GB2392721A

GB2392721A - Gas sensors

Info

Publication number: GB2392721A
Application number: GB0220351A
Authority: GB
Inventors: Graham Paul Hopkins
Original assignee: e2v Technologies UK Ltd
Current assignee: Teledyne UK Ltd
Priority date: 2002-09-03
Filing date: 2002-09-03
Publication date: 2004-03-10
Also published as: WO2004023113A1; GB0220351D0; AU2003260767A1

Abstract

A gas sensor 1 comprises an optical source 2 and detector means 4 sensitive to light from the source. The source and detector are electrically and physically connected to a circuit board (PCB) 3 which, together with a cover 5, forms part of a housing 6 for the source and detector. The sensor further comprises means arranged, in use, to admit gas into the housing, such as a porous cover or apertures in the PCB. The mounting of the source and detector onto a circuit board greatly simplifies the manufacture of such gas sensors. Advantageously, the sensor further includes a temperature sensor arranged to detect temperature inside the housing. Signals from the temperature sensor can be used to compensate for changes in the gas sensor output with changes in ambient temperature.

Description

1 2392721

GAS SENSORS

This invention relates to apparatus for, and methods of, sensing gasses. The invention particularly relates to such methods and devices in which optical radiation is transmitted through a gas and subsequently detected to provide information concerning the gas.

In a typical gas monitor, an infrared source is arranged to emit radiation, which passes through a gas to be monitored. Infrared radiation is absorbed by the gas and that remaining is subsequently detected by an infrared detector. A comparison is made between the source intensity and the intensity of radiation detected following passage 0 through the gas to give the concentration of a target gas.

The present mention seeks to provide a gas monitor having improved' characteristics over those previously known.

5 The invention provides a gas sensor comprising an optical source and detector means sensitive to light from the source, the source and detector being electrically connected to a circuit board which forms part of a housing for the source and detector, and the sensor further comprising means arranged, in use, to admit gas into the housing.

20 The mounting of the source and detector onto a circuit board greatly simplifies the manut:acture of SUCH gas sensors. I'reviously, it has been thought desirable to mount the source and detector in special chambers, the inner contours of which are arranged to

minimise stray light. Although such a gas sensor has good performance characteristics, the method of manufacturing the chamber and mounting the components therein is time-

consuming and therefore more costly.

5 Preferably, the means for admitting gas into the housing comprises apertures in the housing, which apertures may be formed in the circuit board. Alternatively, or additionally, part of the housing may comprise sintered material.

Advantageously, the sensor further includes a temperature sensor arranged to detect to temperature inside the housing. The output from the temperature sensor may be input to a control system arranged to provide a signal to compensate for changes in the sensor output with ambient temperature.

The invention lends itself to the monitoring of carbon dioxide levels and Is therefore IS suitable for the detection of the presence of humans or animals in an environment.

T hercDorc, the invention may be employed in a variety of security and rescue applications. The invention will now be described, by way of example, with reference to the 20 accompanying drawings, in which: Figure I is a sectional view of a gas sensor constructed according to the invention;

Figure 2 is a sectional view of an alternative gas sensor constructed according to the invention; Figure 3 is a sectional view of another alternative gas sensor constructed 5 according to the invention; Figure 4 is a sectional view of a further alternative gas sensor constructed according to the invention; lo Figure 5 is a sectional view of a further alternative gas sensor constructed according to the invention; Figure 6 is a sectional view of a further alternative gas sensor constructed according to the invention; and Figures 7a and 7b are cross sectional views of channels suitable for inclusion in the sensor of Figure 6.

Like reference numerals relate to like parts throughout the specification.

With reference to Figure 1, a gas sensor constructed according to the invention is illustrated and indicated generally by the reference numeral 1. The gas sensor I comprises a source 2 of infrared (JR) radiation, electrically connected to, and physically mounted on, a printed circuit board (PCB) 3. The sensor I further comprises an infrared

detector 4, which includes a bandpass filter. The detector 4 is also electrically connected to, and physically mounted on, the PCB 3. Suitable detectors include photodiodes, thennopiles and pyroelectric devices.

5 A cover 5 is provided for the source 2 and detector 4. The cover 5, together with the PCB 3 forms a housing 6 for the components of the sensor 1. The interior surfaces of the housing 6 tong an optical cavity. In order to obtain good reflectance of IR radiation in the optical cavity, the interior surfaces of the cover are coated with a metallic layer, for example gold. Any material that is highly reflective to IR radiation may be, lo employed. The surface of the PCB onto which the components are mounted may also be coated with IR reflective material.

The cover 5 has a tapered wall 7 with a curved end portion 8 arranged so that, in the sectional view of Figure 1, the cover 5 resembles the shape of a thimble. In this 15 embodiment, at least a portion of the cover 5 comprises a sinter, to allow gas to be admitted into the housing 6 by diffusion.

In use, the source 2 produces broadband IR radiation, which is reflected by the surfaces of the cavity and absorbed by the gas in the housing to a degree proportional to the 20 amount of gas present. A range of wavelengths of the broadband IR radiation not absorbed by the gas is detected at the detector 4. The detector 4 generates an electrical signal corresponding to the strength of the detected IR radiation. This signal is input to processing electronics (not shown) arranged to determine the concentration of gas

l s present in the housing. The concentration is related to the intensity by the following equation: -eel I = lo e where I is the intensity of radiation detected by the detector, lo is the intensity of radiation emitted at the source, e is effectively a constant which is dependent on the particular gas being monitored, c is the gas concentration and I is the distance travelled by the radiation through the gas.

