GB2184229A - Optical temperature sensing apparatus for sensing liquid levels - Google Patents

Abstract

An optical temperature sensing arrangement for sensing the level a body of liquid 3 comprising an optical fibre sensing element 1 which is immersed in said liquid over part or parts of its length, the temperature measuring arrangement being adapted to detect and/or measure a significant difference in temperature of the fibre at the point(s) where it becomes immersed in the liquid. The part or parts of the optical fibre not contacted by the liquid may be heated or cooled to a temperature significantly higher or lower than the remainder of the optical fibre immersed in the liquid. The temperature measuring arrangement uses optical tine domain reflectometry with Rayleigh or Raman back-scattered light or returned fluorescent light. <IMAGE>

Description

SPECIFICATION Improvements relating to optical sensing apparatus This invention relates to optical apparatus for sensing the presence of liquids and has especial application to the sensing of liquid levels or depths of liquid in liquid tanks or containers, for example.

According to the present invention in its broadest aspect a body of liquid is sensed by an optical temperature sensing arrangement including an optical fibre temperature sensing element which is contacted by said liquid over at least part of it length, the temperature measuring arrangement being adapted to detect and/or measure a significant difference in temperature of the fibre where it is contacted by the liquid.

The significant difference in temperature referred to may occur naturally between the liquid temperature and the ambient temperature in the case of some liquids (e.g. cryogenic liquids).

However, in other cases a significanttemperature difference may be generated by providing the optical fibre with heating or cooling means which heats, or cools,that partor parts ofthe optical fibre not contacted by the liquid to a temperature significantly higher or lowerthan the remainder ofthe optical fibre contacted by the liquid. The heating means may, for example, comprise electric heating means attached to the optical fibre, or it may comprise heated gas or vapour derived from any convenient source.

The temperature measuring arrangements for detecting or measuring the temperature distribution along the optical fibre may utilise optical time domain reflectometrywith Rayleigh or Raman back-scattered light, or reflected fluorescent light, produced by the optical fibre being suitably detected and/or measured. In this connection attention is hereby directed to our G B PatentApplications Nos.

2140554Aand 2156513A.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a basic liquid level sensing arrangement according to the invention; Figure2 shows a liquid level sensing arrangement for use with cryogenic liquified gas; Figure3shows a liquid level sensing arrangement in which the optical fibre sensor has electric heating means associated with it; Figure4shows a liquid level sensing arrangement providing improved liquid height measurement resolution; and, Figure5showsa liquid level sensing arrangement for measuring average liquid levels.

Referring to Figure 1 ofthe drawings, the liquid level sensing arrangement illustrated comprises an optical fibre sensor 1 which extends into a liquid tank or container 2 which contains cold liquid 3 the level of which isto be measured bythearrangement.

For the measurement oftemperature at points along the optical fibre sensor 1 ,thereby enabling the liquid level to be determined by the sudden significant change in temperature ofthe fibre at the surface ofthe liquid, optical timedomain reflectometrytechniques may be utilised involving launching short coherent light pulses into the upper end ofthe sensor fibre 1 and measuring variations in the intensity ofthe total back-scattered light received at the launch end ofthefibre. Since the level oftotal back-scattered light from points along the fibre sensor varies with temperature, the temperature of such points can be measured.One particular temperature measuring arrangement of th is kind forms a subject of our patent application No.

21 40554A to which attention Is hereby directed.

Anotherarrangementwhich may be used is disclosed in ourco-pending patent application No.

215651 3A. In this arrangement, the optical fibre sensor is doped along its length with material that absorbs light in dependence upon temperature and contemporaneous light pulses of two different wavelengths are launched into the sensor and variations with time in the back-scattered light of the two wavelengths returned along the fibre are compared to provide an indication oftemperature distribution along the sensor. In yet another temperature measuring arrangement also disclosed in our patent application number2156513Athe optical fibre sensor is doped along its length with material that fluoresces in response to the absorbtion of light in dependence upon temperature.Light pulses are launched along the sensor and means is provided for detecting the variation with time in the levels offluorescent light emitted by the doped material in response to the light pulses which return to the launch end ofthe sensor.

Whatevertemperature sensing arrangement is employed,the light pulse generating means and the meansfordetecting light returning along the sensor fibre to the launch end of the latter are depicted by the box 5 in Figure 1 andtheotherdrawings.

