GB2130739A

GB2130739A - Moisture measurement

Info

Publication number: GB2130739A
Application number: GB08232819A
Authority: GB
Inventors: Andrew Peter Harrison
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-11-17
Filing date: 1982-11-17
Publication date: 1984-06-06
Also published as: DE3340283A1; GB2130739B; JPS5992332A

Abstract

The technique of attenuated total reflection absorption spectroscopy used for I.R. analysis of liquids is modified to be applicable to gases by the use of an optical fibre as a sensor element to enable the measurement of the moisture content of gases. A large number of turns 30 of an unclad optic fibre are arranged in a chamber 23 through which the gas is circulated. Specifically fluoride glass is used for the fibre. <IMAGE>

Description

SPECIFICATION Moisture measurement This invention relates to the measurement of moisture levels, particularly moisture levels in gases.

According to the present invention there is provided an instrument for measuring moisture content, particularly the moisture content of gases, which instrument includes means for determining at a wavelength absorbed by water vapour the attenuation of infra-red light launched into a length of optical fibre whose core is exposed to the ambient over a substantial portion of that length.

The invention also resides in a method of measuring moisture content, particularly the moisture content of gases, which measurement is derived from a measure of the optical attenuation at a wavelength in the infra-red absorbed by water vapour of a length of optical fibre whose optical core is exposed to the ambient over a substantial proportion of that length.

There follows a description of the background to the intention and of a preferred embodiment thereof. The description refers to the accompanying drawings in which: Figure 1 depicts a schematic diagram of the sensor of a prior art attenuated total reflection spectroscopy (ATR) system which may be used for measuring the moisture content of liquids Figure 2 is a block diagram of a moisture measurement system according to the present invention, and Figures 3 and 4 depict schematic diagrams of the sensor of Figure 2.

Attenuated total internal reflection spectroscopy (ATR) is an established analytical technique for determining the concentrations of components in liquids by reference to their infrared absorption characteristics. The ATR sensor is normally in the form of a crystal shaped so that a beam of infra-red radiation launched into one end is 'totally' internally reflected a number of times before re-emerging and being detected. If the crystal is immersed in a liquid containing a component posessing an absorption band at the wavelength of the 'totally' internally reflected light, the reflection coefficient at each 'total' internal reflection is not unity because the radiation field is not confined exclusively to the crystal, but an evanescent term which extends a short way into the surrounding liquid.Therefore the beam will be attenuated at each reflection by an amount related to the concentration of the absorbing component and to the absorptivity of that component at that wavelength. To make the measurement it is typical for two wavelengths to be selected from the output of the infra-red source, one registering with an absorption peak of the component of interest and the other at a nearby wavelength that is not absorbed to any appreciable extent. The proportional signal difference produced by a given level of the component being measured is detected, amplified, and converted into a concentration reading.

Water absorbs strongly in the wavelength band between 2.7 and 2.9 microns due to the resonance of the OH bond within this band. A method of measuring water levels in liquids, such as oil, using a silicon or germanium ATR crystal by monitoring the changes in the 2.9 micron signal level has become routine in laboratories and is now being adopted in industry. The shape of a typical crystal for this purpose is depicted in Figure 1. Radiation 10 enters the crystal 11 through facet 12, makes a number of reflections proceeding down the crystal, one at the remote end 1 3 of the crystal, and then a further set of reflections up the crystal to emerge by way of facet 14. This arrangement in which the radiation enters and leaves the crystal by way of the same end facilitates the suspension of the crystal in a liquid medium.The dimensions of a typical ATR crystal are 50 mm long by approximately 10 mm in diameter.

By this technique dissolved water may be detected and measured in methanol, ether, acetone, glycol etc. down to less than 1% SO by weight. In oils water levels of less than 0.02% may be determined.

ATR crystals are however, not sufficiently sensitive to detect and monitor the presence of moisture in gases owing to the very much lower density of molecules in a gas than in a liquid. Thus the typical water in oil sensitivity of 100 ppm by weight for a conventioal ATR crystal corresponds to about 5 x 1 0-2 9 water per litre of oil, whereas to detect 100 ppm by weight of moisture in air requires a detection sensitivity of about 8x 10-5 g/L.

The present invention discloses how the sensitivity of ATR can be very significantly improved by replacement of the crystal sensor with an optical fibre sensor. This offers a considerable gain in sensitivity mainly because a long thin fibre will possess a much increased length to diameter ratio over a typical ATR crystal (e.g. by a factor of 104 to 106 thus permitting a much greater number of reflections). Typically this will involve changing from germanium which has a high refractive index of about 4.1 to a glass having a refractive index of about 1.5. In making the transition from a germanium/oil to a glass/air system it will be noted that the critical angles are different in the two instances and also the skin depth i.e. the distance from the interface at which the evanescent field is reduced to l/e of its value at the interface.The former effect tends to reduce the sensitivity because the critical angle is greater, and hence fewer reflections are made per unit length; while the latter effect tends to increase the sensitivity because, for an angle of incidence greater than the critical angle by a specific amount, the skin depth is greater.

In order that the absorption of power from the evanescent field outside the confines of the fibre itself shall not be marked by absorption of power within the fibre it is necessary to use a relatively low loss material from which to manufacture the fibre.

