GB912984A - - Google Patents

Abstract

912,984. Indicating liquid levels. MINNEAPOLISHONEYWELL REGULATOR CO. Dec. 24, 1958, No. 41729/58. Class 123 (1). [Also in Group XXX] In a fluid-level indicator comprising a modified bridge circuit 15 with a sensing arm formed by a negativetemperature coefficient resistor 40, which can be mounted, e.g. in a fluid container 49 at such a level as to be able to unbalance the bridge whenever the fluid passes that level, a compensating element 48, consisting of a second negative-temperature coefficient resistor 45 sealed in an air-filled envelope, is incorporated in the bridge so that it can operate effectively even when dealing with relatively hot liquid and cold air. The bridge comprises impedance arms 20, 30, 35 and the resistor 40 which, when immersed in liquid, has a voltage drop across it to maintain the bridge balanced so that no current flows from a point 26 to a point 21 through a detector arm having a relay 55 and a rectifier diode valve 56. When the liquid level falls below the resistor 40 the voltage across the latter, now in air, decreases to unbalance the bridge so that current, flowing in the detector arm, actuates the relay 55. The compensating circuit comprises a diode 50 which, irrespective of the amount of current flowing through it, provides a substantially constant voltage drop between the point 26 and a point 31 between an impedance 30 and the sensing resistor 45. The compensating resistance 45 at substantially the same temperature as the resistor 40 when it is immersed in liquid, raises the potential of the point 31 and thus the higher potential at the more positive side 26 of the diode 50 by an amount equal to the voltage drop across the resistor 40 in air. This compensates for the otherwise lack of potential difference between the points 21, 26 when the voltage across the resistor 40 does not drop sufficiently, upon emerging from the liquid, to actuate the relay 55. The potential drop across the diode may be adjusted by selecting a diode of the correct impedance or by inserting several diodes in series until the voltage level of the point 26 is properly located with respect to the point 31 that is so that it is higher or lower than the point 21 whenever the resistors 40, 45 are in air or immersed in liquid respectively.