GB2401184A - Liquid level detector - Google Patents

Abstract

Apparatus for detecting the level of liquids which have their vapour phase at a different temperature to their liquid phase. The apparatus comprising a fibre optic distributed temperature sensor, which is coiled to increase the resolution of the sensor. The apparatus is also claimed as working where the temperature difference is not inherent but manually produced by either heating or cooling the liquid. Also the apparatus is claimed where the vapour and liquid have their temperature changed at different rates.

Description

Liquid Level Detector The present invention relates to measurement of the

amount of a liquid in a container, and particularly relates to measurement of that amount by detection of the position of the level of the surface of the liquid in the container.

The measurement of liquid levels in containers has many variants.

Container shapes can take many forms. One popular way is to render the surface liquid visible and to measure the position of the surface of the liquid, the level of the surface being related to the overall volume of the liquid, by means of prior calibration or by use of knowledge of the geometry of the container. However, rendering the surface level of the liquid visible has the disadvantage that a frangible glass or ceramic element has to be used if pressure of any significant kind is to be endured, and the presence of the ceramic or glass element makes seals essential, and seals are perishable and require periodic replacement and maintenance. There are also liquids which, by their very nature, can cause devastating damage and injury if allowed to escape, thus making the use of seals highly undesirable.

One area for which the present invention seeks to provide improvement is in the storage of liquid ammonia. The present art has ammonia, most usually, stored in spherical storage tanks.

Differential pressure measurements, in both the vapour and liquid phases, are commonly used to determine the level of liquid in the tank. This, however, is a far from reliable technique, for which the present invention provides an alternative solution.

Where pressure measurement techniques are used, seals are also required to externalise cables, the seals, transducers and cables posing a problem concerning chemical inertia and corrosion.

The present invention seeks to provide a method and apparatus for measuring the amount of hazardous liquid in a container without requiring any breach of the initial integrity of the container and automatically free from chemical inertia or corrosion problems.

The amount of liquid in a container can be enormous, whereas the sensitivity of the measurement process can be verging on the unacceptably inaccurate. For example, a cylindrical container may be metres in diameter, making every centimetre of liquid in the container potentially hold approximately 785.39 litres. So, knowledge of the liquid level in the container say, to plus or minus 0.1 cm, gives an error in the measurement of the amount of liquid in the container of plus or minus 78.5 litres. This may or may not be acceptable, but illustrates the accuracy problem. Simple liquid level detection systems, however they are achieved, all have this problem. The present invention seeks to provide a method and apparatus for detecting the level of a liquid in a container whose sensitivity is automatically enhanced to provide a knowledge of the level of the surface of the liquid which is more accurate than with other methods.

According to a first aspect, the present invention consists in a method for measuring liquid in a container, the temperature of the vapour phase differing from the temperature of the liquid phase, said method comprising the steps of: deploying a linear temperature transducer in thermal contact with the liquid; obtaining a temperature profile along the length of the linear temperature sensor; and detecting the position in said linear temperature transducer where a temperature transition occurs.

According to a second aspect, the present invention consists in an apparatus for measuring liquid in a container, the temperature of the vapour phase differing from the temperature of the liquid phase, said apparatus comprising; a linear temperature transducer deployed in thermal contact with the liquid; transducer monitoring means operative to obtain and to provide as output a temperature profile along the length of the linear temperature sensor; and temperature change detection means, operative to monitor said output of said monitoring means to detect the position in said linear temperature transducer where a temperature transition occurs.

The invention further provides that the thermal contact with the liquid can comprise thermal contact with a portion of the exterior surface of the container The invention further provides for relating the position in said linear temperature transducer of the temperature transition to an amount of liquid in the container.

The invention further provides that relating the position in said linear temperature transducer of the temperature transition to an amount of liquid in the container involves relating the position in said linear temperature transducer to a position on the exterior surface of the container, and relating the position on the surface of the container to an amount of liquid in the container.

The invention further provides for relating the position in said linear temperature transducer to a position on the exterior surface of the container.

The invention further provides that the linear temperature transducer can be disposed in a vertically inclined path on the surface of the container, that the vertically inclined path can comprise a coiled path providing a coil of linear temperature transducer going around the container one or more times, that the diameter of the coil can be constant, that the diameter of the coil can be varied to conform with the container, that the inter-turn spacing of the turns on said coil can be constant, and that the inter-turn spacing on said coil can be adapted to the geometry of the container for equal lengths along the linear temperature transducer to represent equal incremental amounts of liquid in the container.

