GB2293007A

GB2293007A - Waveguide sensor

Info

Publication number: GB2293007A
Application number: GB9417005A
Authority: GB
Inventors: Fenghong Zhang; Patricia Jane Scully; Elled Lewis
Original assignee: Liverpool John Moores Univ
Current assignee: Liverpool John Moores Univ
Priority date: 1994-08-23
Filing date: 1994-08-23
Publication date: 1996-03-13
Anticipated expiration: 2014-08-23
Also published as: GB2293007B; GB9417005D0

Abstract

An apparatus for measuring the concentration of particulate matter suspended in a fluid (e.g. yeast particles suspended in water) has first and second optical fibres 16, 18 secured in a parallel relationship and spaced from each other. A portion of each of the optical fibres is treated (e.g. by means of a coating or by removing a cladding layer) such that radiation from a radiation source 20 passes along the first optical fibre 16 and into the fluid. The radiation is scattered by the particles suspended in the fluid and a portion of the scattered light enters the treated area of the second optical fibre 18. The radiation entering the second optical flbre is detected by a detector 22, 24 located at one or both ends of the second optical fibre. The amount of radiation received by the second optical fibre 18 is a function of the particle concentration (see fig 3). The apparatus thus provides a noninvasive, virtually instantaneous and continuous measure of particle concentration. <IMAGE>

Description

DESCRIPTION WAVEGUIDE SENSOR The present invention relates to waveguide sensors, and in particular, but not exclusively, to optical fibre sensors for use in detecting concentrations of particulate matter (such as yeast particles) suspended in liquid.

At present, if it is desirable or necessary to determine the concentration of particulate matter suspended in liquid it is necessary to remove a sample of the liquid and analyse it, for example by counting the number of particles in the sample by means of a microscope. One disadvantage of this is that the removal and analysis of the sample is a time-consuming and relatively tedious process, which is exacerbated if it is necessary to determine particle concentrations repeatedly. A second disadvantage is that the analysis takes a considerable time to perform, with the result that there is a time lag between removing the sample and determining the particle concentration. Therefore, it is impossible to monitor particle concentrations in "real time".

These problems are particularly acute in many industrial processes where an accurate measure of particle concentration can be vital in order to monitor a chemical or biochemical reaction.

It is an object of the present invention to provide a method and apparatus for detecting concentrations of particulate matter in liquid which do not involve the analysis of sample removed from the liquid and which provide an almost instantaneous measure of particle concentration.

In accordance with a first aspect of the present invention, a method of measuring the concentration of particulate matter suspended in a fluid comprises transmitting radiation along a first waveguide which projects into the fluid and detecting the radiation received by, and transmitted along, a second waveguide spaced from the first waveguide and which also projects into the fluid.

Such a method is based on the discovery that the amount of radiation received by the second waveguide is related to the amount of scattered radiation, which varies in accordance with particle concentration. The method does not require analysis of samples withdrawn from the fluid, is non-invasive and provides a virtually instantaneous and continuous measure of particle concentration.

It is also intrinsically safe since it is remote from any electrical or electronic equipment and is thus ideal for use with flammable fluids. The method is also immune to electromagnetic interference and is particularly suited to medical use.

Advantageously, the waveguides comprise optical fibres, which are readily available and relatively inexpensive.

Preferably, the two waveguides are fixed in position relative to one another (e.g. they are aligned parallel to one another) during measurement.

The radiation received by the second waveguide is propagated in both directions along the waveguide.

The radiation may be measured at one end or at both ends of the waveguide. If the radiation is measured at both ends the two measurements can be ratioed to give a normalised parameter which is immune to variations such as radiation source intensity.

Preferably, the intensity of the radiation should be measured by the or each detector. The first and second waveguides should be held in a fixed position relative to one another during measurement of particle concentration. If the waveguides comprise optical fibres, they may be treated (e.g. they may be coated and/or the cladding layer may be removed) to permit the light to be transmitted from the first optical fibre to the fluid and received by the second optical fibre.

In accordance with a second aspect of the present invention, an apparatus for detecting the concentration of particulate matter suspended in a fluid comprises a first waveguide for transmitting radiation from a radiation source to the fluid and a second waveguide spaced from the first waveguide for receiving radiation transmitted from the first waveguide through the fluid and transmitting it to a radiation detection means.

Advantageously, the waveguides comprise optical fibres.

