GB2094003A

GB2094003A - Detecting the level of an electrically conductive liquid

Info

Publication number: GB2094003A
Application number: GB8106691A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-03-03
Filing date: 1981-03-03
Publication date: 1982-09-08
Also published as: GB2094003B

Abstract

A device for detecting the level of an electrically conductive liquid, comprises a first probe 5 contacting the liquid when the liquid level is at or above an upper limit, a second probe 6 contacting the liquid when the level is at or above a lower limit, at third probe 7 contacting the liquid when the level is below the lower limit and a circuit comprising an AC power supply 1, a current operated switch such as a relay 14, 15 operated by a current at or above a first limit and maintained in its operated condition by a current which is at or above a second, lower limit, and two gate-controlled semiconductor switches such as triacs 3, 4 in series between the power supply 1 and relay 14, 15. When the first and second probes 5, 6 are in the liquid both the triacs conduct by virtue of current passing through the liquid to operate relay 14, 15, and when the second probe 6 is immersed but the first probe 5 is uncovered a current supplied via the liquid and the gate of triac 4 is sufficient to maintain, but insufficient to operate, relay 14, 15. <IMAGE>

Description

SPECIFICATION A device for detecting the level of an electrically conductive liquid The present invention relates to a device for detecting the level of an electrically conductive liquid.

It is often necessary to maintain the level of a liquid in a container between upper and lower limits.

This requires a device capable of detecting when the liquid level goes above the upper limit or below the lower limit. The output of the device may be used for example to control a pump to supply more liquid into the container when the level drops below the lower limit, or to draw liquid from the container when the level rises above the upper limit.

A liquid level detector device is known which provides the required level indication. This device relies upon detecting current passed through the liquid to a first probe located at the upper level, and to a second probe located at the lower level, from a third probe which is located below the lower level. Current through the first probe indicates that the liquid is at or above the upper level, and no current through the second probe indicates that the liquid is below the lower level. Alternating current is used to avoid polarisation problems at the probes.

The known device performs satisfactorily but is relatively complex, requiring significant numbers of components to sense and respond to the probe currents. This means that the device is relatively expensive and also bulky, both of which factors are of significance, particularly when considering a system requiring a substantial number of such devices.

It is an object of the present invention to provide a liquid level sensing device which is extremely simple and which can be contained within a relatively small enclosure.

According to the present invention, there is provided a device for detecting the level of an electrically conductive liquid, comprising a first probe arranged to electrically contact the liquid when the liquid level is at or above an upper limit, a second probe arranged to electrically contact the liquid when the liquid level is at or above a lower limit, a third probe which is arranged to electrically contact the liquid when the liquid level is below the lower limit, and a circuit connected to the three probes for detecting when the liquid is at or above the upper limit and when the liquid is below the lower limit, characterised in that the circuit comprises an AC power supply, a current operated switch which is arranged to be operated by a current at or above a first current limit and to be maintained in its operated condition by a current which is at or above a second lower current limit, and two gate-controlled semiconductor switches connected in series between the power supply and the current controlled output switch, the gate of the semiconductor switch located between the power supply and the other semiconductor switch being connected to the first probe, and the gate of the other semiconductor switch being connected to the second probe, whereby when the first and second probes are immersed in the liquid both the semiconductor switches are rendered conductive by current passing through the liquid to operate the output switch, and when the second probe is immersed but the first probe is uncovered by the liquid current supplied via the liquid and the gate of the said other semiconductor switch is sufficient to maintain the output switch operated but insufficient to operate the output switch.

Preferably, the AC source is a mains powered transformer and the semiconductor switches are triacs.

The second and third probes may be connected by a resistor to provide increased gate current to the said other semiconductor switch.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings.

Referring to the accompanying drawing the liquid level detector device comprises a mains powered transformer 1, the secondary winding 2 of which is connected in series with two triacs 3 and 4. The triacs 3 and 4 are controlled by the current through their respective gates G which are connected to respective ones of a high liquid level probe 5 and a lower liquid level probe 6. Each of the probes 5 and 6 is electrically connected through the liquid to a further, permenantly submerged probe 7 when immersed by the liquid. The further probe 7 is connected to the junction of the secondary winding 2 and the triac 3 and, via a resistor, to earth. The probes 6 and 7 are also connected by a resistor 8.