The invention further provides that the container can comprise a main body and a measuring extension, that the portion of the container, with which the linear temperature transducer is in thermal contact can be the measuring extension, that the measuring extension can comprise a vertical linear tubular portion, and that the linear temperature transducer can be wound in a vertical axis helix about the vertical linear tube portion.

The invention further provides that the main body can be substantially spherical and that the measurement extension can provide a connection between the top and bottom of the main body.

The invention further provides that the linear temperature transducer can comprise one or more fibre optic cables for Distributed Temperature Sensing (DTS).

The invention further provides that one or more alarms can be triggered in response to different amounts of liquid.

The invention further provides that the liquid can be ammonia.

The invention further provides that the container can be a double- skinned tank.

The invention further provides that the linear temperature transducer can be disposed within the container or tank.

The invention further provides that the linear temperature transducer can be wound to be inserted within a thermowell in a cryogenic storage tank and can be used for detecting the surface of liquid within the cryogenic storage tank, and can further be used for measuring temperature profiles within the cryogenic storage tank.

The invention further provides that the fluid can be heated or can be cooled, thereby to create a temperature transition between the thermally conductive liquid phase and the relatively thermally non- conductive vapour phase. The invention further provides that the heated or cooled fluid can be run through a pipe and the temperature s profile of the pipe taken and that the temperature profile can be used to detect the position of the surface of the liquid in the storage tank.

S The invention further provides that the linear temperature transducer can be provided in the interior of a thermally conductive fluid conduit carrying a stream of thermally conductive fluid for the thermally conductive fluid to have its temperature changed at a first rate as said thermally conductive conduit passes through the liquid phase of the liquid and at a second rate as said thermally conductive conduit passes through the vapour phase of said liquid for a knee in the temperature versus depth plot along the linear temperature transducer to indicate the position of the surface of the liquid.

The invention further provides that the linear temperature transducer can be used to find a first position of the knee in the temperature versus depth plot along the linear temperature transducer with the thermally conductive liquid flowing in a first sense through said thermally conductive conduit, that the linear temperature transducer can be used to find a second position of the knee in the temperature versus depth plot along the linear temperature transducer with the thermally conductive liquid flowing in a second sense, opposite to said first sense, through said thermally conductive conduit, and that the mean between said first position of the knee and the second position of the knee can be taken as the true position of the knee.

The invention further provides that said thermally conductive fluid can be cooled or heated by passage through said thermally conductive conduit.

The invention is further explained, by way of example, by the following description, to be read in conjunction with the appended drawings, in which: Figure 1A is a cross-sectional side view, with added detail of the fibre optic cable, of a first embodiment of a tank and a first disposition of the fibre optic cable for use in a first embodiment of the present invention.

Figure 1B is a cross-sectional view of a distributed temperature sensor.

Figure 1C is a cross-sectional side view of the external pipe, the distributed temperature sensor and the monitor 30, otherwise shown in Figure 1A, illustrating another embodiment of the invention where the combination of a high sensitivity distributed temperature sensor and a high sensitivity monitor 30 permit the distributed temperature sensor 24 to be disposed in a vertical orientation.

Figure 2 is a cross-sectional side view, with added detail of the fibre optic cable, of a second embodiment of a tank and a second disposition of the fibre optic cable for use in a second embodiment of the present invention.

Figure 3 is an angled projected view of the tank, otherwise shown in cross-section in Figure 2.

Figure 4 is a side view of a spherical tank, showing how a fibre optic cable can be wound thereabout in such a manner as to give linear reading of the volume of liquid in the tank.

Figure 5 shows a third embodiment of the invention where a cryogenic tank having a thermowell has the depth of fluid measured therein by the present invention detecting a thermal transition.

Figure 6 is a cross-sectional drawing of a fourth embodiment of the present invention where a heating or cooling apparatus is employed.

Figure 7 is a cross sectional view of a fifth embodiment of the invention where the apparatus of Figure 6 is supplemented by provision of a flow pipe where-about a linear temperature transducer can be wound. And

Figures 8A to 8C show a further embodiment of the invention where thermally conductive fluid can be ducted along a fluid conduit to have its temperature measured as heat is transferred from a liquid or gaseous phase of a stored liquid.

