GB2070773A

GB2070773A - Indicating liquid levels

Info

Publication number: GB2070773A
Application number: GB8006785A
Authority: GB
Original assignee: COMMW SMELTING Ltd
Current assignee: COMMW SMELTING Ltd
Priority date: 1980-02-28
Filing date: 1980-02-28
Publication date: 1981-09-09

Abstract

A method and apparatus for measuring the level 3 of an electrically- conductive liquid in a container 2, involves using a conducting weight 6 on the end of an electrically-conducting cable 4, paid out from a winch drum, and measuring the length of cable paid out when the weight makes contact with the liquid surface, resulting in a flow of electrical current through the cable. <IMAGE>

Description

SPECIFICATION Improvements in or relating to level-measuring devices This invention relates to level-measuring devices and more especially to devices for measuring the levels of electrolytes in containers.

A known level-measuring instrument has a weight or float, suspended from the end of a cable, which is lowered into a tank or hopper by means of electrically-driven winch drum. When the weight or float contacts the surface of the material in the container, the cable becomes slack and the winch drum motor is automatically reversed. At this point, the length of cable paid out is automatically measured and the level of the material in the container is computed.

The cable is then wound again on to the winch drum and the operation can be repeated. Such an instrument is usually mounted on the roof of the container.

Where the material in the container is a liquid and more especially a corrosive liquid, e.g. a mineral acid, certain problems arise associated with the corrosive nature of the liquid and the tendency for the liquid to stick to and corrode the float, cable and drum. Further, there is difficulty in detecting instantaneous contact between the weight or float and the material in the container.

It is an object of the present invention to at least partially overcome or ameliorate the above listed disadvantages.

This invention, in one aspect, consists in a method of measuring the level of an electrically-conductive material in a container, wherein a weight or float at the end of a cable is lowered into the container to make contact with the upper surface of said material, characterised in that the cable incorporates an electrical conductor, that the weight or float is also electrically conducting and that surface contact is indicated by a flow of electrical current through the cable.

In a second aspect, the invention consists in apparatus for measuring the level of an electricallyconductive material in a container, comprising an electrically-conducting weight or float suspended from the end of an externally-insulated electricallyconducting cable, a winch-drum for winding said cable in and out, a reversible electric motor for driving said winch drum, means for stopping said motor and for determining the length of cable unwound from said drum when an electrical current is detected flowing through said cable.

Preferably the end of the cable remote from the weight (or float) is connected to an insulated slipring and hence via a contact arm to a source of electrical potential.

Preferably the winch drum is attached to a pulse counter which measures the distance of rotation of the drum as the cable is unwound from the winch drum.

Alternatively an analogue device may be connected to the cable drum to produce an analogue display of level.

Preferably a marker is attached to the insulated cable, in order to actuate a "park switch" when the weight is close to the top of the container (or in any desired reference position).

Preferably the unwound cable from the drum passes over a spring-supported pulley, so that should the cable become jammed by any means, the spring supported pulley is pulled down and actuates an override switch to cut out the drive motor of the cable winch. The advantages of the level-measuring device according to this invention, compared with the prior art device, can be summarized as follows: (1) It is more precise. In the prior art device detection of slack in the cable is not instantaneous whereas detection of an electrical current is instantaneous.

Further, the accuracy of the instrument is limited only by the design of the pulse counter which can be designed to record e.g. 1 pulse per inch or 1 pulse per centimetre.

2. With the weight sensitive device, the instrument head needs to be positioned above the access hole in the container. This is because the heed for additional pulleys and longers run of cable necessary to mount the instrument in an alternative position can give rise to spurious level detections.

Using the conductivity device described above, the weight is only used to keep a tension on the cable and the head unit can be mounted in any desired position. This is particularly useful when the instrument is used to measure the level in a dusty of fumytank, e.g. a sulphuric acid tank. The instrument can be mounted at the base of the tank, away from any dust or fume.