Preferably, the optical fibres are treated to permit the transmission of radiation into the fluid from the first optical fibre and the receipt of radiation by the second optical fibre. The treatment may comprise the provision of a layer of material of greater refractive index than that of the cladding of the optical fibres, e.g. polymer-organic glass such as optical epoxy resin, on the surface of the fibres.

Another option is to remove the cladding layer of the optical fibre and not provide any additional coating.

Preferably, means are provided for securing a portion of the waveguides (e.g. optical fibres) in a fixed position relative to each other during measurement. Preferably, a portion of each of the waveguides are mounted in parallel relationship. The portions of the waveguides may be mounted in a rigid support member. The mounting may conveniently be achieved by securing the waveguides to the support member using the layer of material for treating the waveguides to enable the transmission and detection of radiation. The transmitting and receiving waveguides may be mounted on respective support members whose relative positions are adjustably securable.

In one embodiment the waveguides pass into and out of the fluid so that both ends of the fibres are outside the fluid, thus providing a distributed measurement arrangement which measures continuously along the length of the detection area. This also allows two detectors, one at either end of the second waveguide, to be used if desired.

In another embodiment one end of each of the optical fibres terminates in the fluid, thus providing a point sensor arrangement.

In one arrangement the first and second waveguides are formed into a plurality of separate, spaced apart detectors whose measurements can be obtained by means of optical time domain ref lectometry.

In accordance with a third aspect of the present invention, an apparatus for detecting the concentration of particulate matter suspended in a fluid comprises a radiation source, a first waveguide for transmitting radiation from the radiation source to the fluid, a second waveguide spaced from the first waveguide for receiving radiation transmitted from the first waveguide through the fluid and radiation detection means for detecting the radiation received by the second waveguide.

Preferably, the radiation detecting means are adapted to measure the intensity of radiation.

By way of example only, specific embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a first embodiment of optical fibre sensor in accordance with the present invention; Fig. 2 is a schematic illustration of a second embodiment of optical fibre sensor in accordance with the present invention; Fig. 3 is a graph illustrating the approximate variation of detector output signal with particle concentration, in particular for a suspension of yeast particles in water; Fig. 4 is a schematic side view of a third embodiment of optical fibre sensor in accordance with the present invention; Fig. 5 is a front view of the sensor of Fig. 4; and Fig. 6 is a schematic illustration of a fourth embodiment of optical fibre sensor in accordance with the present invention.

Referring firstly to Fig. 1, the first embodiment of optical fibre sensor comprises a stainless steel backing plate 10 to one face of which two lmm diameter PMMA core, PMMA-fluorine doped cladding, step index optical fibres 12,14 are permanently secured in parallel, spaced relationship by means of a respective layer 16,18 of optical epoxy resin. The polymer fibres 12,14 are conventional and comprise a core having a refractive index of 1.48 and a cladding layer having a refractive index of 1.40. Other fibres, such as polymer-clad silica core (PCS) fibres, can of course be used. The refractive index of the optical epoxy resin layers is 1.56 and the region of the epoxy resin is approximately 5cm long.Alternatively, the cladding may be removed and the optical fibre used without any additional layer, such that the core of the fibre contacts the monitored fluid, as shown schematically in Fig. 2. PCS fibre is more suitable in this application. If no additional layer is used, the optical fibres are held in place mechanically, e.g. by means of clips 19.

As shown in Figs. 1 and 2, light is transmitted along one of the optical fibres 12 from a light source 20. Light entering the other fibres 14 is transmitted along the fibre and can be detected by a detector 22, e.g. a photodetector, located at one end of the fibre 14 (and optionally by a second detector 24 located at the other end of the fibre 14).

In use, the apparatus is located in a vessel containing the liquid or gas whose particle concentration is to be measured, the transmitting and receiving fibres passing sealingly through the wall of the vessel. The refractive indices of the core and cladding (and the additional layer if present) are matched with the refractive index of the fluid to maximise transfer of radiation into and out of the fluid. Fluid whose particle concentration is to be measured is thus present between the optical fibres 12,14. The light source 20 (e.g. transmitting at 850nm) and the detector 22 (and optionally the detector 24) are switched on, causing light to be transmitted along the optical fibre 12 with minimal loss.As the light passes through the section of the optical fibre 12 coated with epoxy resin 16 or from which the cladding has been removed a portion of the light is radiated out of the fibre 12 into the fluid surrounding the fibre. The light radiated from the fibre 12 is scattered by particles in the fluid surrounding the fibre and a portion of the scattered light is received by the other optical fibre 14 through the epoxy resin layer 18 (if present), the scattered light then being transmitted along the fibre 14 where its intensity is detected by means of the light sensor 22,24 at one end or both ends.