Attention is first drawn to Figure 1, showing a side cross sectional view of an apparatus, according to a first embodiment of the present invention.

A spherical tank 10 contains a liquid 12 having a level 14 above which vapour 16 exists. An external pipe 18 is connected between the top 20 of the spherical tank and the bottom 22 of the spherical tank. The external pipe 18 is substantially vertical and is closed to the outside so that the vapour 16 above the liquid 12 in the spherical tank 10 communicates with the vapour 16 above the liquid 12 in the external pipe 18. The liquid 12 attains the same level in the external pipe 18 as it does in the spherical tank 10.

A fibre optic Distributed Temperature Sensor (DTS) 24 comprises one or more fibre optic cables 26 held within a stainless steel pipe 28.

The stainless steel pipe 28 protects the fibre optic cable 26 against chemical attack and mechanical damage. The stainless steel pipe 28 also is thermally conductive, and conducts heat to and from outside the stainless steel pipe 28 to bring the fibre optic cable 26 towards the temperature of the exterior environment of the stainless steel pipe 28 at each point along the stainless steel pipe 28. The DTS 24 provides a very subtle means for detecting temperature difference, capable of detecting a temperature transition of a fraction of a degree.

As is well known in the art of measuring temperatures at remote positions down hydrocarbon wells, the fibre optic cable 26 can be interrogated to know its temperature at any point in its length.

The DTS 24 is wound in a helical fashion about the external pipe 18 so that the temperature of the outside of the external pipe 18 is transferred to the fibre optic cable 26. A monitor 30 has the ends 32 of the DTS 24 connected thereto and is operative to measure and provide a temperature profile along the fibre optic cable 26. The DTS 24 and the pipe 18 can, within the spirit of the invention, be included inside or outside of the tank 10.

The example shown in Figure 1 has, as the contained liquid 12, ammonia, which is normally stored in liquid 12 form. Liquid 12 ammonia has a gaseous or vapour 16 phase which has a temperature range, typically, from -33 degrees Celsius to +25 degrees Celsius, and a liquid 12 phase whose temperature ranges, typically, from -33 degrees Celsius to +5 degrees Celsius. In any event, the temperature difference between the gaseous or vapour 16 phase and the liquid 12 phase is always in the region of 20 Celsius degrees.

The monitor 30 creates a temperature profile along the fibre optic cable 26 and finds the point, on the fibre optic cable 26, where a temperature transition is indicated by change between the low temperature of the vapour 16 phase and the warmer liquid 12 phase at the liquid level 14.

This measurement is made more sensitive because the DTS 24 has a sloping path. If the DTS 24 were only vertical and attached vertically to the exterior of the external pipe 18, the range of positions along the fibre optic cable 26 available for the position detection of the temperature transition would be along only a very short length of fibre optic cable 26. When the fibre optic cable 26 is completely vertical, a 0.1 cm change in the position of the liquid level 14 is reflected in only a 0.1 cm change in the position, along the fibre optic cable 26, where the monitor 30 detects the temperature transition. However, in the example of the invention here given, if the external pipe 18 is 10 cm in circumference, and the DTS 24 is wound about the external pipe 18 with a vertical pitch of 0.5 cm, each rise in the liquid level 14 of 0.1 cm represents a fifth of a turn of the DTS 24 about the external pipe 18 and the detected position of the temperature transition moves by 2.0 cm, a sensitivity magnification of twenty to one.

Greater or lesser magnifications can be achieved using different sizes of circumferences and different pitches of winding.

The sensitivity of the measurement of the amount of liquid 12 in the spherical tank 10 is very much dependent upon the sensitivity of the distributed temperature sensor 24 and the resolution and sensitivity of the monitor 30. A very high resolution machine can be built, thereby allowing the distributed temperature sensor 24 to be deployed vertically, rather than in a coil. The present invention does not exclude the use of the distributed temperature sensor 24 in a vertical orientation that has a high resolution. Such a machine is shown in Figure 1C where the external pipe 18 has the distributed temperature sensor 24 affixed in a vertical orientation to its exterior. The monitor 30 is of a high resolution type allowing for precise measurements of the level 14 of the liquid 12.

