GB2119101A

GB2119101A - Liquid level sensor

Info

Publication number: GB2119101A
Application number: GB08311485A
Authority: GB
Inventors: Christopher David Dobson
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-04-29
Filing date: 1983-04-27
Publication date: 1983-11-09
Also published as: GB8311485D0

Abstract

Apparatus for determining the level of liquid nitrogen in a container 18 comprises a detector 11 and combined power source and meter 12 connected by cables 13. The detector 11 includes a length of electrical conductor 14, which is held in a position such that a first portion 15 projects above and a second portion 16 lies below the surface level. When a current is passed through the conductor differential cooling occurs in portions 15 and 16 and the resultant change in resistance in the conductor is detected to give an indication of the height of the liquid in the container. The conductor 14 is preferably formed by winding a circular coil and then twisting the turns about a diameter of the coil so that it may be inserted in a tube 22. <IMAGE>

Description

SPECIFICATION Apparatus and method for determining the level of a liquid in a container This invention relates to apparatus and methods for determining the level of a liquid in a container and to detectors for use in such apparatus and methods.

Liquid nitrogen is used in many industrial proces ses. It is often crucial that the nitrogen does not run out and frequently it is only possible to detect the level of the nitrogen in a container by means of a remote sensor. However, as the boiling point of nitrogen is 77"K, many conventional liquid level detectors will not operate. At present it is common to use a pair of thermocouples located at respective ends of the container. The object is to detect the emptying of the container by monitoring the temper ature difference detected by the thermocouples.

However, this arrangement does not work particular ly effectively, because the temperature of the gaseous nitrogen is not significantly higher than that of the liquid nitrogen. In addition this arrangement only indicates when the nitrogen has fallen below withe level of the lower thermocouple and does not provide a constant indication of liquid level.

It is an object of the invention to provide an improved detector for determining the level of a liquid in a container.

From one aspect the invention consists in appar atus for determining the level of a liquid in a container, comprising an electrical conductor dimen sioned to project above a predetermined liquid level in a container, means for passing a current through the conductor such that a first portion of the conductor, which extends above the surface of the liquid, heats more rapidly than the second portion which is immersed in the liquid and means for measuring the total resistance of the conductor or an indication thereof to obtain an indication of the length of the second portion and hence the depth of the liquid.

The conductor may be constituted by an elongate sensor and it may be bent or wound so that the length of the conductor is greater than the length of the sensor. In this case the length of the conductor may be between 20 and 200 times the length of the sensor. In a preferred embodiment the conductor is a coil wound about an axis extending generally orthogonally to the axis of the sensor and the coil is twisted along a diameter thereof.

If the conductor is wound as a coil it may either be wound about the axis of the conductor or about an axis transverse to the conductor's axis. Alternatively the conductor may be formed in a zigzag or may be concertinaed.

Preferably the conductor has an electrically in sulating coating, for example a ceramic coating. The conductor may have a temperature coefficient of resistance in the range 3.5 to 4.0 x 10-3"C-'. The conductor may be copper or iron or any other conductor with a high thermal coefficient of resis tivity.

From a second aspect the invention consists in a detector for determining the level of liquid in a container comprising an elongate sensor constituted by an electrical conductor bent or wound so that the length of the conductor is greater than the length of the sensor.

The conductor may have a constant cross-section, for example it may have diameter of between 0.1 and 0.5 mm. In a preferred embodiment the diameter may be 0.15. In a preferred embodiment the conductor is a coil would about an axis generally orthogonal to the axis of the sensor and the coil is twisted about a diameter. The conductor is contained within a protective tube, which is open at one end. It will be appreciated that in this construction the diameter of the coil must be at least equivalent the maximum depth of liquid, which is required to be measured.

From a third aspect the invention consists in a container for containing liquid nitrogen incorporating apparatus as defined above or a detector as defined above.

From a fourth aspect the invention consists in a method of determining the level of liquid in a container comprising inserting an electrical conductor into the container so that a part of the conductor projects above the level of liquid in the container, passing current through the conductor and monitoring the resistance of the conductor or an indication thereof to obtain an indication of the length of the part immersed in the liquid and hence the depth of the liquid.

The invention may be performed in various ways, a specific embodiment of which, with modifications, will now be described with reference to the accompanying drawings, in which: Figure 1 is a view of apparatus for determining the level of a liquid in a container, with the detector of the apparatus inserted in a container; and Figures 2, 3 and 4 show various embodiments of the conductor of the apparatus of Figure 1.

Referring to Figure 1 the apparatus for determining the level of a liquid in a container is generally indicated at 10. This apparatus comprises a detector 11 and a combined power source and meter 12. The detector is connected to the power source/meter 12 by cables 13.

As will be described in greater detail below, the detector 11 includes a length of electrical conductor 14, which is held in a container, in a predetermined position, such that a first portion 15 projects above the level of the liquid in the container and a second portion 16 lies below that level.

For the purpose of this example the liquid 17 in the container 18 is liquid nitrogen, but it will be appreciated that the detector can be used with any suitable liquid.

