GB1309542A - Liquid level sensors - Google Patents

Abstract

1309542 Liquid level sensor W B BARTON 18 March 1970 [19 Dec 1968] 60440/68 Heading G1H A liquid level sensor includes a pair of electrodes 2 for contacting a liquid in a container; means for applying alternating current across said pair of electrodes 6 a first rectifying circuit arranged to pass one half cycle of the alternating current when the level of the liquid contacts both electrodes; a second rectifying circuit arranged to pass the other half cycle of the alternating current; means for balancing the two rectifying circuits so as to produce a balanced series of positive and negative half cycles of current through the electrodes; and means for indicating that current is passing through the liquid. A pair of liquid level sensing electrodes 2 are connected in series with the secondary coil of step down transformer 6 and the base electrode of transistor 12 through rectifier 8 and resistor 11. The base circuit of the transistor is completed through resistor 18 to the secondary of transformer 6. A second rectifier 10 and variable resistor 14 are connected in shunt across the rectifier 12, resistor 11 and rectifier 8 so that with liquid conducting between the electrodes only positive half cycles pass through transistor 12 but positive and negative half cycles pass through the electrodes thus reducing corrosion of the electrodes. Variable resistor 14 (and a second transistor (20) of different temperature characteristics, Fig. 2 (not shown)) provides an impedance which may be matched to that of the base-emitter circuit to cause the positive and negative cycles passing the electrodes to be equal. In Fig.3 (not shown) the electrodes are connected in parallel to rectifier 8, transistor 12 and to rectifier 10 so that transistor 12 normally conducts but is short circuited (and thus switches off) when liquid bridges the electrodes.