GB999750A - Capacitance measuring apparatus - Google Patents

Abstract

999, 750. Capacitance measurement; determining liquid level. STANDARD TELEPHONES & CABLES Ltd. Oct. 16, 1963, No. 40843/63. Headings G1N and G1U. [Also in Division H3] An apparatus for measuring a capacitance C x in the presence of a large static capacitance C s comprises a pulse generator (GEN) which provides a continuous train of pulses of given polarity to charge the capacitor to be measured, which is contained in the charge circuit 14, and also to charge a second capacitor C b in a second charge circuit 18. The charge circuits are such that the capacitors are fully charged during each pulse. Discharge circuits 16 and 20 for the capacitors C x and C b are arranged such that the capacitors will be fully discharged between pulses, and it is arranged that the discharge current for C x and the charge current for C b pass in opposite directions through a common path which includes the circuit of a direct current meter 6. The capacitor C b is adjusted so that its charge current will balance the discharge current of the static capacitance of the system contained in parallel with C x , so that the reading of the meter 6 indicates the value of C x , the capacitance to be measured. Diodes 26, 36, 56 and 62 are included in the various charge and discharge circuits to ensure that charge and discharge currents flow only in the allotted paths. For the purpose of adjusting the meter, a test capacitor of known value may be incorporated into the circuit in place of C x by means of a relay. The meter range may be adjusted by means of a tapped potentiometer 48. The pulse generator is suitable a transistorised multivibrator (see Division H3). The apparatus is particularly adapted for use in a liquid level measuring arrangement. Liquid level is detected by means of a capacitive probe 77 which is attached to a housing 76 containing most of the circuitry of the invention, including the pulse generator embedded in plastic in the unit 23, and balance and test capacitors. This renders the circuitry rugged and reliable and not affected by humidity changes. The balance capacitor C b may be adjusted from outside the housing by a shaft 122, and the test capacitor C t is similarly adjustable. A cable 78 from the housing passes to a unit 79 which contains the meter and its associated circuit components, the power supply for the generator, and the necessary control switches.