In use current is passed through the conductor 16 from the meter/power source 12. As is well known when current is passed through a conductor, the conductor heats in accordance with the magnitude of the current squared times the resistance of the conductor. Those parts of the conductor which are immersed in the liquid nitrogen have little or no temperature change as a result of this heating, because the heat is immediately removed, by conduction, into the liquid nitrogen. However, the first portion of the conductor 15 is heated, because the gaseous nitrogen in the upper half 20 of the container 18 can only cool by the less efficient process of convention etc. in a gas. The first portion 15 of the conductor thus becomes hotter than the second portion 16.The resistance of most conductors changes with temperature, and with pure metals this temperature change can be quite significant.

Hence as the first portion of the conductor is heated the resistance of the conductor changes. A steady state is soon reached, in which the total resistance of the conductor is directly dependent upon the length of the conductor in the first portion.

As the level of liquid 17 in the container 19 drops the first portion becomes larger and the resistance of the conductor 14 increases. The resistance of the conductor can be monitored on meter 12 and a gauge 21 on that meter can be calibrated to show the liquid level equivalent to the measured resistance.

As can be seen in Figure 1 the detector comprises a length of conductor 14 held within an open mouthed tube 22. It will be appreciated that the degree of measurable change is dependent on the length of the conductor. It is thus desirable to wind or bend the conductor 14 in a manner to increase its length.

Conveniently the conductor can be wound about a generally horizontal axis, as shown in Figure 2. The diameter of the windings should be approximately the required length of the detector 11 and the size of the conductor can be reduced, for insertion in the tube 22, by twisting the windings together two or three times about their vertically extending diameter. Alternate forms of "winding" are shown in Figures 3 and 4.

The conductor is preferably formed of metal and conveniently has a temperature coefficient of resistance in the range 3.5 to 4.0 x 1 o-3Dc-1. Preferably the conductor is made of copper or iron wire, which is covered with an insulating coating such as a ceramic coating or organic coating. The conductor may have a constant cross-section and its diameter may, for example, be 0.15 mm.

The magnitude of the current supplied to the coil depends on the liquid, with which the detector is being used. The colder the liquid and/or the greater the temperature difference between the gas and the liquid, the less the current has to be. For liquid nitrogen a current between 0.1 and 1 amps may be used, and acceptable readings have been obtained using a current of 0.2. amp.

Claims

1. Apparatus for determining the level of a liquid in a container, comprising an electrical conductor dimensioned to project above a predetermined liquid level in a container, means for passing a current through the conductor such that a first portion of the conductor, which extends above the surface of the liquid, heats more rapidly than the second portion which is immersed in the liquid and means for measuring the total resistance of the conductor or an indication thereof to obtain an indication of the length of the second portion and hence the depth of the liquid.

2. Apparatus as claimed in claim 1 wherein the conductor constitutes an elongate sensor and is bent or wound so that the length of the conductor is greater than the length of the sensor.

3. Apparatus as claimed in claim 2, wherein the length of the conductor is between 50 and 200 times the length of the sensor.

4. Apparatus as claimed in claim 1 or claim 2 wherein the conductor is a coil wound about an axis extending generally orthogonally to the axis of the sensor and the coil is twisted along the diameter thereof.

5. Apparatus as claimed in any one of claims 1 to 3 wherein the conductor is a coil.

6. Apparatus as claimed in claim 5 wherein the coil is wound about an axis transverse to the conductor's axis.

7. Apparatus as claimed in claim 5 wherein the coil is wound about the axis of the conductor.

8. Apparatus as claimed in any one of claims 1 to 3 wherein the conductor is formed in a zigzag or is concertinaed.

9. Apparatus as claimed in any one of the preceding claims wherein the conductor has an electrically insulating covering.

10. Apparatus as claimed in any one of the preceding claims wherein the conductor may have a temperature coefficient of resistance in the range 3.5 to 4.0 x 10-3 C-l.

11. Apparatus as claimed in any one of the preceding claims wherein the conductor is copper, iron or any other conductor with a high thermal coefficient resistivity.

12. A detector for determining the level of a liquid in a container comprising an elongate sensor constituted by an electrical conductor bent or wound so that the length of the conductor is greater than the length of the sensor.

13. A detector as claimed in claim 12 wherein the conductor has a diameter of between 0.1 and 0.5 mm.

14. A detector as claimed in claim 12 or claim 13 wherein the conductor is a coil wound about an axis generally orthogonal to the axis of the sensor and the coil is twisted about a diameter.

15. A detector as claimed in any one of claims 12 to 14 wherein the conductor is contained within a protective tube which is open at one end.

16. Acontainerforcontaining liquid nitrogen incorporating apparatus for determining the level of liquid in the container as claimed in any one of claims 1 to 11 or a detector as claimed in any one of claims 12 to 15.

17. A method of determining the level of liquid in a container comprising inserting an electrical conductor into the container so that a part of the conductor projects above the level of liquid in the container, passing current through the conductor and monitoring the resistance of the conductor or an indication thereof to obtain an indication of the length of the part immersed in the liquid and hence the depth of the liquid.

18. Apparatus for determining the level of a liquid in a container substantially as hereinbefore described with referende to the accompanying drawings.

19. A detector for determining the level of liquid in a container substantially as hereinbefore described with reference to the accompanying drawings.

20. A method of determining the level of liquid in a container substantially as hereinbefore described with reference to the accompanying drawings.