GB2186076A - Chemical monitor - Google Patents

Abstract

A chemical monitor has a solid- state source 12 of radiation (e.g. an LED) and a solid-state detector 16 for the radiation within a sample chamber 14. Gas is drawn through the sample chamber 14 and the source 12 activated by circuitry 20. The wavelength of the radiation is selected to correspond to an absorption wavelength of a component of the gas (e.g. methane) and the variation in radiation detected by the detector 16, measured by circuitry 22, then gives a measure of the amount of the component of the gas present. The sample chamber may be a straight or convoluted hollow tube, or a rectangular or square box with an elongate optical path then being achieved by mirrors within the box. The use of a solid-state source enables the source to be immersed in the gas, even if the gas is explosive or inflammable, thus eliminating the distortion or loss which occurs in systems where the source and gas must be separated. <IMAGE>

Description

SPECIFICATION Chemical monitor The present invention relates to a chemical monitor in which electromagnetic radiation is passed through a gas, and the effect of that gas on the radiation investigated, and to a method of using that monitor to monitor gas.

When electromagnetic radiation, and in particular, infra-red radiation is passed through a gas, radiation of specific wavelengths will be absorbed by the gas, the frequencies absorbed depending on the nature of the chemical compounds present in the gas i.e. in dependence on the components of the gas. By detecting the absorption it is possible to analyse the gas. Of course, to provide a full analysis it would be necessary to provide infrared radiation over a wide spectrum of wavelengths, but very often what is needed is to detect one, or at most a few, chemical compounds in the gas and hence only a few wavelengths are hecessary. For example, such a monitor may be used in a mine.to detect the presence of methane, or other dangerous gases.

In existing gas monitors using electromagnetic radiation, the radiation is generated from a tungsten heater, which produces a wide spectrum of infra-red radiation. The radiation is then passed through filters, with each filter passing a particular band of wavelengths. The radiation from one filter is passed to the gas, and detected. Changes in the radiation intensity detected correspond to changes in concentration of a compound which absorbs that wavelength in the gas, provided a second compound with similar infra-red absorption characteristics (interfering compound) is not present. By changing the filter, usually achieved by mounting the filters on a moving wheel which rotates in front of the heater, the wavelength of the radiation passing through the gas may be changed, so enabling more complex analysis to be made.

The big disadvantage, however, of this known type of chemical monitor is its size, power consumption and fire danger. Typically, the monitor weighs around 10 Ibs., and uses 30 w of power. Furthermore, the rotating filter wheel, being a moving part, is prone to failure and the speed at which wavelength values may be altered is limited. Finally, the fact that a tungsten heater is used means that firstly the fire danger is increased and secondly the infra-red source cannot emit directly into the gas, but that there must be something separating the two. The second feature introduces the possibility of distortion of the results by absorption of radiation before the radiation gets to the gas.

The present invention seeks to provide a chemical monitor, which overcomes, or at least ameliorates, these defects. In its most general form, the present invention involves the use of a solid-state radiation generator, such as a light-emitting diode (LED). It is possible to fabricate light-emitting diodes so that they emit specific wavelengths or at least a narrow band of wavelength of infra-red radiation, and so one LED can replace both the tungsten heater and one filter. Preferably, a plurality of such electromagnetic radiation generators are provided, each generating a specific frequency.

It is known to use solid-state radiation sources in chemical monitors. For example, the article by K. Chan et al entitled "Optical Remote Monitoring of CH4 Gas Using Low Loss Optical Fiber Link and In-Ga-As-P Light Emitting Diode in 1.33 ism Region" in App.

Phys. Lett. 43(7) (1st October, 1983) there is disclosed a monitor in which the output of an In-Ga-As-P LED is fed to an optical fibre (said to be 1 km long). The light output from the end of the optical fibre remote from the LED is fed to a cell for gas to be monitored. However, despite the use of low-loss fibres, there is some distortion significant loss of light energy due to reflection and absorption.

Therefore, the present invention proposes that the solid-state radiation source be placed directly within a sample chamber for the gas, so that there can be no distortion of the radiation before it encounters the gas, and maximum efficiency can be achieved.

Solid-state radiation sources such as LEDs, may maintain a low temperature whilst generating light. This enables the present invention to be used without hazard with an explosive gas. With the prior art systems it was inevitable that light distortion and loss occurred, as there had to be a physical barrier ("window") between the source and the gas, but with the present invention the source may be immersed in the gas.

The radiation detector is preferably a solidstate detector, such as a photodiode. Again, this is placed within the sample chamber.

