Device for checking the level of a liquid in a tank

Claim 2

Apparatus for monitoring the level of a liquid in a tank, comprising: an electrical resistance probe having a resistance value which increases with temperature, said probe arranged to be immersed to a level dependent upon the liquid level in said tank such that, when said probe is supplied with a constant current, the temperature and the resistance of said probe increase in dependence on the amount of said probe which is immersed; means energizable for supplying a constant current through said probe to produce a voltage across said probe dependent on the temperature of said probe; and means for comparing the initial voltage across said probe upon energization of said constant current supplying means with the voltage across said probe at a predetermined time after energization of said constant current supplying means, and for providing an indication dependent on said comparison, said indication being representative of the liquid level in said tank and being substantially independent of ambient temperature.

Claim 3

The apparatus of claim 1, wherein said comparing means comprises means for detecting and storing a representation of said initial voltage, and for detecting said voltage across said probe at said predetermined time; and a comparator for receiving and comparing said voltages and providing said indication.

Claim 4

The apparatus of claim 2, wherein said detecting and storing means provides an output voltage which decays in time from said initial voltage, and wherein said comparing means compares said voltage measured across said probe at said predetermined time with said output voltage which decays in time.

Claim 5

The apparatus of claim 3, wherein said detecting and storing means comprises a resistance-capacitance discharge circuit, said capacitance being charged to said initial voltage.

Claim 6

The apparatus of claim 2, wherein said detecting and storing means comprises an analog memory for storing said initial voltage value, and said comparator indication is continuously provided and comprises the difference between the instantaneous voltage across said probe and said stored representation of said initial voltage, said monitoring apparatus further comprising means for comparing said comparator indication with a predetermined number of reference voltages corresponding to respective different liquid levels.

Claim 7

The apparatus of claim 1, wherein said comparing means comprises: a source of clock pulses; first and second controllable means for counting said clock pulses; means for converting the digital output of said first counting means to an analog signal; a comparator having a first input coupled to receive the voltage across said probe, a second input coupled to receive the analog output of said converting means, and an output coupled to an input of a controlling means; and means responsive to the output of said comparator for selectively controlling said first and second counting means such that said controlling means initially enables said first counting means upon energization of said constant current supplying means until the output of said comparator indicates that the output of said converting means equals said initial voltage, and then disables said first counting means such that a count representative of said initial voltage is stored in said first counting means, and such that said controlling means, at said predetermined time after energization of said constant current supplying means, enables both said first and second counting means until the output of said comparator indicates that the output of said converting means equals the voltage across said probe at said predetermined time after energization of said constant current supplying means, whereby said second counting means provides a digital representation of the liquid level in said tank, substantially independent of ambient temperature.

Claim 8

The apparatus of claim 6, wherein said comparing means further comprises delay means coupled to said source of clock pulses and to said controlling means for defining the interval of said predetermined time.

Claim 9

The apparatus of claim 1, further comprising means for displaying the measured liquid level as provided by the indication from said comparing means.

Claim 10

The apparatus of claim 1, further comprising means for giving a warning if the measuring liquid level is below a predetermined level.

Claim 11

The apparatus of claim 8, further comprising means for holding the indication provided by said comparing means at the end of a measurement.