GB2076960A - Liquid sensor - Google Patents

Abstract

An optical liquid level sensor arrangement comprises a corner cube reflector in a sensor head 15, which is arranged at a predetermined level in a liquid container, and to which light is directed and from which reflected light is received via single optical fibre 14. Immersion of the reflector in the liquid destroys total internal reflection. Beam splitter 13 receives light from an L.E.D. or semiconductor laser 11 via branch fibre 12, and branch fibre 16 conveys the reflected light to a detector 16. There is a lens between fibre 14 and the reflector. A plurality of sensor elements may be used to give a digital indication of liquid level. <IMAGE>

Description

SPECIFICATION Fibre optic switches This invention relates to liquid level sensors, and in particular to optical sensors of the total internal reflection type.

Liquid level sensors are employed in a variety of industrial and non-industrial applications. In many cases such sensors are used to determine the level of hazardous and/or inflammable liquids, such as petroleum spirit ortetraethyl lead. In such cases purely mechanical arrangements cannot be readily employed as it is extremely difficult to provide an hermetic seal between the liquid or its vapour and the external atmosphere whilst providing for insertion of the arrangement into the liquid container. It is generally necessary therefore to provide some form of electromechanical remote sensing arrangement.

However, because of the risk of fire or explosion, very effective electrical insulation of the sensor arrangement is mandatory. This of course can prove relatively expensive. One simple solution to this problem has been the provision of a vertically arranged transparent, usually glass, tube communicating with the liquid container. As the liquid level in the tube is the same as that in the container the volume of the liquid in the container is readily determined. However such an arrangement is fragile and can thus lead to spillage of the liquid.

The object of the invention is to minimise orto overcome these disadvantages.

According to one aspect of the invention there is provided a sensor arrangement for indicating the presence of a liquid, the arrangement including at least one transparent corner cube reflector element coupled to a light source via an optical fibre, collimating means disposed between the fibre and the reflector such that the incident light beam from the fibre end is reflected back thereto, a branch fibre coupled to the fibre for receiving reflected light signals from the reflector, and detector means for detecting said reflected signals, the arrangement being such that when the reflector is immersed in the liquid refraction of incident light into the liquid takes place thus extinguishing the reflected light beam to the detector.

According to another aspect of the invention there is provided a liquid sensor element, including a tubular housing, a corner cube reflector mounted in the housing and coupled to an optical fibre via lens means, the fibre and lens being so disposed that light from the fibre and incident on the reflector is directed back to the fibre over a substantially identical path.

According to a further aspect of the invention there is provided a liquid level sensor arrangement, including a plurality of corner cube reflector sensor elements which elements, when the arrangement is in use, are disposed each at a different predetermined level in a vessel containing the liquid, optical fibres one associated with each said sensor element, means for directing light via the fibres to the sensor elements, branch fibres are coupled to each said optical fibre for carrying reflected light signals from the sensor elements, means for detecting the presence or absence of a reflected light signal in each said branch fibre, the reflected signal being absent when the respective sensor element is immersed in the liquid, and digital logic means for determining from the number of reflecting and non-reflecting sensor elements the position of the liquid surface.

It is well known that at any interface between two media of different refractive index refraction of light takes place, the refraction conditions being defined by Snell's law. A particular feature of such refraction is that there exists a critical angle of incidence, for light in the medium of higher refractive index, such that total internal reflection occurs. Thus, by providing a transparent body with a surface against which light impinges at a suitable angle of incidence, a sensitive liquid level sensor can be obtained. In par ticularthis surface can be provided by a corner cube reflector which has three orthogonal plane surfaces.

This has the unique property that any incident light beam is reflected back along a paralleled path thus greatly simplifying the optical alignment of arrangements using such a device.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. lisa schematic diagram of an optical liquid level sensor arrangement; Fig. 2 shows a corner cube sensor element for use with the arrangement of Fig. 1; Fig. 3 shows a multi-level sensor arrangement.

Referring to Fig. 1,the sensor arrangement includes a light source 11, typically an LED or a semiconductor laser, coupled to an optical fibre 12.

This fibre 12 is coupled via a beam splitter 13 to a main optical fibre 14 which latter fibre carries light to and from a sensor head 15. Light reflected from the sensor head 15 is guided back along the fibre 14 via the beam splitter 13 to a branch fibre 16 leading to a photodetector arrangement 17.

Fig. 2 shows a sensor head for use with the arrangement of Fig. 1. This consists of a corner cube reflector 21 mounted in a housing 22, e.g. of glass, and coupled to the optical fibre 14 via a beam spreading and collimating lens 23, the end of the fibre 14 being disposed at the focus of the lens 23. Advantageously the lens is formed integral with the reflector.

The critical angle of incidence for an interface between an optical medium and air will of course depend on its refractive index. Advantageousiy such a reflector 21 is made of a glass the refractive index of which will generally be within the range 1.4 to 1.7.