Thus, the source may be selected in accordance with one or more particular components of the gas which it is desired to monitor. Normally the wavelengths absorbed are in the infra-red, and suitable photodiodes may be obtained commercially. The radiation is then passed through the ,gas and the amount of radiation received by the source is monitored.

Changes in the output of the detector will result from changes in the concentration of the absorbing components of the gas and hence those components can be analysed. By suitable calibration, it may be possible to detect accurately the amount of particular components of the gas present in any sample. It is also important that the monitor, or at least those parts of it in contact with the gas, are chemically inert to the gas. Otherwise the results obtained will be modified by that reaction, and be of little or no value.

As mentioned above, the gas, or other fluid, will encounter the radiation within a sample chamber. Many different designs of sample chamber are possible. Thus, for example, the chamber may consist of an elongate hollow tube with the radiation emitter and the radiation detector remote from each other in the tube. The tube is closed except for a gas inlet and oulet at or adjacent opposite ends of the tube, and gas is pumped through the tube and the variation in the radiation detected is determined.Alternatively, the sample chamber may contain perforations arranged so as to allow gas to enter from the ambient-atmosphere possibly by diffusion without the aid of a pump but to exclude interfering light reaching the IR detector; or the light source and detector may be mounted on a rigid support ("optical bench"), positioned in the centre of a large gas-containing plenum chamber: The output from the detector may be used to generate an analogue display of gas concentration and flow rate, or can be converted to a digital signal for subsequent processing by e.g. a computer. The tube forming the sample chamber may be straight, but it is also possible, by use of mirrors formed by specular flat glass with a gold surface, to guide the infra-red radiation over a convoluted path.This enables the optical path of the radiation from the emitter to the detector to be increased without substantially increasing the size of the monitor housing.

In another embodiment the sample chamber is a rectangular or square box containing the source(s) and detector. Mirrors may then provide an elongate optical path between source and detector, and this enables a compact construction to be achieved. Lenses may also be provided within the box to collimate the radiation from the source and focus the radiation to the detector.

Embodiments of the present invention will now be described by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows a diagrammatic view of a first embodiment of a chemical monitor according to the present invention; Figure 2 shows a schematic view of a second embodiment of a chemical monitor according to the present invention; Figure 3 shows a plan view of a practical example of the second embodiment; Figure 4 shows a side view of the example of Fig. 3; Figure 5 shows a plan view of a third embodiment of a chemical monitor according to the present invention; Figure 6 shows a side view of the embodiment of Fig. 5; and Figure 7 shows an LED for use in embodiments of the present invention.

Referring firstly to Fig. 1, a chemical monitor 10 has a sample chamber 14 which is in the form of a straight, hollow, elongate tube, for example a 2 inch stainless steel tube. At (or adjacent) one end of the chamber 14 is a gas inlet 26, and at (or adjacent) the other end is a gas outlet 28.

Gas to be investigated is drawn through the chamber 14 by a pump 30 connected to the outlet 28 (to prevent the pump contaminating the gas). Thus a gas sample from e.g. a sample reservoir 38, passes firstly through a valve 34 and then an absorber 36 for drying the gas before being drawn through the chamber 14. A second valve 35 is situated between the chamber 14 and the pump 30.

The apparatus may be initially calibrated by the passage of a calibration gas through the chamber 14. This gas enters the system via an inlet 39 adjacent the valve 34 and is drawn through the chamber 14 by the pump 30. For ease of construction, the inlet 26 and outlet 28 are mounted in end caps 32, 33 respectively which seal the ends of the chamber 14.

Also mounted in the end cap 32, adjacent the gas inlet 26, is a solid-state electromagnetic radiation source 12, embedded in a heat sink 18. The source 12 emits infra-red radiation at a unique wavelength (or at most a narrow range of wavelengths). Light-emitting diodes (LEDs), can be designed to emit a specific wavelength which is dependent on their allow composition and therefore may be used as the source 12. For example, LEDs based on compounds of the Ill-V series have been developed, such as those produced by Plessey Ltd., which have a film of In-Ga-As-P compound, on an In-P substrate, which are particularly suitable for use in the present invention. The LEDs may, in some cases, be covered with a corrosion-preventing film to prevent attack of the LED by the gas.

The source 12 is connected to a low-voltage power supply 20, typically of 12 V, giving 70 mA current. The current is preferably chopped electrically and the detector "tuned" to the chopping frequency. Passage of current through the LED gives rise to infra-red radiation which passes through the sample chamber 14 to a detector 16. The detector 16, for instance a Ge photodiode of 1 mm diameter, is mounted in the end cap 33 adjacent the gas outlet 28. The detector 16 generates a signal which is processed by circuitry 22. The circuitry 22 filters and amplifies the signal and will usually also contain means for adjusting fluctuations in the signal due to temperature changes in the apparatus. The signal output from the circuitry 22 may then be fed to a display 24, such as a 30 segment bar-graph indicator and/or an alarm.

