GB1601233A - Gas detection - Google Patents

Description

(54) GAS DETECTION (71) We, J. & S. SIEGER LIMITED, a British Company, of 35 St. Thomas Street, London SEl 9SN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the detection of gases using infra-red radiation.

In a known apparatus of this type, an emitter of infra-red radiation is situated at the focus of a mirror, which collimates the radiation and produces two beams that pass respectively through an object and a reference cell. The reference cell is sealed and contains usually dry air or nitrogen. The object cell is provided with two ports through which sample gas is pumped. The radiation emerging from the two cells is incident on a second mirror and is focussed on to an infra-red detector. A rotating chopper alternately allows energy from the reference and the object cell to reach the detector. A synchronising device detects the position of the chopper and signals this position to controlling electronics, which process the pulses of information coming from the detector.The output of the system is determined according to the decrement in the energy of the beam which has passed through the object cell when compared to that emerging from the reference cell. If the object cell contains gas which absorbs infra-red radiation in the response band of the instrument, the radiation reaching the detector via the object cell will be reduced.

The two alternate pulses of energy reaching the detector will therefore be of different magnitudes, and the processing electronics will output a D.C. voltage or current level which will be a measure of the decrement of energy transmitted through the object cell and hence a measure of the infra-red absorbing gas in the object cell. This technique is used extensively for the detection of many gases, notably hydrocarbons, which absorb radiation with wavelengths of approvimately 3.4 ,u.

Disadvantages of this known type of apparatus are the need for focussing optical parts that can be upset by any movement of the apparatus and the relatively low proportion of the energy emitted by the infra-red emitter that is usefully employed. The energy dissipated heats the apparatus, so cooling is often necessary. Moreover, the time taken for the apparatus to reach a working condition from switch-on is relatively lengthy.

It is an object of the present invention to provide an apparatus in which these disadvantages are reduced.

According to the present invention there is provided apparatus for detecting gases by absorption of infra-red radiation compris mg a source ot mtra-red radiation arranged at one end of an optical paths of the apparatus at the other end of which is an infra-red detector, a plurality of cells arranged to be moved, in turn, into the optical path, the cells being such that when a cell is positioned on the optical path the infrared radiation passes through that cell before reaching the detector, and electronic means for comparing the infra-red radiation received by the detector when one cell is positioned on the optical path with the infra-red radiation received by the detector when a selected other cell is positioned on the optical path and for producing a signal dependent on the said comparison, the cells being housed within an enclosure and including one or more sealed reference cells each containing a known concentration of a gas, and an object cell that has openings in its wall facing generally along the path of movement CL the cell whereoy an atmosphere to be tested is pumped or allowed to pass into the enclosure and is pumped through the object cell by the motion of that cell through the enclosure.

Preferably, the cells are mounted for rotary motion about an axis parallel to the optical path of the apparatus and are moved successively into the optical path by said motion. Alternatively, the cells could be mounted as a pendulum or slide and moved in that way.

In at least its preferred form, the present invention has the following advantages over the known apparatus.

All focussing optics are dispensed with, thus giving an improvement in stability and robustness.

A much greater proportion of the energy emitted by the infra-red source is utilised, thus allowing a reduction in the power supplied to the source for an equivalent signal at the detector. A reduction to approximately one fifth of that supplied to a conventional source is easily possible, thus easing considerably the heat dissipation and warm-up problems associated with conventional equipment. A useful output can be obtained from the device in under two minutes from initial switch which compares very favourably with the warm-up time of many conventional instruments.

The self-pumping action of the object cell allows the device to be used as a direct monitor of the amosphere of any particular area, since gas may be allowed to diffuse from such an area to the enclosure. Thus the external pump required in the conventional device is not required. Alternatively, the sample may be obtained from a distant area by using an external pump to draw the gas to be tested into the enclosure. Thus the installation of the device may be simplified and easily tailored to the particular application.

As mentioned above, there may be more than two cells. The additional cells could well be filled with samples of a variety of gases of interest, for example, methane, profane, butane, ethane, each at the alarm level(s) of interest. An external switch would allow the user to select whichever cell(s) are appropriate for the particular application. A synchronising device forming part of the said electronic means then signals the processing electronics whenever the selected alarm level cell is at the optic axis, and thus gain and zero levels may be adjusted automatically by the electronics allowing the device to operate simply as a comparator, thus giving an extremely reliable alarm function.

If required, the single, static, infra-red filter may be replaced by a dual or multiple pass-band filter, thus allowing zero span and alarm adjustments to be made by the processing electronics from the detector outputs derived from a single cell. This would prevent cell window obscuration from causing errors due to outputs taken from different cells. The multiple infra-red filter system could be driven in synchronism by any suitable mechanical or electrical drive system.

The invention will now be described, by way of example, with reference to the drawings accompanying the provisional specification, in which: Fig. 1 is a plan of an infra-red gas detection apparatus embodying the invention; Fig. 2 is a section on the line II-II in Fig.