The following table shows the relationship between critical angle and refractive index for this index range.

TABLET

Refractive Index 1.4 1.5 1.6 13 Critical Angle 145.58 leo 41.81 | 38.68 | 36.03 The corner cube reflector 21 has three orthogonal plane reflecting surfaces which provide it with a uni que property that any incident light beam is reflected back along a parallel path. If the direction of the inci dent beam is parallel to the diagonal ofthe cube then the angle of incidence on any of the plane surface is 54.749 This is clearly greater than the critical angle for any glass refractive index in the table shown above.Thus, if the corner cube is in air all light parallel to the diagonal of the cube is totally internally reflected and is received by the detector 17 (Fig. 1).

If the sensor head is immersed in a liquid of refractive index greater than n sin' 54.74 , where n is the refractive index of the liquid, there is then no total internal reflection and substantially all of the light is refracted into the liquid. Thus there is substantially no light received by the detector. The critical refractive index of I iquids with various cube material indices in the range 1.4 to 1.7 is illustrated in the following table.

TABLEII

Sensorlndex 1.41.5 1.6 1.7 Liquid Index 1. 151.221.311A Thus, by ensuring that the cube reflector 21 has a suitable refractive index in relation to the liquid to be detected, a compact and inert level detector can be provided. Furthermore, if the cube reflector index is suitably chosen then there is total internal reflection for liquids of refractive index less than the critical valuen sin 54.74" and refractive light loss (extinction) in liquids of higher refractive index. Thus the level of an interface between two immiscible liquids of differing refractive index can be determined. For example, if the cube reflector is made of sapphire with an index of 1.7 then the liquid critical index is 1.4.Thus, for instance, water (index 1.33) would permit total internal reflection whereas oils (index greaterthan 1.4) would provide a refractive light path.

It is preferred that the light directed into the corner cube reflector 21 be collimated into a parallel beam.

A divergent beam has many angles of incidence some of which are not critical in air. Furthermore the reflected beam will also be divergent and difficult to collect. This necessary collimation is achieved by the lens. The end of the fibre is adjusted to the focus of the lens, using standard fibre optic alignment techniques, and is then sealed in position with a body of an epoxy resin or low melting point glass. As the diameter of a typical optical fibre is of the order of 30 microns or less the fibre end approximate to a point source there is very little divergence of the collimated expanded beam. This small divergence is given by the expression sin (df) where d is the fibre diameter and fthe focal length of the lens.Thus it will be clearthat by choosing a suitably long lens focal length in comparison with the fibre diameter this divergence can be reduced to negligible propor- tions.

In an alternative arrangement, which permits the use of a relatively large diameter fibre, the fibre end is placed further away from the lens at a point coincident with the conjugate image through the lens.

Using the standard lens formula we have u=v= Df D-2f where f is the focal length and D is the distance between the lens and the cube reflector. For example, if D = 3f then u = v = 3f, so a typical sensor could be made using a 50 micron fibre positioned 6 mm from a lensed corner cube off = 2 mm. The corner cube would be placed 6 mm from the lens.

In use the sensor head is placed at a predetermined level in a tank containing the liquid, the fibre 14 providing for remote sensing. As the sensor head has no moving parts and carries no electrical signals the arrangement is intrinsically safe when used with hazardous liquids. Also by using a high transmission single fibre system measurement can be effected over a relatively long distance.

Fig. 3 shows an arrangement with a plurality of sensor elements 31 which may be arranged at different levels in a vessel or tank containing a liquid so as to give a digital reading corresponding to the liquid level. Each element 31 is coupled to a common light source 32 via a respective fibre 33 and to a detector unit 34 via a respective branch fibre 33a. Each sensors element is coupled to its own photodetector in the detector unit 34.

In a typical arrangement the detector unit 34 is coupled to a multiplexer 35 to which the signals from the individual photodetectors are fed, control of the multiplexer being effected in a time division multiplex manner via a free running sequencer control circuit 36. This circuit selectively enables each channel of the multiplexer 35, thus interrogating each fibre 33a in turn, and at the same time provides an output code signal identifying the individual fibre 33a being interrogated. The output signal from each respective photodetector, together with the fibre idendification code, is fed to an output logic circuit 37 which circuit provides a digital indication of the liquid level from the information received from the various fibres 33a, this indication being displayed on a display device 38, which device may for example be of the liquid crystal type. Advantageously the output logic circuit 37 includes a verification circuit for compensating for a non-functioning sensor element or fibre should the sensor element fibre become damaged. The logic output circuit may also have adjustable calibration means so as to provide an output reading in terms of liquid volume measure. Typically the logic output circuit comprises a suitably programmed microprocessor. In some applications the output logic circuit can be synchronised with the sequencer thus obviating the need for the identification code.

In a further modification each fibre 33 may be provided with its own light source in place of the com mon source 32, the light sources being operated in a pulse mode and each being synchronised with the sequencer 36. In this embodiment the multiplexer 35 can thus be dispersed with.