(3) When the known instrument is used to measure the level of a corrosive liquid, and is placed above the access hole, the payed out cable becomes wet with condensate and this contaminated cable is re-wound into the instrument with consequent difficulties due to corrosion of the internal parts of the instrument.

If the instrument is mounted at the base of the tank, as an instrument incorporating this invention can be, then the payed out cable is never re-wound into the instrument.

The invention will now be further described with reference to the accompanying drawings in which Figure 1 is a perspective view of apparatus according to the invention and Figure 2 is a view of the location of such apparatus when used to measure the level of liquid in a storage tank.

With reference to Figure 2, ievel-measuring apparatus, generally designated 1, is mounted at the base of a storage tank 2 containing an electrically conducting liquid up to level 3. A length of cable 4 from the measuring apparatus 1 passes over pulleys 5 and has attached to the free end thereof an electrically conducting weight or bob 6.

With reference to Figure 1 the other end of the cable is attached to and is wound on to winch-drum 7, which is driven, via the shaft 8 by the reversible electric motor 9. The other end of shaft 8 carries a counting disc 10 which revolves with the shaft, within the slotted switch 11. The drum-end of the cable is connected to the insulated slip-ring 12 which makes spring-contact with source of electric poten tial via the contact arm 13. A "park switch" is provided at 14 and an over-ride limit switch at 15.

The cable 4 passes over pully 16 which is mounted on pivoted arm 17 which is upwardly spring-biassed by means of spring 18.

In operation, the electric motor 9 is caused to rotate in the sense which allows cable 4 to unwind from the drum 7. The bob or weight 6 is thus lowered within the tank 2 until it makes contact with the liquid at level 3. Provided that the liquid is electricallyconducting, an electrical current passes via slip-ring 12 and contact arm 13 through cable 4 and weight 6 and to earth via liquid in tank 2. As soon as this current flows, the motor is switched off and the count of the counting disc 10 as measured by slotted switch 11 is recorded. This is directly proportional to the length of cable unwound from drum 7 and is therefore a measure of the distance of fall of weight 6 from a fixed reference point In this way the level of liquid 3 in tank 2 is computed relative to the said fixed reference point The "park switch" 14 can be actuated by a marker or contact on cable 4. The override switch 15 is actuated, should the cable 4 become jammed by any means, by the spring-biassed pulley 16 being pulled down to actuate the switch 15 and cut out the electric motor 9. The "park switch" can be used to set the reference point from which the length of cable paid out from drum 7 is measured.

Claims

1. A method of measuring the level of an electrically-conductive material in a container, wherein a weight or float at the end of a cable is lowered into the container to make contact with the upper surface of said material, characterised in that the cable incorporates an electrical conductor, that the weight or float is also electrically conducting and that surface contact is indicated by a flow of electrical current through the cable.

2. Apparatus for measuring the level of an electrically-conductive material in a container, comprising an electrically-conducting weight or float suspended from the end of an externally-insulated electrically-conducting cable, a winch-drum for winding said cable in and out, a reversible electric motor for driving said winch drum, means for stopping said motor and for determining the length of cable unwound from said drum when an electrical current is detected flowing through said cable.

3. Apparatus as claimed in claim 2 wherein the shaft driving the winch drum is connected to a pulse counter which serves to measure the distance of rotation of the drum as the cable is unwound therefrom.

4. Apparatus as claimed in claims 2 or 3 in which the end of the cable remote from the weight is connected to an insulated slip-ring and hence via a contact arm to a source of electrical potential.

5. Apparatus as claimed in claims 2 to 4 in which a marker or contact is attached to a point on the insulated cable to actuate a park switch when the weight is close to the top of the container, or some other desired reference position.

6. Apparatus as claimed in claims 2 to 5 in which the cable leaving the drum passes over a springsupported pulley which, in the event of cable jamming, is pulled down to actuate an override switch which cuts out the drive motor.

7. A method of measuring the level of an electrically-conductive liquid in a container substantially as herein before described.

8. Apparatus for measuring the level of an electrically-conductive liquid in a container substantially as hereinbefore described with reference to the accompanying drawings.