The liquid 12, shown in this example, is ammonia. Ammonia has the advantage of having a naturally high temperature difference between liquid 12 and vapour 16 phases. The embodiment of the invention shown is also eminently suitable for use with, but is not restricted to use with, Liquid Natural Gas (LNG), Liquid Petroleum Gas (LPG), or ethylene. In fact, the invention can be used for any fluid for which a temperature difference exists between the liquid 12 and gaseous or vapour phases 16. While the examples quoted all have vapour 16 phases which have a lower temperature than the liquid 12 phases, the present invention can equally well be practiced where the vapour 16 phase has a higher temperature than the liquid 12 phase. All that is required is to detect a temperature transition.

Attention is next drawn to Figure 2, showing a cross-sectional side view, with added detail of the fibre optic cable, of a second embodiment of the invention, showing a double-skinned ammonia tank and a second disposition of the fibre DTS 24 for use in the second embodiment of the present invention. Attention is also drawn to Figure 3, which shows an angled projected view of the tank, otherwise shown in cross section in Figure 2.

A double-skinned ammonia tank 34 comprises an inner skin 36 and an outer skin 38, between which is an annular void known as the annulus 40, which is often not insulated. Ammonia is stored in a central vessel 42. The construction of the roof (not shown) of the double- skinned ammonia storage tank 34 is such that ammonia vapour can enter and condense in the annulus 40, causing a build up of liquid ammonia.

The second embodiment of the present invention provides a means for measuring the amount of liquid ammonia within the annulus 40. In the past, this has been monitored only by placing a small number of thermocouples near the top of the annulus 40. In the second embodiment of the present invention, a section of open pipework 44 has the DTS 24 wound thereabout in a helical path, and is placed, vertically, within the annulus 40. The fibre optic cable 26, within the DTS 24, is protected by the stainless steel pipe 28 as the section of open pipework 44 is partially immersed in liquid ammonia i 46 in the annulus 40 so that the monitor 30 can measure the position of the level 48 of condensed ammonia in the same manner as for the first embodiment shown in Figure 1.

Attention is next drawn to Figure 4, showing another embodiment of the invention where a spherical tank 50 has the DTS 24 wound thereabout in such a manner that equal heights of movement of the detected liquid level 14 indicate equal changes in volume of fluid in the spherical tank 50. A zero height, or position of the temperature transition in the fibre optic cable 26, designates that; the spherical tank 50 is empty. The pitch of the winding is increasingly dense at the centre portion of the spherical tank 50, where the change in the amount of stored liquid is large for each change in height of the level 14 of the liquid. The pitch of the winding becomes more and more widely spaced towards the top 20 and bottom 22 of the spherical tank, in sympathy with the change in the amount of stored liquid becoming smaller for change in height of the level 14 48 of the liquid 12. The pitch of the winding of the DTS 24 varies continuously, even within a turn. The same principle can be applied to any geometric shape, to accommodate a linear scale for storage vessels of all shapes.

Attention is next drawn to Figure 5, showing a cryogenic storage tank 52 for the storage of cryogenic fluids at an extremely low temperature, such as, but not limited to, liquid nitrogen, liquid oxygen, liquid helium and so on. The cryogenic storage tank 52 comprises a thermowell 54 which is used, in the prior art, specifically for the purposes of temperature profiling. The thermowell 54 is a tube, open at the top and sealed from the contents of the tank. In the example shown in Figure 5, the thermowell 54 is of sufficient diameter that a pre-coiled distributed temperature sensor 56 can be dropped into the inside of the thermowell 54 so that the pre-coiled distributed temperature sensor can be used, as earlier described in the other embodiment to find the amount of liquid in the cryogenic storage tank 52 and can also be used to monitor the temperature gradient within the cryogenic storage tank 52.

Attention is drawn to Figure 6 showing a holding tank 58 which contains a liquid 12 which does not require to be stored in a cryogenic state. The liquid 12 can be a petroleum product where a significant temperature difference does not naturally exist between the liquid 12 and the vapour 16 phase.

A heater 60 is provided in the lower section of the holding tank 58 to heat the liquid phase 12 of the stored substance. A distributed temperature sensor 24 is disposed either within the holding tank 58 or externally to the holding tank 58 in thermal contact therewith.