1, and Fig. 3 is a block diagram of the electronic circuitry used in the apparatus shown in Figs. 1 and 2.

Referring firstly to Figs. 1 and 2, the apparatus comprises an enclosure 11 within which is mounted, for rotation about a horizontal axis, a rotor 12 that is driven by a motor 13. The rotor 12 carries a cylindrical object cell 14 and a similar cylindrical reference cell 15. The cells 14 and 15 have infrared transmitting end walls 16 in register with windows 17 on the end plates of the rotor, which end walls are so disposed that when each cell is at its lowermost position as seen in Fig. 2, its end walls 16 are in register with infra-red transmitting entry and exit windows 18 and 19 in the walls of the enclosure 11. The windows 18 and 19 lie on an optical path of the apparatus at one end of which, outside the enclosure, is a source 21 of infra-red radiation and at the other end of which, also outside the enclosure, is a detector 22 of infra-red radiation.

The reference cell 15 is sealed and contains nitrogen, or dry air. The object cell 14 has openings 23 in its sides positioned adjacent to its ends. As the cell 14 is moved through the enclosure, gas is pumped through it by means of the openings 23, so that the gas in the cell 14 is continually changed and is always effectively the same as that within the enclosure 11, which receives gas through the ports 24.

A air of oticalsensors 25 and 25' on the floor of the enclosure cooperate with light sources 28 and 28' and with notches 27 and 27' in flanges 26 and 26' of the cells respectively to detect which cell is, at any particular time, on the optical path of the apparatus. Notch 27 is adjacent the reference cell 15 in the left-hand flange 26 as seen in Fi. 2, and notch 27 is adjacent the object cerl 14 in the right hand flange 26'. Fig. 3 shows the electronic circuitry for providing an output for the apparatus.

The two gas cells 14 and 15 passing the optical path allow two pulses of infra-red radiation to reach the detector 22 per rotation of the rotor 12. The two pulses outputted from the detector will, therefore, correspond to the signal obtained (1) via the object cell 14, and (2) via the reference cell 15. The optical synchronising circuitry shown in Fig. 3 includes the sensors 25 and 25' the outputs of which are each applied to a respective Schmitt trigger circuit 30, 30'. A second Schmitt trigger circuit 31, 31' is connected to the output of each of these first Schmitt circuits respectively via a capacitor 32, 32'.

The output of each of the first Schmitt circuits is connected to the sampling switch 33, 33' of an appropriate sample and hold circuit 34, 34'. That is to say, switch 33 and circuit 34 together with a buffer amplifier 40 constitute sample-and-hold circuit for the reference signal and switch 33' and circuit 34' together with a buffer amplifier 40' constitute a sample-and-hold circuit for the object signal.

The output of detector 22 is pre-amplified in a pre-amplifier 35 and then passed to a main automatic gain control (A.G.C.) amplified 36. The notches 27 and 27' are so positioned as to ensure that triggering pulses from the Schmitt circuits 30, 30' close the appropriate switch 33, 33' when the corresponding cell is positioned in the optical path of the apparatus. In this way the reference and object signals are passed to two different halves of the circuit. The output of the sample and hold circuits 34, 34' are approximate D.C. voltage levels, which are "refreshed" once per rotation of the rotor.

Before each new voltage pulse is passed from the detector, the appropriate holding capacitor is partially discharged by a "discharge" signal originating from the second Schmitt circuit 31, 31'. This allows the circuitry to rapidly follow falling voltage signals.

The voltage level from the reference sample and hold circuit 34 is fed back to be compared in a comparator 37 to a reference potential VREF, and thence to a light emitting diode 38 which is coupled to a light dependent resistor 39, which adjusts the gain of the A.G.C. amplifier 36 to ensure that the reference output signal is maintained at a constant level, irrespective of changes in emitter or detector efficiency or cell window obscuration. Also, this voltage level is compared in a comparator 41 with the corresponding level from the object sample and hold circuit 34', the difference between the two levels constituting the output signal 42.

Since the object and reference signals are at all times subjected to the same amplification factors, and since the reference voltage is held at a constant level, the difference between the two signals is equivalent to a ratio signal, and thus is an accurate measure of the decrement in the infra-red radiation transmitted through the object cell compared with that transmitted through the reference cell. The output signal is, therefore, a measure of the infra-red absorbing gas present in the obiect cell.

WHAT WE CLAfM IS: 1. Apparatus for detecting gases by absorption of infra-red radiation comprising a source of infra-red radiation arranged at one end of an optical path of the apparatus at the other end of which is an infra-red detector, a plurality of cells arranged to be moved, in turn, into the optical path, the cells being such that when a cell is positioned on the optical path the infra-red radiation passes through that cell before reaching the detector, and electronic means for comparing the infra-red radiation received by the detector when one cell is positioned on the optical path with the infra-red radiation received by the detector when a selected other cell is positioned on the optical path and for producing a signal dependent on the said comparison, the cells being housed within an enclosure and including one or more sealed reference cells each containing a known concentration of a gas, and an object cell that has openings in its wall facing generally along the path of movement ot the cell whereby an atmosphere to be tested is pumped or allowed to pass into the enclosure and is pumped through the object cell by the motion of that cell through the enclosure.