In the field of low loss optical fibres much work has been carried out in respect of fibres made of silica. However, silica is not ideal for the present purpose partly because transmission losses begin to increase rapidly with wavelengths longer than 1.7 microns due to phonon absorption caused by the Si-O bond, and partly because of problems caused by residual hydroxyl contamination of the material of the fibre. In this context it is to be noted that, on account of the much higher power density within fibre than in the evanescent field region surrounding it, a given level of hydroxyl contamination in the fibre will cause for greater attenuation of the propagating signal than the same level in the atmosphere surrounding the fibre.However, although silica has a high affinity for hydrogen, much research has been undertaken in connection with the manufacture of telecommunications fibre into ways of removing hydroxyl contamination.

The potential problems of hydroxyl contamination are reduced by using fibre made of one of the new fluoride glasses being developed for mid infra-red long haul optical fibre transmission. A typical glass composition is 57 mole % HfF4, 36 mole % BaF2, 4 mole % AIF3, 3 mole % LaF3. Problems of eliminating hydroxyl contamination from such glasses are equivalent to those in silica, but higher residual hydroxyl contamination can be tolerated in the fibre because the hydroxyl absorption peak in the glass is not coincident with the same absorption peak in the surrounding air. Thus for instance in the case of fluorohafnate glasses the hydroxyl absorption peak is at 2.9 microns, whereas free water vapour peaks at 2.7 to 2.75 microns. Other glasses, including chalocogenide glasses, may also prove suitable.

Referring to Figure 2, the basic components of an optical fibre moisture meter consist of an optical attenuation measuring device 21 containing an optical source and associated drive circuitry for launching substantially mono chromatic radiation into one optical fibre of a cable 22 and a photodetector and associated circuitry for measuring the amount of radiation emanating from another optical fibre of the cable 22. The other ends of these two optical fibres are connected to the opposite ends of the sensor fibre which is housed in a chamber 23. Referring to Figures 3 and 4, the sensor fibre consists of a large number of turns 30 of unclad fibre wound in a single layer open helix upon a frame 31 which hold the turns in position with the minimum contact length between the frame and the fibre.

The chamber 23 is cylindrical or toroidal in shape, and is provided with tangential inlet and output ports 32 for the gas whose moisture level is to be determined. Since the fibre is an unclad fibre it is necessary for the portions of the frame with which it comes into contact to be made lower refractive index. This is conveniently achieved by facing the fibre supporting surfaces of the frame with low refractive index plastics material.

Alternatively the sensor may be constructed from fibre provided with an optical cladding which, after the fibre has been wound on the frame, is selectively removed by etching so as to leave the cladding intact where the fibre passes over each support point of the frame, and expose the optical core over substantially the whole of the intervening regions. Typically the sensor fibre is about 100 microns in diameter (core diameter in the case of a clad fibre), and about 40 metres long. The support surfaces of the frame may be grooved to facilitate the even spacing of adjacent turns, and the fibres may be fixed in position by the application of a bead 34 of low index cement running along the length of each support surface.

This has the advantage of enabling the provision of a fully wetted joint between the fibre and its supports rather than a dry joint.

Claims

1. An instrument for measuring moisture content, particularly the moisture content of gases, which instrument includes means for determining at a wavelength absorbed by water vapour the attenuation of infra-red light launched into a length of optical fibre whose core is exposed to the ambient over a substantial portion of that length.

2. An instrument as claimed in claim 1, wherein the fibre is an unclad fibre wound in separated helical turns upon a frame whose surface at the points of contact with the fibre has a lower refractive index than that of the fibre.

3. An instrument as claimed in claim 2, wherein at the points of contact with the frame, the fibre is embedded in a resin of lower refractive index than that of the fibre.

4. An instrument as claimed in claim 1, wherein the fibre is wound in separated helical turns upon a frame and is an optically clad fibre from which the optical cladding has been removed in regions between it is supported by the frame.

5. An instrument as claimed in claim 2, 3, or 4, wherein the fibre and its frame is supported in a substantially cylindrical chamber.

6. An instrument as claimed in claim 2, 3 or 4, wherein the fibre and its frame is supported in a substantially toroidal chamber.

7. An instrument as claimed in any preceding claim, wherein the composition of the fibre is such that the hydroxyl contamination absorption peak of the material of the fibre is significantly displaced with respect to that of water vapour.

8. An instrument as claimed in claim 7, wherein the fibre is made of a fluoride glass.

9. An instrument for measuring the moisture content of gases, which instrument is substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.

10. A method of measuring moisture content, particularly the moisture content of gases, which measurement is derived from a measure of the optical attenuation at a wavelength in the infrared absorbed by water vapour of a length of optical fibre whose optical core is exposed to the ambient over a substantial proportion of that length.

1 A method as claimed in claim 10, which employs a fibre made of a material whose hydroxyl contamination absorption peak is significantly displaced with respect to that of water vapour.

12. A method wherein the gas is caused to flow through a chamber containing the fibre.

1 3. A method of measuring moisture content, which method is substantially as hereinbefore described with reference to Figures 2, 3 and 4 of the accompanying drawings.