The distributed temperature sensor 24 is monitored, as indicated in earlier examples, by the monitor 30. In this example the distributed temperature sensor 24 is shown in a straight and vertical orientation. There is nothing to stop the distributed temperature sensor 24 being used in the coiled mode as shown in Figures 1A and 2.

The heater 60 heats the liquid phase 12 which in turn heats the vapour phase 16. While the liquid phase is heating, it is generally warmer than the vapour phase 16 and thus the monitor can detect the temperature transition indicative of the level 14 of the liquid 12.

Once thermal equilibrium has been reached, the liquid phase 12 is generally more efficient at heat energy transfer than the vapour phase 16. A temperature transition will thus still be apparent at the liquid level 14 between the liquid phase 12 and the vapour phase 16.

The heater 60 can equally well be a refrigeration apparatus which will cool the liquid phase 12 rather than heat it. The distributed temperature sensor 24 can be used, as earlier stated, to locate the temperature transition from hot to cold or from cold to hot to indicate the level 14 of the liquid phase 12 in the holding tank 58.

Attention is next drawn to Figure 7 showing a variation of the embodiment shown in Figure 6 where a flow pipe 62 feeds the contents of the holding tank 58 in and out as indicated by arrow 64. Here the flow pipe has a distributed temperature sensor 24 wrapped spirally there around. The distributor temperature sensor 24 could equally well be disposed within the flow pipe 62 and/or could be provided in a purely vertical orientation as shown in Figures 1C and 6. The heater 60 is here shown as being within the holding tank 58.

The embodiment of the invention shown also provides that the heater 60 (or equivalent cooling device) can be situated elsewhere providing the fluid to flow in and out of the holding tank via the flow pipe 62.

As before, it is generally true that the vapour phase is poorer at thermal conduction than the liquid phase, so that a temperature transition will be detected by the monitor 30. Should total thermal equilibrium be reached, in which case there will be no thermal transition, it is perfectly possible to heat or cool the liquid phase 12 and to move it within the holding tank 15 and the flow pipe 62 to once again readily re-establish a temperature transition.

Likewise, at each instance of extraction or addition of liquid 12 from the holding tank 58, thermal equilibrium will be broken, I allowing a measurement of the amount of liquid 12 in the holding tank 58 to be made at that instant.

Figure 8A shows a further embodiment of the present invention where a fluid conduit 64 has a thermally conductive fluid 66 passing therethrough as indicated by arrow 68. The fluid conduit 64 is thermally conductive and conducts the thermally conductive fluid 66 through liquid 12. Heat is transferred from the vapour phase 16 to the thermally conductive fluid 66, and from the liquid phase 12 to the thermally conductive fluid 66. The fluid conduit 64 also acts as the housing for a fibre optic cable 26 which goes from one end to the other of the fluid conduit 64, immersed in the thermally conductive fluid 66.

Figure 8B shows the graph which is obtained of temperature versus depth for the fibre optic cable 26 within the fluid conduit 64. The rise in temperature of the thermally conductive fluid 66 is greater while the fluid conduit 64 is beneath the surface 14 of the liquid 12 because the liquid phase 12 is generally a better conductor than the vapour phase 16. The graph of temperature versus depth exhibits a knee 70 as the fibre optic cable 26 passes through the surface 14 of the liquid 12. Because flow of the thermally conductive fluid 66 in the fluid conduit 64 can effectively move the apparent position of the knee 70, the invention also encompasses the act of finding the position of the knee 70 when the thermally conductive fluid 66 is flowing in a first direction, finding the position of the knee 70 when the thermally conductive fluid 66 is flowing in a second, opposite, direction, and taking the average there-between to give the true position of the surface 14 of the liquid 12.

Figure 8C shows a further embodiment over that shown in Figure 8A where the thermally conductive fluid conduit 64 is disposed within the liquid 12 in a generally spiral array 72, thereby permitting greater positional sensitivity to be obtained.

The thermally conductive fluid 66 can be cooled or heated by the liquid 12 or vapour 16 phases, and the apparatus can include means for heating, cooling and temperature equalizing the thermally conductive fluid 66.

Typically, but not exclusively, the liquid 12 can be petroleum spirit of some type and the thermally conductive fluid can be water.