Referring now to Figure 2 ofthe drawings, this shows a liquid level sensing arrangement eminently suitable for measuring the level of cryogenic liquified gas. As can be seen tileliquified gas 6 is contained within a vented tank 7 located within thermal insulation 8. Boiled off gas vapour is arranged to rise in a tube 9 which extends belowthe level of the liquified gas 6. The gas vapourwithin the tube passes through a heat exchanger section 10 of the tube so that it receives heat from the relatively warm ambient atmosphere. The warmed gas then passes down the tube leg section 11 which extends below the level ofthe liquified gas 6 and which is provided with a series of vent holes 12 at spaced intervals along its length.The optical fibre sensor 1 is introduced into the tube leg section 11 and extends virtuallytothe bottom of the tank 7. As will be appreciated, the heated gas vapour passing down the tube section 11 heats that part of the fibre sensor above the surface of the liquified gas so that there is a significant change in temperature of the optical fibreatthesurfaceofthe liquified gas. This change in temperature of the optical fibre at the liquid surface will be detected by the detecting means within the box Sand the liquid level withinthetank7 duly determined.

In the Figure 3 arrangement the optical sensor fibre 1 extends through a tubular resistive heating element 13 which is arranged to be electrically energised for heating the optical fibre 1. The heat from that part ofthe heating element below the surface ofthe liquid 14will be readily dissipated by the relatively cold liquid whereas the heat radiated bythe upper part ofthe heating element will significantly raise the temperature ofthe optical fibre 1 abovethe liquid surface so that a significant changeintemperaturewill occuratthe liquid surface which can accordingly be determined by the detecting means 5.An alternative arrangement to thatshown in Figure3would beto providethe optical fibre sensor with a conductive coating which could then be electrically energised in order to heat the fibre.

Referring to Figure 4 ofthe drawings, this shows a liquid level sensing arrangement using an optical fibre having a relatively poortemperature monitoring resolution with distance along the fibre.

As can be seen, the length ofthe optical fibre sensor 15 is substantially increased by winding it into the form of hel ix which may be supported on a cylindrical former 16. This helical form of optical fibre allows constant level resolution enhancement to be achieved by the ratio between the length ofthe fibreonasingleturnofthehelixandthepitchofthe helix. An approximation to this helical arrangement may be achieved by serrosoidal (zig-zag) or sinusoidal horizontal displacement of the optical fibre sensor as a function of the height ofthe liquid tank.

In applications where the liquidtankorcontainer will be subjected to mechanical movement (e.g.

vehicle tanks) the average level of the agitated liquid may be measured by the arrangement depicted in Figure 5. As shown, the optical fibre sensor 17 may be of generally zig-zag form and will be provided along its length with associated heating means (not shown) sothatthefibre has a very shortthermal time constant. This allows the parts ofthe fibre not immersed in the liquid to be heated rapidly so that when the liquid becomes disturbed or agitated those parts of the fibre that become exposed to the ambient atmosphere are quickly heated. Significant temperature changes at points 18 along the length of thefibre occurwhere the fibre becomes immersed in the liquid and these points will be determined bythe detectig means to provide an average liquid level indication.

Although in the embodiments shown in Figures 3 and 5 ofthe drawings the optical fibre sensor has associated heating means it will be appreciated that optical fibre cooling means may be provided in cases where relatively the level of hot liquid isto be sensed.

Claims (7)

1. An optical temperature sensing arrangement for sensing a body of liquid comprising an optical fibre sensing elementwhich is immersed in said liquid over part or parts of its length, thetemperature measuring arrangement being adapted to detect and/or measure a significant difference in temperature ofthefibre at the point(s) where it becomes immersed in the liquid.

2. An optical temperature sensing arrangement as claimed in claim 1, in which a significant difference in temperature is generated by providing the optical fibrewith heatingorcooling means which heats orcoolsthatpartof partsofthe opticalfibre not contacted by the liquid to a temperature significantly higher or lowerthan the remainderof the optical fibre immersed in the liquid.

3. An optical temperature sensing arrangement as claimed in claim 2, in which the heating means comprises electric heating means attached to the optical fibre.

4. An optical temperature sensing arrangement as claimed in claim 3, in which the heating means comprises heated gas or vapour derived from any convenientsource.

5. An optical temperature sensing arrangement as claimed in any preceding claim, in which the temperature measuring arrangement utilises optical time domain reflectometry with Rayleigh or Raman back-scattered light or returned fluorescent light being suitably detected and/or measured.

6. A liquid level measuring arrangement including an optical temperature sensing arrangement according to any ofthe preceding claims.

7. A liquid level sensing arrangement substantially as hereinbefore described with reference to any one of the accompanying drawings.