GB2255405A

GB2255405A - Atmospheric liquid contact sensor

Info

Publication number: GB2255405A
Application number: GB9109628A
Authority: GB
Inventors: Graham James Harden; Paul Roderick Norman; Edward Crowson
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1991-05-03
Filing date: 1991-05-03
Publication date: 1992-11-04
Also published as: GB9109628D0

Abstract

A liquid content sensor has a length of sheathed optical fibre (11) with a modulated (preferably pulsed) light source (10) at a first end and a light intensity measuring device (13) at a second end. The optical fibre has a length (30) bared for exposure to an atmosphere, and impingement of liquid droplets (17) thereon causes a reduction in passage of light through the fibre (14) (provided that the refractive index of the fibre is not greater than that of the liquid). The reduction in transmitted light is compared with a calibration. <IMAGE>

Description

LIQUID CONTENT TESTING The present invention relates to apparatus and methods for testing the liquid content of an atmosphere, and is particularly concerned in testing for the content of liquid other than water in an atmosphere where water droplets might also be present.

It is frequently desired to know the amount of a particular liquid present in an atmosphere. This can be for health and safety reasons or for control of an industrial process. The liquids of interest are frequently of the type where their presence in the atmosphere consists of liquid droplets rather than vapour, and it is also very often the case that water droplets are also present. In a known method of carrying out such testing a sample of the atmosphere is passed through a filter to remove the liquid, the filter being weighed before and after the sample has been passed. This method can, of course, work only when a known particular liquid is present, and will almost certainly give imprecise results if water droplets are present in the atmosphere.In another method samples of the atmosphere are passed through a laser beam and light reflected from liquid particles is analysed to distinguish the numbers of liquid particles present, and to distinguish liquid particles from solid particles such as dust and pollen. Using this method it is frequently possible to distinguish certain liquids by their characteristic particle sizes. Examples of laser testing methods are given in PCT/GB88/00974, PCT/GB88/00972 and PCT/GB88/00975. The apparatus used in these laser methods is expensive and complicated, and also comparatively fragile, and when used for field testing, as is frequently desired, forms a bulky and delicate package.

The present invention provides a simple, comparatively cheap testing apparatus.

According to the present invention a liquid content sensor includes an optical fibre at least a length of which is bared for exposure to an atmosphere, a monochromatic modulated light source at a first end of the fibre and a light intensity measuring device at a second end of the fibre.

When the sensor is to be used with a particular liquid the refractive index of the optical fibre will be substantially the same as but not greater than that of the liquid.

The modulated light source is preferably pulsed.

A number of fibres might be used in parallel. The fibres might advantageously have a number of different refractive indices, these being substantially the same as but not greater than the refractive indices of various liquids for which it might be desired to test.

According to another aspect of the invention a method of measuring the content of a particular liquid in an atmosphere includes the steps of: Exposing a bared length of optical fibre to the atmosphere; Supplying a beam of monochromatic pulsed light to a first end of the fibre; Measuring the light intensity supplied at a second end of the fibre; and Comparing the light intensity at the second end of the fibre before exposure of the fibre to the atmosphere and the light intensity obtained after exposure with a calibration.

The optical fibre preferably has a refractive index substantially the same as but not less than that of the liquid whose content is being measured.

Some embodiments of the invention, and the manner in which the invention works, will now be described, by way of example only, with refence to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation of apparatus according to the invention, Figure 2 is a plan view of part of the apparatus of figure 1, Figure 3 is a detail, in section of part of a sheathed optical fibre used in the apparatus, Figure 4 is an elevation of a bared optical fibre, Figure 5 is an elevation of the fibre of figure 4 upon which a droplet of liquid has impinged, and Figure 6 is a graph of light intensity passing through an optical fibre against refractive index of the fibre.

An apparatus according to the invention (Figure 1, Figure 2) has an optical source 10 capable of providing a pulsed monochromatic light to a sheathed optical fibre 11. The sheathed optical fibre 11 extends through an open box 12 to a light intensity measuring unit 13, a length 30 of fibre which extends across the box 12 being bared. The sheathed optical fibre (Figure 3) has a translucent core 14 having a refractive index n covered by a coating 15 having a refractive index nc.

g As is well known in the art when nc is less than n light passing along the fibre 14 g will be reflected at every interface between fibre 14 and coating 15, and virtually all light entering the fibre 14 at one end will be emitted at the other end. Should the coating 15 be removed, to leave the bared fibre 14 which passes across the box 12 (Figure 4) light will be reflected without loss from the interface between the fibre 14 and air 16 provided the refractive index na of the air is lower than that ng of the fibre, which will usually be the case. However if the refractive index nc of the coating 15 or na of the air 16 were greater than that ng of the fibre 14 light would not be reflected and would pass out of the fibre 14.

Similarly, when a drop of liquid 17 (Figure 5) impinges on the bared fibre 14, light passing along the fibre 14 which hits the interface of the fibre 14 and the drop 17 will not be reflected, but will escape as indicated at 18, if the refractive index n1 of the droplet 17 is greater than the refractive index ng of the fibre 14.