Claims (76)

1. A method for measurlog liquid in a container, the temperature of the vapour phase differing from the temperature of the liquid phase, said method comprising the steps of: deploying a linear temperature transducer in thermal contact with the liquid; obtaining a temperature profile along the length of the linear temperature transducer; and detecting the position in said linear temperature transducer where a temperature transition occurs.

2. A method, according to claim 1, wherein said thermal contact lo with the liquid comprises thermal contact with a portion of the exterior surface of the container

3. A method, according to claim 1 or claim 2, including the step of relating the position in said linear temperature transducer of the temperature transition to an amount of liquid in the container.

4. A method, according to claim 3, including the step of relating the position in said linear temperature transducer to a position on the exterior surface of the container.

5. A method, according to claim 4, including the step of relating the position on the surface of the container to an amount of liquid in the container.

6. A method, according to any one of the preceding claims, including the step of disposing said linear temperature transducer in a vertically inclined path.

7. A method, according to claim 6, wherein said vertically inclined path comprises a coiled path.

8. A method, according to claim 7, including the step of said coiled path providing contact between said linear temperature transducer with the outer surface of the container, said coiled path providing a coil of linear temperature transducer going around the container one or more times.

9. A method, according to claim 8, wherein the diameter of the coil is constant.

10. A method, according to claim 8, wherein the inter-turn spacing of the turns on said coil is constant.

11. A method, according to claim 7, including the step of said coiled path providing said coil within the container, said coiled path providing a coil of linear temperature transducer having one or more turns.

12. A method, according to claim 11, wherein the diameter of the coil is constant.

13. A method, according to claim 12, wherein the inter-turn spacing of the turns on said coil is constant.

14. A method, according to claim 8, wherein the inter-turn spacing on said coil is adapted to the geometry of the container for equal changes in length along the linear temperature transducer for the detection of the temperature transition to represent equal changes of amount of liquid in the container.

15. A method, according to any one of the preceding claims, including the steps of: providing said container with a main body and a measuring extension; providing that said linear temperature transducer is in thermal contact with said measuring extension; providing that said measuring extension comprises a vertical linear tubular portion; and providing that said linear temperature transducer is wound in a vertical axis helix about the vertical linear tubular portion.

16. A method, according to claim 15, wherein said main body is substantially spherical and said measurement extension provides a connection between the top and bottom of said main body.

17. A method, according to any one of the preceding claims, wherein said linear temperature transducer comprises one or more fibre optic cables for Distributed Temperature Sensing (DTS).

18. A method, according to claim 17, including the step of housing said one or more fibre optic cables within a stainless steel tube.

19. A method, according to any one of the preceding claims, including the step of triggering one or more alarms in response to different amounts of liquid in said container.

20. A method, according to any one of claims 1 to 14, or according to any one of claims 17, 18 or 19 when not dependent upon claim 15 or claim 16, wherein said container is a cryogenic storage tank having one or more thermowells, and including the step of deploying said linear temperature transducer within said one or more thermowells.

21. A method, according to claim 20, including the step of employing said linear temperature transducer to obtain a temperature profile within the cryogenic storage tank.

22. A method, according to any one of claims 1 to 14, or according to any one of claims 17, 18 or 19 when not dependent upon claim 15 or claim 16, wherein said container is a double-skinned tank.

23. A method, according to claim 22, for used in measuring liquid between an inner skin and an outer skin in said double- skinned tank.

24. A method, according to any one of the preceding claims, wherein the liquid is ammonia.

25. An apparatus for measuring liquid in a container, the temperature of the vapour phase differing from the temperature of the liquid phase, said apparatus comprising; a linear temperature transducer deployed in thermal contact with the liquid; transducer monitoring means operative to obtain and to provide as output a temperature profile along the length of said linear temperature transducer; and temperature change detection means, operative to monitor said output of said monitoring means to detect the position in said linear temperature transducer where a temperature transition occurs.

26. An apparatus, according to claim 25, wherein said thermal contact with the liquid comprises thermal contact with a portion of the exterior surface of the container

27. An apparatus, according to claim 25 or claim 26, comprising position relation means, operative to monitor the output of said temperature change detection means and relate the position in said linear temperature transducer of the temperature transition to an amount of liquid in the container.