Thus, in the embodiment of Fig. 1, the amount of radiation detected when the calibration gas is in the chamber 14 is determined, and then the sample gas fed to the chamber 14. Absorption of the infra-red radiation by specific molecules in the sample gas will cause a decrease in the amount of radiation detected, so enabling the amount of those molecules in the sample gas to be investigated.

In the embodiment of Fig. 1, both the source - 12 and the detector 16 are mounted within the gas stream. Thus distortion and loss of the signal due to reflection, refraction or absorption outside of the sample chamber 14 may be minimised.

In the present example there is an optical path of about 300 mm to 1000 mm between source 12 and detector 16, although this size is not critical and will naturally depend on the desired sensitivity to gas concentration and the overall size of the monitor. However, the optical path should not, normally, be less than 300 mm, as otherwise the sensitivity of the system will be too low.

In the above example, an analogue signal and a straight tube for the sample chamber are employed. The example shown in Fig. 2 uses a convoluted sample chamber 40, consisting of a stainless steel tube with polished walls. The tube has corners at which light is reflected by mirrors 41 to give an optical path from source 12 to detector 1 6 down the centre line of the tube. The source 12 is driven by a pulsed power supply 42. However, any combination of power supply and sample chamber shape may be used within the present invention. The advantage of the convoluted and mirrored tube of Fig. 2, however, is that it permits a longer optical path to be achieved within the same overall monitor length. The increase in optical path increases the absorption for any particular concentration of gas, so making the monitor more sensitive.

Referring to Fig. 2, the pulsed power supply 52 operates the source 12 (formed preferably by a plurality of LEDs), and also supplies a synchronising signal to circuitry 44 receiving the signal output from the detector 16. The signal from the detector 16 is amplified and digitised, with respect to the pulsed power input by the circuitry 44, and may then be passed through an interface 46 to a computer 48 for display or further processing.

In practice, the embodiment of Fig. 2 may be produced as shown in the example of Figs.

3 and 4. In this example, the tube 40 consists of four straight sections 40a, 40b, 40c, and 40d with corner units 50a, 50b, and 50c interconnecting the sections at right angles.

Each of the corner units 50a, 50b and 50c contains a mirror of e.g. specular flat glass covered with a gold surface, equivalent to the mirrors 41 of Fig. 2. Thus light passing down the tube sections 40a, 40b, 40c and 40d follows a convoluted or "folded" path.

The radiation source, e.g. four LEDs are mounteed in a head 51 with an input 52 for gas. It is desirable to use more than one LED, as this permits a multi-wavelength system to be used. By suitable selection of the wavelengths of the LEDs, several different components of the gas may be monitored, by switching between the sources, and/or one LED may be selected so that its light is unaffected by the gas and so can act as a reference for the other LEDs. Alternatively, several LEDS of the same wavelength may be used, to increase the amount of radiation, and so increase the signal detected. The detector is mounted in a second head 53, with a gas output 54 connected to the pump 30 (equivalent to the corresponding part in Fig. 2).The power supply 52 shown in Fig. 2 is, in the example of Fig. 3, formed by a driver circuit 55 and an LED selection circuit 56 which are themselves powered by a power supply 57 of e.g. 12 V. The output of the detector in the second head 53 is passed to an amplifier 58 and thence to processing circuitry 59 which includes a display 60 for displaying the amount of the component of gas that is present. The processing circuitry 59 of Fig. 3 will be generally equivalent to the circuitry 44, of Fig. 2 but instead of outputting the result to a computer, a direct display is given, enabling the example of Fig. 3 to be used as a portable monitor.

As shown in Fig. 4, the monitor has a housing with upper 61 and lower 62 parts.

The upper part 61 is hinged on the lower part 62 so that it can act as a cover when the monitor is not in use.

By using a convoluted tube, the overall dimensions of the monitor may be kept relatively small, yet achieve a sufficiently long optical path between source and detector (300 mm to 1000 mm) for accurate results to be achieved.

In another embodiment of the present invention, illustrated in Figs. 5 and 6, the sample chamber is a rectangular (or possibly square) box 70. A radiation source 71 is provided e.g. part-way along one side of the box 70 and a radiation detector 72 at a similar point along the opposite side. Radiation from the source 71 is reflected between mirrors 73,74 on opposite sides of the box, so that the optical path has a large number of "passes" across the interior of the box 70. In this way a suitably elongate optical path may be achieved, but with the overall length of the sample chamber much less than needed in the embodiment of Fig. 1. Lenses 75,76 may focus the radiation from the source 71 and to the detector 72 respectively.

