GB2254429A

GB2254429A - Liquid level sensor

Info

Publication number: GB2254429A
Application number: GB9107056A
Authority: GB
Inventors: L A Birt
Original assignee: BIRT ELECTRONIC SYSTEMS Ltd
Current assignee: BIRT ELECTRONIC SYSTEMS Ltd
Priority date: 1991-04-04
Filing date: 1991-04-04
Publication date: 1992-10-07
Also published as: GB9107056D0

Abstract

The liquid level circuit comprises two or more probes PR1, PR2 and a sensing circuit, the oscillation frequency of which is altered when the probes contact the liquid. The probes are electrically isolated from the circuit and connected across the secondary winding S2 of a transformer T1. The primary winding P1 of the transformer T1 forms a part of a tuned circuit 10 which oscillates at its resonant frequency except when the probes PR1, PR2 are immersed in the liquid 11: in the latter case the load impedance of the transformer T1 is altered and this causes a change in the frequency of oscillation, e.g. a cessation of oscillation. This change in oscillation is detected by an output circuit which delivers a DC output signal. <IMAGE>

Description

LIOUID LEVEL SENSOR This invention relates to a liquid level sensor.

It is known to sense the level of liquids using two electrically conductive probes. As the level of the liquid being sensed rises, the probes become immersed, and an AC current flows through the liquid between the two probes.

Sensing this current determines the presence of the liquid.

Lifeforms, bacteria, etc. present in the liquid may be very sensitive to this electric current flow. Also in flammable conditions it is undesirable to use electrical apparatus which may spark and ignite the liquid.

In the event of a fault in the sensing circuitry connected to the probes, the liquid may become live and thereby present a safety hazard.

We have now devised a liquid level sensor which eliminates these problems in so far as the sensor probes are isolated from the sensing circuitry and can be earthed if required.

In accordance with this invention there is provided a liquid level sensor comprising a sensing circuit connected to two or more probes and arranged to give a change in an electrical output from the sensing circuit once at least two of the probes contact the liquid, wherein immersing said probes in the liquid being sensed alters the frequency of oscillation of a portion of the sensing circuit.

Preferably the probes are electrically isolated from the sensing circuit, and are connected in pairs across the terminals of a transformer winding. The sensing circuit is preferably connected across a different winding of the transformer.

The transformer winding connected to the sensing circuit preferably forms a part of a tuned circuit which determines said frequency of oscillation.

Immersing a pair of probes in the liquid being sensed alters the load impedance across the transformer winding to which the probes are connected. This change in load impedance of the probe winding causes a change in the frequency of oscillation.

This change in frequency is detected and thus the presence of liquid at the probes is sensed.

In a preferred embodiment the oscillator is arranged to oscillate at its resonant frequency when the probes are not immersed. However, immersing the probes alters the impedance of the tuned circuit such that oscillation cannot be sustained.

Sensing the oscillation of the circuit therefore determines the absence of liquid at the probes, and once the probes are immersed the oscillation stops. In this embodiment only a negligible AC current will flow through the liquid due to the absence of oscillation of the sensing circuit.

Preferably a circuit is provided which gives a DC output signal according to the level of the liquid being sensed.

An embodiment of this invention will now be described by way of example only, and with reference to the accompanying drawing, the single figure of which is a circuit diagram of a liquid level sensor in accordance with the invention.

Referring to the drawing, there is shown a transistor oscillator having a tuned collector. The inductive component of the tuned circuit 10 comprises the primary winding P1 of a transformer T1; a tuning capacitor C1 is connected in parallel across the primary winding P1. One terminal of a first secondary winding S1 is fed back to the base of the transistor TR1. A bias resistor R1 is connected to the opposite terminal of the secondary winding S1.

Two electrically conductive sensor probes PR1, PR2, are connected across a second secondary winding S2 of the transformer T1.

An oscillation detecting circuit is coupled to the collector of the transistor TR1 via a DC blocking capacitor C2, and comprises diodes D1, D2, capacitor C3 and resistor R2.

In use, the tuned circuit 10 connected to the transistor TR1 output is arranged to oscillate at a high frequency e.g. 40 kHz. A portion of the output signal is taken via transformer secondary S1 and resistor R1 to the base of the transistor TR1. The total phase shift between transistor base and collector is 3600 and therefore oscillation is set up. The overall loop gain of the system is determined by the turns ratio of transformer primary to secondary winding Pl:S1, resistor R1 and the total load impedance of the tuned circuit 10. The loop gain of the system is chosen such that when the probes PR1 and PR2 are not immersed the circuit oscillates at the resonant frequency of transformer T1 and capacitor C1.

This signal is detected by C2, C3, D1, D2 and R2 to provide a DC output signal representing a LIQUID NOT PRESENT condition.

Typically water or water based liquids, e.g. 11, present a load impedance between the probes PR1 PR2 of approximately 1500 ohms at 40 kHz. When the probes PR1, PR2 are immersed the load impedance is presented across the terminals of the transformer secondary winding S2. This change in load impedance is reflected to the transformer primary winding in relation to the square of the turns ratio. This ratio is chosen such that the reflected load on transistor TR1 causes the loop gain to be decreased below the point where oscillation can be sustained. When oscillation stops the DC output signal falls to zero, indicating a LIQUID PRESENT condition. In this state, since the circuit is not oscillating, virtually no current flows in the secondary transformer winding S2 through the probes PR1, PR2 or through the liquid.

It will be appreciated that the output voltage of the detecting circuit may be amplified and conditioned so that any desired equipment can be controlled. The system may comprise a plurality of probes placed at required depths within a tank to allow automatic filling and depth control to be carried out.

Alternatively the probes may be positioned to inhibit the action of heating elements in the tank should insufficient liquid be present.

Because the probes are isolated from the sensing circuitry they can either be earthed or set at any potential.

Claims

1) A liquid level sensor comprising a sensing circuit connected to two or more probes and arranged to give a changeinan electrical output from the sensing circuit once at least two of the probes contact the liquid, wherein immersing said probes in the liquid being sensed alters the frequency of oscillation of a portion of the sensing circuit.

2) A liquid level sensor as claimed in Claim 1, in which the probes are electrically isolated from the sensing circuit, and are connected in pairs across the terminals of a transformed winding.

3) A liquid level sensor as claimed in Claim 2, in which the sensing circuit is connected across another winding of the transformer.

4) A liquid level sensor as claimed in Claim 3, in which the transformer winding connected to the sensing circuit forms a part of a tuned circuit which determines said frequency of oscillation.

5) A liquid level sensor as claimed in any preceding claim, in which said portion of the sensing circuit is arranged to oscillate at its resonant frequency except when the probes are immersed in the liquid.

6) A liquid level sensor as claimed in claim 5, in which said portion of the sensing circuit is arranged to oscillate except when the probes are immersed in the liquid.

7) A liquid level sensor as claimed in any preceding claim, comprising a circuit which gives a DC output signal according to the level of the liquid being sensed.

8) A liquid level sensor as claimed in any preceding claim, comprising a plurality of probes placed at selected depths within a tank for liquid.

9) A liquid level sensor substantially as herein described with reference to the accompanying drawing.