GB642933A - Improvements in or relating to liquid level indicators - Google Patents

Abstract

642,933. Automatic control systems for liquid levels. SANDERS (ELECTRONICS), Ltd., W. H. March 24, 1948, No. 8690. [Class 38 (iv)] [Also in Groups XVIII and XXXVI] In a control or indicating system for liquid levels of the type wherein insulated control &c. electrodes are inserted in a tank, inaccuracies in operation due to the presence of leakage currents across the insulation of the electrodes is nullified by the provision of a second leakage path having the same electrical characteristics as the insulation of the electrodes. As shown in Fig. 3, this second path is provided by the insulation of a " guard " electrode 15. The level of liquid in a tank 31 is maintained between two levels determined by electrodes 16, 17 by a pump 32 driven by a motor 33 which is controlled by a relay 55 in the output circuit of a valve 53. The grid potential of the valve is controlled by a Wheatstone bridge 64 ... 67, the arms 66, 67 of which are controlled by the " guard " electrode 15 and the control electrodes 16, 17 respectively. The insulation resistances of the " guard " electrode and control electrodes 16, 17 with respect to the tank are arranged to be equal (as represented by dotted resistances 76, 75) so that the effects of leakage across the control electrode are balanced out in the bridge. When the level of the liquid reaches electrode 16, the balance of the bridge is upset to deenergize relay 55 and release contacts 46, 47, 73, 74. The pump 32 is thereby switched on and the bridge circuit transferred to electrode 17 to maintain the pump in operation until the level falls below electrode 17 when the bridge is rebalanced and relay 55 de-energized to stop the pump. Instead of providing two control electrodes, a single-level electrode may be used. Moreover as an alternative to automatic control, the system may be employed to operate one or more level indicators.