The output of the triac circuit described is bridge rectified by diodes 9 to 12 and smoothed by a capacitor 13. The resultant current is supplied to a relay coil 14 controlling relay contacts 15. If the current through the coil 14 reaches a first predetermined level, the contacts are operated. The contacts remain operated until the current level drops below a second predetermined level which is lower than the first. The switch 15 can be used to control the level of liquid by switching on or off, for example, a pump which raises or lowers the level of liquid.

Operation of the above described embodiment of the present invention is as follows.

When the level of liquid in a container (not shown) is such as to immerse both probes 5 and 6, currents are supplied to the respective gates G of triacs 3 and 4 through the liquid. Since the currents applied to the gates of triacs 3 and 4 are in phase with the supply on terminals T1 of triacs 3 and 4 both triacs 3 and 4 conduct As a result an AC voltage is applied to the bridge rectifier circuit of diodes 9 to 12. The resultant DC current supplied to coil 14 causes the relay contacts 15 to switch. The relay contacts 15 when the switched may switch off a pump to prevent the liquid level being raised further or switch on a pump to actively lower the liquid level.

As the liquid level falls the high liquid level probe 5 is uncovered and the supply of current to the gate of triac 3 ceases. As a result triac 3 switches off, which disconnects the voltage applied to terminal T1 of triac 4 which in turn also switches off.

Despite both triacs 3 and 4 switching off the relay contacts 15 are still held in position by a "hold on" current above the second predetermined level which is supplied through the gate G and terminal T2 junction of triac 4. The "hold on" current is supplied to the gate of triac 4 via probes 6 and 7, electrically connected through the liquid, and resistor 8. The "hold on" current ensures that the relay contacts 15 are held in position.

As the liquid level continues to fall it eventually uncovers probe 6 which as a result is disconnected from probe 7. The current supplied through the resistor 8 is insufficient by itself to hold on the relay contacts 15 which as a result switch. Once the relay contacts 15 switch they may switch on a pump which starts to raise the liquid level or switch off a pump which is actively lowering the liquid level.

As the liquid level begins to rise it immerses probe 6 again, and as a result current again flows via probes 6 and 7 through the liquid to the gate G of triac 4. However, the current through the gate G and terminal T2 junction of triac 4 is below the first threshold level and is thus not sufficient to switch the relay contacts 15 and as a result the liquid level continues to rise.

When the liquid level reaches probe 5, both triacs 3 and 4 conductthe relay contacts 15 switch again.

The above described sequence of operations is then repeated.

It will be appreciated that the relay circuit for switching on and off the liquid level raising or lowering means is conventional. In this respect in the embodiment shown the relay is a DC relay because DC relays are physically small, however AC relays may also be used. Alternatively an appropriate semiconductor switch could be used.

The circuitry may be mounted on a conventional liquid level controller plug-in module to form a direct replacement. The space required for the components is less than in a conventional controller and accordingly the casing may be much reduced in size.

Claims

1. A device for detecting the level of an electrically conductive liquid, comprising a first probe arranged to electrically contact the liquid when the liquid level is at or above an upper limit, a second probe arranged to electrically contact the liquid when the liquid level is at or above a lower limit, a third probe which is arranged to electrically contact the liquid when the liquid level is below the lower limit, and a circuit connected to the three probes for detecting when the liquid is at or above the upper limit and when the liquid is below the lower limit, characterised in that the circuit comprises an AC power supply, a current operated switch which is arranged to be operated by a current at or above a first current limit and to be maintained in its oper ated condition by a current which is at or above a second lower current limit, and two gate-controlled semiconductor switches connected in series bet ween the power supply and the current controlled output switch, the gate of the semiconductor switch located between the power supply and the other semiconductor switch being connected to the first probe, and the gate of the other semiconductor switch being connected to the second probe, whereby when the first and second probes are immersed in the liquid both the semiconductor switches are rendered conductive by current passing through the liquid to operate the output switch, and when the second probe is immersed but the first probe is uncovered by the liquid current supplied via the liquid and the gate ofthe said other semiconductor switch is sufficient to maintain the output switch operated but insufficient to operate the output switch.

2. A device according to claim 1, wherein the AC source is a mains powered transformer.

3. A device according to claim 1 or 2, wherein the semiconductor switches are triacs.

4. A device according to any preceding claim, wherein the second and third probes are connected by a resistor.

5. A device according to any preceding claim, wherein the current operated switch is a DC relay connected to the output of a bridge rectifier.

6. A device according to any preceding claim, wherein the third probe is connected to earth via a resistor.

7. A device for detecting the level of an electrically conductive liquid, substantially as hereinbefore described with reference to the accompanying drawing.