28. An apparatus, according to claim 25 or 26, comprising position relating means, operative to monitor the output of said temperature change detection means to relate the position of the temperature transition in said linear temperature transducer to a position on the exterior surface of the container.

29. An apparatus, according to claim 28, including means to relate the position on the surface of the container to an amount of liquid in the container.

30. An apparatus, according to any one of claims 25 to 29, wherein said linear temperature transducer is disposed in a vertically inclined path.

31. An apparatus, according to claim 30, wherein said vertically inclined path comprises a coiled path.

32. An apparatus, according to claim 31, wherein said coiled path provides thermal contact between said linear temperature transducer with the outer surface of said container, said coiled path providing a coil of linear temperature transducer going around the container one or more times.

33. An apparatus, according to claim 31 or 32, wherein the diameter of the coil is constant.

34. An apparatus, according to claim 31, 32 or 33, wherein the inter-turn spacing of the turns of said coil is constant.

35. An apparatus, according to claim 31, wherein said coiled path lies within the container, said coiled path providing a coil of linear temperature transducer having one or more turns.

36. An apparatus, according to claim 35, wherein the diameter of the coil is constant.

37. An apparatus, according to claim 35 or 36, wherein the inter turn spacing of the turns of said coil is constant.

38. An apparatus, according to claim 32, wherein the inter-turn spacing on said coil is adapted to the geometry of the container for equal changes in length along the linear temperature transducer for the detection of the temperature transition to represent equal changes of amount of liquid in the container.

39. An apparatus, according to any one of claims 25 to 38, wherein said container comprises a main body; wherein said main body comprises a measuring extension; wherein said linear temperature transducer is in thermal contact with said measuring extension; wherein said measuring extension comprises a vertical linear tubular portion; and wherein said linear temperature transducer is wound in a vertical axis helix about the vertical linear tubular portion.

40. An apparatus, according to claim 39, wherein said main body is substantially spherical and said measurement extension provides a connection between the top and bottom of said main body.

41. An apparatus, according to any one of claims 25 to 40, wherein said linear temperature transducer comprises one or more fibre optic cables for Distributed Temperature Sensing (DTS).

42. An apparatus, according to claim 41, wherein said one or more fibre optic cables are house within a stainless steel tube.

43. An apparatus, according to any one of claims 25 to 42, including alarm means, operative to monitor said temperature change detection means and to trigger one or more alarms in response to different amounts of liquid in said container.

44. An apparatus, according to any one of claims 25 to 38, or according to any one of claims 41, 42 or 43 when not dependent upon claim 39 or claim 40, wherein said container is a cryogenic storage tank having one or more thermowells, and wherein said linear temperature transducer is deployed within said one or more thermowells.

45. An apparatus, according to claim 44, wherein said linear temperature transducer is operative to obtain a temperature profile within the cryogenic storage tank.

46. An apparatus, according to any one of claims 25 to 43, wherein said container is a double-skinned tank.

47. An apparatus, according to claim 46, operative to measure liquid between an inner skin and an outer skin in said double skinned tank.

48. An apparatus, according to any one of claims 25 to 47, wherein the liquid is ammonia.

49. A method for measuring liquid in a container, comprising the steps of: employing temperature changing means to alter the temperature of the liquid; deploying a linear temperature transducer in thermal contact with the liquid; obtaining a temperature profile along the length of the linear temperature transducer; and detecting the position in said linear temperature transducer where a temperature transition occurs.

50. A method, according to claim 49, wherein said means to alter the temperature of the liquid is a heater.

51. A method, according to claim 49, wherein said means to alter the temperature of the liquid is a cooler.

52. A method, according to claim 48, 50 or 51, including the step of passing the liquid through a flow pipe, and employing said linear temperature transducer to obtain a temperature profile along the length of the flow pipe.

53. A method, according to any one of claims 49, 50, 51 or 52, wherein said liquid is hydrocarbon.

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54. A method, according to any one of claims 49 to 53, wherein said liquid is ammonia.

55. A method, according to any one of claims 49 to 54, wherein said linear temperature transducer comprises a fibre optic cable.

56. An apparatus for measuring liquid in a container, said apparatus comprising; a linear temperature transducer deployed in thermal contact with the liquid; temperature changing means, operative to alter the temperature of the liquid; transducer monitoring means operative to obtain and to provide as output a temperature profile along the length of said linear temperature transducer; and temperature change detection means, operative to monitor said output of said monitoring means to detect the position in said linear temperature transducer where a temperature transition occurs.