The amount of light scattered, and hence the amount of light transmitted into the second optical fibre 14, is a function of the concentration of particles in the liquid or gas under test. Thus, as seen in Fig. 3, the detector output signal varies in accordance with particle concentration, the graph being drawn for yeast particles suspended in water.

Thus, by suitable processing of the signals from the or each detector 22,24 and calibration of the apparatus a virtually instantaneous value of particle concentration can be obtained.

The first embodiment is particularly suitable for distributed measurement of particle concentrations in flow-through applications such as pipes. The third embodiment, shown in Figs. 4 and 5, is more suited to vessel-type reactions such as fermentation where point measurement is made.

The second embodiment comprises a solid base 26 to which a first fixed stainless steel, fibre mounting body 28 is secured. The mounting body 28 has an enlarged head portion 30 of the same length and width as the base 28 and a narrower, fixed plate 32 projecting down from the head portion such that an overhang 34 is formed below the head 30. A second stainless steel plate 36 is adjustably secured to the fixed plate 28, with its face parallel to the face of the plate 32, by means of four bolts 37 passing through the plate 36 and threadedly received in bores in the fixed plate 28. The position of the plate 36 is thus adjustable.

A lmm diameter transmitting, optical fibre 38 (identical to the fibres in the first embodiment) passes sealingly through the base 26 and through the head portion 30. The optical fibre 38 is then fixedly secured to the inner face of the fixed plate 32 by means of a layer 40 of optical epoxy resin of refractive index 1.56. Similarly, an identical, receiving, optical fibre 42 passes sealingly through the base 26 and head portion 30 and is secured to the inner face of the movable plate 36 by means of a layer 44 of optical epoxy resin of refractive index 1.56.

As for the first embodiment, the region of the epoxy resin layers 40,44 is about 5cm long. Alternatively, the cladding may be removed and the optical fibre used without any additional coating.

A light source 46 is connected to the upper end of the transmitting optical fibre 38 and a light sensor 48, e.g. a photodetector, is connected to the upper end of the receiving optical fibre 42. In contrast to the first embodiment, the transmitting and receiving optical fibres 38,42 do not pass right through the apparatus but terminate on the respective plates 28,36.

In use, the position of the second stainless steel plate 36 is adjusted, depending upon the conditions, e.g. constituents of the liquid under test, likely particle concentrations, etc., and is secured in position at the desired location. The apparatus is then fitted to a reaction vessel such that in use the plates 28,36 project into the liquid contained therein. The shape of the base 26 is adapted to be sealingly received in a complimentarilyshaped aperture in a wall of the vessel, and is secured in position. The light source 46 and sensor 48 are then switched on, and as for the first embodiment in the region of the epoxy resin layers a portion of the light passing along the fibres 38 is radiated from the fibre into the fluid. The light is scattered by particles in the fluid and received by the fibre 42 which transmits the light to the sensor 48 where its intensity is measured. By processing the signal from the sensor 48 a virtually instantaneous value of particle concentration can be obtained.

Of course, the transmitting fibre 38 need not necessarily be secured to the fixed plate 28.

Instead, the receiving fibre could be secured to the fixed plate 28 and the transmitting fibre connected to the adjustable plate 36.

A further embodiment is illustrated in Fig. 6, which is a modification of the embodiment of Figs. 1 and 2. It will be described with reference to Fig. 1 but can be modified to replace the optical fibres of Fig. 1 with those of Fig. 2.

Instead of having a single detection location, several detection locations are provided at spaced apart intervals along the fibres, each location being a repetition of the arrangement shown in Fig. 1, there being only two optical fibres for all of the detection locations. By using optical time domain reflectometry a measurement of particle concentration can be obtained separately for each of the detection areas.

The invention is not restricted to the details of the foregoing embodiments. For example, waveguides other than optical fibres may be used and the wavelength of radiation may be varied as appropriate.

Also, the references to fluid are intended to cover liquids and gases. The refractive indices of the various components may also differ from those described above.

The detectors need not just sense the intensity of radiation but may, for example, be wavelengthsensitive.