2. Apparatus according to claim 1, wherein the cells are mounted for rotary motion about an axis parallel to the optical path of the apparatus and are moved successively into the optical path by said motion.

3. Apparatus according to claim 1 wherein the cells are mounted as a pendulum or slide.

4. Apparatus according to any of the preceding claims, wherein the said openings are located in the side wall of the obJect cell near its two ends, the openings at opposite ends facing in opposite directions.

5. Apparatus according to any of claims 1 to 4 including a plurality of sealed reference cells each filled with gas at a respective alarm concentration, a selector switch to enable the sample cells to be compared with any of the reference cells.

6. Apparatus according to any of claims 1 to 5 in which the electronic comparing means include an automatic gain control amplifier connected to the output of the infra-red detector, and a control circuit arranged to compare an output derived from the automatic gain control amplifier when the reference cell is in the optical path, with a reference voltage and adjust the gain of the amplifier to hold the output to the reference voltage level.

7. Apparatus for detecting gases substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. circuit 34 together with a buffer amplifier 40 constitute sample-and-hold circuit for the reference signal and switch 33' and circuit 34' together with a buffer amplifier 40' constitute a sample-and-hold circuit for the object signal. The output of detector 22 is pre-amplified in a pre-amplifier 35 and then passed to a main automatic gain control (A.G.C.) amplified 36. The notches 27 and 27' are so positioned as to ensure that triggering pulses from the Schmitt circuits 30, 30' close the appropriate switch 33, 33' when the corresponding cell is positioned in the optical path of the apparatus. In this way the reference and object signals are passed to two different halves of the circuit. The output of the sample and hold circuits 34, 34' are approximate D.C. voltage levels, which are "refreshed" once per rotation of the rotor. Before each new voltage pulse is passed from the detector, the appropriate holding capacitor is partially discharged by a "discharge" signal originating from the second Schmitt circuit 31, 31'. This allows the circuitry to rapidly follow falling voltage signals. The voltage level from the reference sample and hold circuit 34 is fed back to be compared in a comparator 37 to a reference potential VREF, and thence to a light emitting diode 38 which is coupled to a light dependent resistor 39, which adjusts the gain of the A.G.C. amplifier 36 to ensure that the reference output signal is maintained at a constant level, irrespective of changes in emitter or detector efficiency or cell window obscuration. Also, this voltage level is compared in a comparator 41 with the corresponding level from the object sample and hold circuit 34', the difference between the two levels constituting the output signal 42. Since the object and reference signals are at all times subjected to the same amplification factors, and since the reference voltage is held at a constant level, the difference between the two signals is equivalent to a ratio signal, and thus is an accurate measure of the decrement in the infra-red radiation transmitted through the object cell compared with that transmitted through the reference cell. The output signal is, therefore, a measure of the infra-red absorbing gas present in the obiect cell. WHAT WE CLAfM IS:

1. Apparatus for detecting gases by absorption of infra-red radiation comprising a source of infra-red radiation arranged at one end of an optical path of the apparatus at the other end of which is an infra-red detector, a plurality of cells arranged to be moved, in turn, into the optical path, the cells being such that when a cell is positioned on the optical path the infra-red radiation passes through that cell before reaching the detector, and electronic means for comparing the infra-red radiation received by the detector when one cell is positioned on the optical path with the infra-red radiation received by the detector when a selected other cell is positioned on the optical path and for producing a signal dependent on the said comparison, the cells being housed within an enclosure and including one or more sealed reference cells each containing a known concentration of a gas, and an object cell that has openings in its wall facing generally along the path of movement ot the cell whereby an atmosphere to be tested is pumped or allowed to pass into the enclosure and is pumped through the object cell by the motion of that cell through the enclosure.

2. Apparatus according to claim 1, wherein the cells are mounted for rotary motion about an axis parallel to the optical path of the apparatus and are moved successively into the optical path by said motion.

3. Apparatus according to claim 1 wherein the cells are mounted as a pendulum or slide.

4. Apparatus according to any of the preceding claims, wherein the said openings are located in the side wall of the obJect cell near its two ends, the openings at opposite ends facing in opposite directions.

5. Apparatus according to any of claims 1 to 4 including a plurality of sealed reference cells each filled with gas at a respective alarm concentration, a selector switch to enable the sample cells to be compared with any of the reference cells.

6. Apparatus according to any of claims 1 to 5 in which the electronic comparing means include an automatic gain control amplifier connected to the output of the infra-red detector, and a control circuit arranged to compare an output derived from the automatic gain control amplifier when the reference cell is in the optical path, with a reference voltage and adjust the gain of the amplifier to hold the output to the reference voltage level.

7. Apparatus for detecting gases substantially as hereinbefore described with reference to and as illustrated in the drawings accompanying the provisional specification.