As shown, the electrodes GE, RE are immersed in a liquid of which the pH value is to be measured and which is earthed. The measuring network consists of an amplifier tube V feeding an amplifier A the output of which feeds a motor M to regulate the output of a potentiometer P. The bucket condenser C1 is connected to movable contacts K1, K2 which move respectively between connections D and B to the two electrodes and D1, B1, the amplifier input terminals. The potentiometer output is applied across B1 and to a further movable contact E1 which also connects the potentiometer output periodically to D1. Initially condenser C1 is connected between contacts B and B and is then transferred to contacts D and D1, the connection to contact D is broken and movable contact E is closed to fixed contact D1. If the voltage across input resistor Rg remains constant during a cycle the potentiometer output voltage is a measure of the difference between the electrode potentials and is independent of their potential to earth. If the potentiometer output differs from the electrode P.D. an alternating signal is produced the phase sense of which depends on whether the potentiometer output is too great or too small and which is used to drive motor M to reset the potentiometer. The movement of the motor is an indication of the required P.D. When the potentiometer has a low internal impedance it may be connected. in the line joining primary contact B1 to Rg (Fig. 3, not shown) or, in series with a further resistor Rb across resistor Rg when the travelling contact may be omitted. In a still further embodiment (Fig. 5, not shown), the potentiometer is inserted in the earth line and a fixed resistor replaces the moving contact to join contact D1 to earth. Specification 695,909 is referred to.