Claims

1. A method of measuring the concentration of particulate matter suspended in a fluid, comprising transmitting radiation along a first waveguide which projects into the fluid and detecting radiation emitted from the first waveguide and received by, and transmitted along, a second waveguide, the second waveguide being spaced from the first waveguide and also projecting into the fluid.

2. A method as claimed in claim 1, wherein the two waveguides are fixed in position relative to one another during measurement.

3. A method as claimed in claim 2, wherein the two waveguides are aligned parallel to one another during measurement.

4. A method as claimed in any of the preceding claims, wherein the radiation is measured at one end of the second waveguide.

5. A method as claimed in any of claims 1 to 3, wherein the radiation is measured at both ends of the second waveguide.

6. A method as claimed in claim 5, wherein the measurements from the two ends of the second waveguide are ratioed to give a normalised parameter.

7. A method as claimed in any of the preceding claims, wherein the intensity of the radiation is measured.

8. A method as claimed in any of the preceding claims, wherein radiation is detected at a plurality of locations on the second waveguide.

9. A method as claimed in claim 8, wherein radiation is transmitted from a plurality of locations on the first waveguide.

10. A method as claimed in claim 8 or claim 9, comprising measuring the radiation received at the plurality of locations on the second waveguide by means of optical time domain reflectometry.

11. A method as claimed in any of the preceding claims, wherein the waveguides comprise optical fibres.

12. A method as claimed in claim 11, wherein the optical fibres are treated.

13. A method as claimed in claim 12, wherein the optical fibres are coated.

14. A method as claimed in claim 12, wherein a cladding layer of the optical fibres is removed.

15. An apparatus for detecting the concentration of particulate matter suspended in a fluid, comprising a first waveguide for transmitting radiation from a radiation source to the fluid and a second waveguide spaced from the first waveguide for receiving radiation transmitted from the first wave guide through the fluid and transmitting it to a radiation detection means.

16. An apparatus for detecting the concentration of particulate matter suspended in a fluid, comprising a radiation source, a first waveguide for transmitting radiation from the radiation source to the fluid, a second waveguide spaced from the first waveguide for receiving radiation transmitted from the first waveguide through the fluid and radiation detecting means for detecting the radiation received by the second waveguide.

17. An apparatus as claimed in claim 15 or claim 16, comprising means for securing a portion of the waveguides in a fixed position relative to each other during measurement.

18. An apparatus as claimed in claim 17, comprising means for mounting a portion of each of the waveguides in parallel.

19. An apparatus as claimed in any of claims 15 to 18, comprising a support member upon which the waveguides are mounted.

20. An apparatus as claimed in claim 19, wherein the wave guides are secured to the support member by means of a layer of material for treating the waveguides to enable transmission and detection of radiation.

21. An apparatus as claimed in claim 19, wherein the first and second waveguides are mounted on respective support members whose relative positions are adjustably securable.

22. An apparatus as claimed in any of claims 15 to 21, wherein the waveguides pass into and out of the fluid so that both ends of the fibres are outside the fluid.

23. An apparatus as claimed in claim 22, comprising two detectors, one located at each end of the second waveguide.

24. An apparatus as claimed in any of claims 15 to 21, wherein one end of each of the optical fibres terminates in the fluid.

25. An apparatus as claimed in any of claims 15 to 24, wherein the second waveguide comprises a plurality of, spaced-apart regions for receiving radiation.

26. An apparatus as claimed in claim25, wherein the first waveguide comprises a plurality of spacedapart regions for emitting radiation.

27. An apparatus as claimed in any of claims 15 to 26, wherein the waveguides comprise optical fibres.

28. An apparatus as claimed in claim 27, wherein the optical fibres are treated to permit the transmission of radiation into and out of the fluid.

29. An apparatus as claimed in claim 28, comprising a layer of material of greater refractive index than that of a cladding of the optical fibres.

30. An apparatus as claimed in claim 29, wherein the layer of material comprises optical epoxy resin.

31. An apparatus as claimed in claim 28, wherein a cladding layer of the optical fibres is removed.

32. A method of measuring the concentration of particulate matter suspended in a fluid, substantially as herein described, with reference to, and as illustrated in, the accompanying drawings.

33. An apparatus for detecting the concentration of particulate matter suspended in a fluid, substantially as herein described, with reference to, and as illustrated in, the accompanying drawings.