The sensor elements 15 of the arrangement shown in Fig. 3 can be mounted at regular intervals through the side wall 39 of a tank containing the liquid. However it is preferred to mount the sensor elements 15 at regular intervals along a tube of an inert material, e.g. glass, the fibre 33 leading to the sensor elements 15 being fed along the bore of the tube to one end thereof. The other end of the tube preferably sealed.

In this way a probe unit is provided which may be inserted into a liquid coating vessel. This obviates the need to provide openings in the side wall of the vessel.

In the arrangements of Figs. 1 to 3 the sensor element or elements may be illuminated by a steady light signal from the light source. However, to minimise the effect of external light, it is preferred to employ a pulsed light source coupled to a synchronised detector system. This has the added advantage of prolonging the working life of the laser or LED light source.

As the arrangement is intrinsically safe it can be used in a variety of remote sensing applications, for example, it can be employed in a tanker vehicle or vessel for sensing the level of oil or a hazardous liquid. Other applications include water level sensing systems in nuclear reactors where some form of remote sensor is essential.

In some applications the lens may be replaced by other collimating means, e.g. a 'selfoc' lens. Such a lens may be a discrete component or it may for part of the fibre termination.

Where the sensor element is disposed in a dirty liquid it may be subject to fouling. This can easily be overcome by applying high pressure bursts of ciean liquid to the sensor element at regular intervals.

Using such a techique the arrangement described can be used for e.g. monitoring effluent levels in drains or for determining the level of an oil-water interface in a tanker vessel.

Claims (20)

1. A sensor arrangement for indicating the presence of a liquid, the arrangement including at least one transparent corner cube reflector element coupled to a light source via an optical fibre, collimating means disposed between the fibre and the reflector such that the incident light beam from the fibre end is reflected back thereto, a branch fibre coupled to the fibre for receiving reflected light signals from the reflector, and detector means for detecting said reflected signals, the arrangement being such that when the reflector is immersed in the liquid refraction of incident light into the liquid takes place thus extinguishing the reflected light beam to the detecfor.

2. A sensor arrangement as claimed in claim 1, and wherein the light beams between the collimat ihg means and the reflector are parallel beams.

3. A sensor arrangement as claimed in claim 1 or 2, and wherein the lens means and the reflector are formed from a single transparent body.

4. A sensor arrangenflen-e as claimed in any one of claims 1 to 3, and wherein the light source provides a pulsed signal, the detector being synchronised with the light source.

5. A sensor arrangement as claimed in any one of claims 1 to 4, and wherein the light source is an LED or a semiconductor laser.

6. A sensor arrangement as claimed in any one of claims 1 to 5, wherein the reflector is formed from sapphire.

7. A sensor arrangement as claimed in any one of claims 1 to 6, wherein the collimating means comprises a selfoc lens.

8. A sensor arrangement as claimed in any one of claims 1 to 7, and which includes means for periodic cleaning of the reflector element.

9. A liquid sensor arrangement substantially as described herein with reference to Figs. 1 and 2 of the accompanying drawings.

10. A liquid sensor element including a tubular housing, a corner cube reflector mounted in the housing and coupled to an optical fibre via lens means, the fibre and lens being so disposed that light from the fibre and incident on the reflector is directed back to the fibre over a substantially identical path.

11. A sensor element as claimed in claim 10, wherein said lens is a selfoc lens.

12. A liquid sensor element substantially as described herein with reference to Fig. 2 of the accompanying drawings.

13. A liquid level sensor arrangement, including a plurality of corner cube reflector sensor elements which elements, when the arrangement is in use, are disposed each at a different predetermined level in a vessel containing the liquid, optical fibres one associated with each said sensor element, means for directing light via the fibres to the sensor elements, branch fibres are coupled to each said optical fibre for carrying reflected light signals from the sensor elements, means for detecting the presence or absence of a reflected light signal in each said branch fibre, the reflected signal being absent when the respective sensor element is immersed in the liquid, and digital logic means for determining from the number of reflecting and non-reflecting sensor elements the position of the liquid surface.

14. A level sensor arrangement as claimed in claim 13, wherein the sensor elements are mounted on an elongate probe, said probe being partially immersed in the liquid when the arrangement is in use.

15. A level sensor as claimed in claim 13 or 14, wherein the output signal from each branch fibre is determined in sequence.

16. A level sensor as claimed in claim 13, or 15, wherein said digital logic means comprises a microprocessor.

17. A level sensor as claimed in any one of claims 13 to 16 and which includes a digital visual display device for indicating the liquid level.

18. A liquid level sensor arrangement substantially as described herein with reference to Figs. 2 and 3 of the accompanying drawings.

19. A method of liquid level measurement substantially as described herein with reference to the accompanying drawings.

20. A tanker vessel or vehicle provided with one or more sensing arrangements as claimed in any one of claims 1 to 9 or claims 13 to 18.