Thus, in the apparatus shown in figure 1, if a signal from the light intensity measuring device 13 is passed to a processing unit 20 which, inter alia, has a display 21 of the light intensity measured by the measuring device 13 the reading indicated by the display 21 will decrease each time a liquid droplet 17 having a refractive index nl greater than ng impinges on and adheres to the bared fibre 14.

It has been found that this reduction in light intensity passing through the bared fibre 14 is very precise and repeatable. Thus, as illustrated in figure 6, the decrease in light intensity transmitted by the apparatus will be reduced by a certain amount when one drop 17 adheres to the bared fibre 14, exactly twice that amount with two drops, 3 times that amount with three drops and so on. Furthermore the reduction in intensity is greatest when the refractive index nl of the liquid forming a drop 17 is the same as the refractive index ng of the optical fibre 14.Therefore if the apparatus is required for testing the liquid content of a particular liquid in an atmosphere it is advantageous if the refractive index ng of the fibre 14 used in the test apparatus be the same as (but usually, in practice, slightly less than) that nl of the liquid in question.

In use the apparatus is calibrated by passing pulsed monochromatic light from the light source 10 through the optical fibre 11,12 to the light intensity measuring device 13 and placing one drop of the liquid in question on the bared fibre 14. The reduction in light intensity measured by the device 13 can be displayed on the display 21 and will be preferably stored in a memory of the calculating device 20. The device is then further calibrated by placing the box 12 and its associated bared fibre 14 into an atmosphere of known liquid content for a suitable time and noting the change in reading of the light intensity measuring device 13. This can be repeated for a number of atmospheres of known liquid levels, the resulting calibration being stored in the device 20 or otherwise.

Once calibrated the device is used by placing it in an atmosphere whose liquid content in terms of the particular liquid it is desired to know for the appropriate period, measuring the reduction in light intensity passed by the measuring device 13 and passing the results to the calculating unit 20. Droplets of liquid other than the liquid of interest which impinge on the bared fibre 14 will, if their refractive index is lower than ng, cause no loss. If a drop of liquid having a refractive index greater than that, ng, of the fibre 14 (and the refractive index n1 of the liquid of interest), such as shown at nl 2 in figure 6, this will, of course, cause a reduction in light intensity transmitted.However the reduction caused by this drop will be less than that caused by a drop of the liquid of interest, as illustrated at d in figure 6, and this difference will be distinguished by the calculating means 20 such that the loss caused by this drop will be ignored. The light intensity drop measured by the measuring device 13 will be processed by the calculating means 20 to exclude unwanted drops (which might, for example, be of water) and compared against the stored calibration, the results then being presented as required; for example by presentation on the display 21.

For atmospheres with a plurality of liquids the above described technique can be followed using a box 12 through which pass a plurality of bared fibres 14, each fibre 14 having a refractive index n 1, n 2, ng3 etc. appropriate to a g g particular liquid. For improved accuracy, of course, a plurality of fibres 14 for each liquid might be used crossing the box 12 (for example, where interest is only in 1 liquid the refractive indices of the fibres 14 shown in figure 2 would be equal).

The pulsed light source may be replaced by a modulated source. By using a pulsed or modulated light source the invention can be carried out in unsteady ambient light conditions. It will be realised, of course, that if the ambient light is artificial and created from an alternating power source the frequency of the pulsed light should be other than that of the power source, or of any harmonic of the power source.

Claims

What is claimed is: 1. A liquid content sensor including at least one optical fibre at least a length of which is bared for exposure to an atmosphere, a monochromatic modulated light source at a first end of the fibre and a light intensity measuring device at a second end of the fibre.
2. A liquid content sensor as claimed in Claim 1 wherein the modulated light is pulsed.
3. A liquid content sensor as claimed in Claim 1 or in Claim 2 for use with a particular liquid and wherein at least one optical fibre has a refractive index substantially the same as but not greater than the refractive index of the liquid.
4. A liquid content sensor as claimed in any one of Claims 1 to 3 wherein there are a plurality of optical fibres.
5. A liquid content sensor as claimed in Claim 1 or in Claim 2 for sensing the content of a plurality of liquids and having a plurality of optical fibres, there being for each liquid at least one optical fibre having a refractive index substantially the same as but not less than that of the liquid.
6. A method of measuring the content of a particular liquid in an atmosphere including the steps of: exposing a bared length of optical fibre to the atmosphere; supplying a beam of monochromatic modulated light to a first end of the fibre; measuring the light intensity supplied at a second end of the fibre; and comparing the light intensity at the second end of the fibre before exposure of the fibre to the atmosphere and the light intensity obtained after exposure with a calibration.
7. A method as claimed in Claim 6 wherein the optical fibre has a refractive index substantially the same as but not greater than that of the liquid.
8. A method as claimed in Claim 6 or in Claim 7 wherein the monochromatic light is pulsed.
9. A liquid content sensor substantially as herein described with reference to Figures 1 to 6 of the accompanying drawings.
10. A method of measuring liquid content of an atmosphere subtantially as herein described with reference to Figures 1 to 6 of the accompanying drawings.