The sensor and processing electronics may be configured to detect an increase ore decrease ('?c) in concentration of the gas being sensed. Alternatively, absolute measurements of the concentration of a particular gas may be determined.

An alternative arrangement of the sensor of Figure I is shown in Figure 2. In this embodiment, the cover 9 comprises a solid shell of pressed metal. Gas is able to diffuse into the housing by means of apertures 10 in the PCB.

Another alternative sensor constructed according to the invention is shown in Figure 3.

20 In this embodiment, the cover 11 is square or rectangular in section. This cover 11 includes porous material, for example a sinter, for the admittance of gas. Alternatively, the cover 11 could comprise a solid shell of pressed metal, in which case the PCB of would incorporate apertures for the gas. The advantage of this embodiment is that the cover is straightforward to manufacture.

2s

In the sensor of Figure 4, the cover 12 comprises a straight cylindrical wall 13 having a dome 14 as a lid. The wall 13 and dome 14 may be of onepiece construction. This sensor includes an apertured PCB for the admittance of gas. The cover of the Figure 4 embodiment is less simple to manufacture than the basic cubic shape of the cover of the s Figure 3 embodiment. However, the domed lid ensures that a greater proportion of light is directed onto the detector.

A further alternative gas sensor is illustrated in Figure 5. In this embodiment, the cover 15 comprises a pipe 16, one end portion 17 of which is arranged to surround the source lo 2 of IR radiation. The other end portion 18 of the pipe is arranged to surround the detector 4. The pipe 16 forms an inverted "U" over the PCB. The pipe 16 may comprise a sinter or else have a plurality of apertures (not shown) in its walls. An advantage of this embodiment is that the pipe provides a predehmed optical path for radiation travelling from the source to the detector. Thus, stray hght is minimised. The 5 inverted "U" shape of the pipe provides a relatively long optical path as well as providing a large surface area for gas to diffuse into the cover.

In the alternative sensor of Figure 6, the source 2 and detector 4 are spaced apart. al he cover 19 comprises a solid metallic shell arranged to provide an elongated channel 20 between the source 2 and the detector4. In this embodiment, diffusion of gas into the housing is effected by means of apertures in the PCB.

The cover 19 of Figure 6 may be square or rectangular in cross section, as illustrated in Figure 7a. Alternatively, the cover may have a curved upper surface so that it is arch

( 7 shaped in cross section as shown in Figure 7b. Of course, the cover could take on any shape required, for example domed, pyramidal, etc. in order to enhance the proportion of radiation incident on the detector.

5 The gas admittance means could comprise apertures in the PCB, apertures in the cover, a sinter forming part of the cover or any combination of these.

Further variations may be made without departing from the scope of the invention. For example, a reflector may be located adjacent the source of IR radiation and arranged to lo reflect light in a desired direction or range of directions.

A particle filter, for example a membrane of Goretex'R0, may be provided over the gas sensor in order to prevent dirt particles, water droplets and other contaminants from entering the sensor.

A temperature sensor (not shown) in the form of a thermistor, for example, may be incorporated in the sensor to provide a signal representing the temperature in the sensor to a control system, which employs suitable algorithms to provide temperature compensation of the output signal from the detector.

The invention is particularly suitable for detecting levels of carbon dioxide in an environment. Alternatively, an increase in concentration of carbon dioxide levels may be detected. The gas sensor constructed according to the invention typically has an optical path length in the range of forty to sixty millimetres approximately. It has been

found that this is suitable for detecting levels of carbon dioxide In the range of 500 ppm to 10,000 ppm. The IR source and detector may be tuned to the absorption band of carbon dioxide at 4.2 microns. A gas sensor configured to detect carbon dioxide is suitable for a wide range of applications as such a sensor can detect the presence of s humans or animals in an environment.

Claims

( 9 CLAIMS

1. A gas sensor comprising an optical source and detector means sensitive to light from the source, the source and detector being electrically connected to a circuit board which forms part of a housing for the source and detector, and the sensor further comprising means arranged, in use, to admit gas into the housing.

2. A gas sensor as claimed in claim 1, wherein the means for admitting gas into the housing comprises at least one aperture in the circuit board.

3. A gas sensor as claimed in claim I or 2, in which the means for admitting gas into the housing comprises at least one aperture in the housing.

4. A gas sensor as claimed in any one of claims I to 3, in which the means for admitting gas into the housing comprises sintered material which forms at least part of the housing.

5. A gas sensor as claimed in any previous claim, in which at least part of the interior of the housing is coated with IR reflective material.

6. A gas sensor as claimed in any previous claim, further comprising a reflector associated with the source.

lo 7. A gas sensor as claimed in any preceding claim, further comprising processing electronics arranged, in use, to determine the presence of a predetermined gas in dependence on signals Tom the detector.

8. A gas sensor as claimed in claim 7, in which the processing electronics is arranged to detect a change in concentration of the predetermined gas.

9. A gas sensor as claimed in claim 7, in which the processing electronics is arranged to determine absolute concentration of the predetermined gas 10. A gas sensor as claimed in any previous claim, further comprising a temperature sensor. A gas sensor as claimed in claim 10, further comprising compensating electronics arranged to compensate for changes in temperature detected by the temperature sensor.

12. A gas sensor as claimed in any previous claim, f'urthcr comprising a particle filter. 13. A gas sensor, substantially as hereinbefore described, with reference to, or as Illustrated in, the accompanying drawings.

14. A carbon dioxide detector including a gas sensor as claimed in any one of claims 1 to9.

15. A gas detection system including a gas sensor as claimed in any one of claims I to 9.