As shown in Figs. 5 and 6, it is convenient if the box 70 is divided into three parts, one part 77 forming the sample chamber, another part 78 below the sample chamber which contains the power supply and processing circuitry 79, and a third part 80 which contains a display 81 for displaying the results of gas analysis effected by the monitor. In this way, a compact construction may be achieved.

The part 77 of the box forming the sample chamber and the part 78 containing circuitry 79 are separated by a wall 82 on which are located supports 83 for the mirrors 75,76 which are preferably movable to permit the lenses to be positioned accurately. The part 77 of the box 70 forming the sample chamber and the part 80 containing the display 81 are separated by a partition 84 which supports the source 71, the detector 72, and one of the mirrors 74. The partition 84 may also support a heat sink 85 for the source 71.

The embodiment of Fig. 7 may be modified in various ways. For example, focusing could be achieved with concave mirrors, rather than lenses. By the addition of additional mirrors within the sample chamber, an optical path involving reflection from four, or even six, internal walls may be achieved. Also, a plurality of radiation sources of different wavelengths may be used (as mentioned with reference to Fig. 2) to enable one to act as a reference for the other(s). This also has the advantage that calibration of the system does not depend on knowledge of the number of "passes" the optical path undergoes within the sample chamber. Furthermore, the optical path may be arranged to be divergent adjacent the detector, to permit the use of large-area infra-red detectors e.g. of PVdF pyroelectric film.This reduces cost not only because such detectors are less expensive than conventional semiconductor infra-red detectors but also the optical components may be reduced.

A chemical monitor according to the present invention is applicable to many purposes, such as detection of poisonous or flammable gases in a mine, for gas control in chemical process plants or as a moisture measuring device for crop and food processing. In any particular case, the molecules of gas to be detected will absorb infra-red radiation at specific frequencies and hence, by suitable selection of the source 12, the apparatus can be designed to detect the specific gas sought.

The structure of an LED suitable for use in the present invention is shown in Fig. 8. This has an active layer 90 of an In-Ga-As-P compound on an In-P layer 91. These two layers 90, 91 are positively or negatively doped respectively to create the P-N junction of the LED. The active layer 90 is connected via an In-P layer 92 to an Au electrode 93, and the layer 91 is connected via a layer 94 to another electrode 95. As shown,- a window region 96 is etched into the In-P layer 94.

Claims (11)

1. A method of monitoring a gas in a chemical monitor having a sample chamber for the gas to be monitored, a solid-state radiation source in the sample chamber for generating radiation corresponding to an absorption wavelength of a component of the gas, and a detector in the sample chamber for detecting the radiation; the method comprising passing radiation from the source to the detector through the gas, and monitoring the amount of radiation detected by the detector, thereby to monitor the amount of the component of the gas present.

2. A method according to claim 1, wherein the radiation source is a first radiation source, and the monitor has a second radiation source in the sample chamber, radiation from the second source not being absorbed by the gas, and the method includes operating the two sources alternately, whereby radiation from the second source acts as a reference against which the radiation detected by the detector is compared.

3. A method according to claim 1 or claim 2, wherein the monitor has a further radiation source for generating radiation corresponding to a further component of the gas, and the method includes monitoring the amount of radiation from the further source detected by the detector, thereby to monitor the amount of the further component of the gas present.

4. A method of monitoring a gas substantially as any one herein described with reference to the accompanying drawings.

5. A chemical monitor for monitoring a gas in accordance with the method of claim 1, comprising a sample chamber for the gas to be monitored, a solid-state radiation source in the sample chamber for generating radiation corresponding to an absorption wavelength of a component of the gas, and a detector in the sample chamber for detecting the radiation.

6. A chemical monitor comprising a sample chamber for a gas to be monitored, a solidstate radiation source in the sample chamber and a detector in the sample chamber for detecting radiation from the source, wherein the optical path between the source and the detector is free from chemically reactive material.

7. A chemical monitor according to claim 5 or claim 6, wherein the sample chamber is an elongate hollow tube.

8. A chemical monitor according to claim 7 wherein the tube is convoluted and has mirrors for reflecting light at corners of the convolutions.

9. A chemical monitor according to claim 6 or claim 7, wherein the sample chamber is a rectangular or square box.

10. A chemical monitor according to claim 9 wherein the box contains mirrors for reflecting radiation between the source and detector.

11. A chemical monitor substantially as herein described with reference to and as illustrated in Fig. 1, or Figs. 2 to 5, or Fig. 6, or Fig. 7 of the accompanying drawings.