57. An apparatus, according to claim 56, wherein said temperature changing means is a heater.

58. An apparatus, according to claim 56, wherein said temperature changing means is a cooler.

59. An apparatus, according to claim 56, 57 or 58, further comprising a flow pipe, for passing the liquid there through, said linear temperature transducer being operable to obtain a temperature profile along the length of the pipe.

60. An apparatus, according to any one of claims 56 to 59, wherein said liquid is hydrocarbon.

61. An apparatus, according to any one of claims 56 to 59, wherein said liquid is ammonia.

62. An apparatus, according to any one of claims 56 to 61, wherein said linear temperature transducer comprises a fibre optic cable.

63. A method for measuring liquid in a container, said method comprising the steps of: providing a linear temperature transducer in the interior of a thermally conductive fluid conduit; causing said thermally conductive fluid conduit to carry a stream of thermally conductive fluid; causing said thermally conductive fluid conduit to pass between the liquid phase and the vapour phase of the liquid for the thermally conductive fluid to have its temperature changed at a first rate as said thermally conductive conduit passes; through the liquid phase of the liquid and at a second rate as said thermally conductive conduit passes through the vapour phase of said liquid; obtaining a temperature profile along the length of the linear temperature transducer; and detecting the position of a knee in the temperature profile along the linear temperature transducer to indicate the position of the surface of the liquid.

64. A method, according to claim 63, including the steps of: using said linear temperature transducer to find a first position of the knee in the temperature profile along the linear temperature transducer with the thermally conductive liquid flowing in a first sense through said thermally conductive conduit; using said linear temperature transducer to find a second position of the knee in the temperature profile along the linear temperature transducer with the thermally conductive liquid flowing in a second sense, opposite to said first sense, through said thermally conductive conduit; and taking the mean between said first position of the knee and said second position of the knee as the true position of the knee.

65. A method, according to claim 63 or 64, wherein said thermally conductive fluid is cooled by passage through said thermally conductive conduit.

66. A method, according to claim 63 or 64, wherein said thermally conductive fluid is warmed by passage through said thermally conductive conduit.

67. A method, according to any one of claims 63 to 66, wherein said liquid is hydrocarbon.

68. A method, according to any one of claims 63 to 66, wherein said liquid is ammonia.

69. A method, according to any one of claims 61 to 64, wherein said linear temperature transducer comprises a fibre optic cable.

70. An apparatus for measuring liquid in a container, said apparatus comprising; a linear temperature transducer inside a thermally conductive fluid conduit carrying a stream of thermally conductive fluid, said thermally conductive fluid conduit passing between the liquid phase and the vapour phase of the liquid for the thermally conductive fluid to have its temperature changed at a first rate as said thermally conductive conduit passes through the liquid phase of the liquid and at a second rate as said thermally conductive conduit passes through the vapour phase of said liquid; transducer monitoring means operative to obtain and to provide as output a temperature profile along the length of said linear temperature transducer; and temperature change detection means, operative to detect the position of a knee in the temperature profile along the linear temperature transducer to indicate the lo position of the surface of the liquid.

71. An apparatus, according to claim 70, including control means, operative to employ said linear temperature transducer to find a first position of the knee in the temperature profile along the linear temperature transducer with the thermally conductive liquid flowing in a first sense through said thermally conductive conduit; operative to employ said linear temperature transducer to find a second position of the knee in the temperature profile along the linear temperature transducer with the thermally conductive liquid flowing in a second sense, opposite to said first sense, through said thermally conductive conduit; and operative to take the mean between said first position of the knee and said second position of the knee as the true position of the knee.

72. An apparatus, according to claim 70 or 71, wherein said thermally conductive fluid is cooled by passage through said thermally conductive conduit.

73. An apparatus, according to claim 70 or 71, wherein said thermally conductive fluid is warmed by passage through said thermally conductive conduit.

74. An apparatus, according to any one of claims 70 to 73, wherein said liquid is hydrocarbon.

75. An apparatus, according to any one of claims 70 to 73, wherein said liquid is ammonia.

76. An apparatus, according to any one of claims 70 to 75, wherein said linear temperature transducer comprises a fibre optic cable.