WO2005015176A1

WO2005015176A1 - Gas detector

Info

Publication number: WO2005015176A1
Application number: PCT/GB2003/003295
Authority: WO
Inventors: David Michael Stuttard
Original assignee: Status Scientific Controls Limited
Priority date: 2003-07-28
Filing date: 2003-07-28
Publication date: 2005-02-17
Also published as: AU2003251361A1

Abstract

A gas detector comprising: a housing; an optical gas sensor unit mounted within the housing; electrical connections external to the housing; an electrical interface module electrically connected between the external electrical connections and the optical gas sensor unit for providing compatibility of the optical gas sensor unit with an external pellistor command and control circuit when connected thereto; wherein the optical gas sensor unit comprises a self-contained component having an optical source, an optical detector and an optical pathway extending therebetween integrated into a single casing.

Description

GAS DETECTOR

The present invention relates to gas detectors and in particular, though not exclusively, to gas detectors used in hazardous environments for the detection of flammable gases.

Gas detectors capable of detecting and giving warning of a build-up of combustible gases are widely used in industry, and in particular in offshore and onshore oil and gas installations. Such gas detectors are often used for the detection of methane.

Conventionally, flammable gas detection has been carried out using catalytic sensors, of the type known as pellistors. Pellistors detect flammable gases by burning the target gas on a heated wire or bead, surrounded by a reaction- enhancing catalyst. The beads are used in pairs, only one of which is treated with the catalyst. The passive bead is used as a compensator.

The burning gas causes the resistance of the active bead to rise and this change of resistance compared to the compensator is sensed by, for example, a Wheatstone bridge circuit. In a typical installation, such as on an oil or gas platform, the detectors are installed at strategic points and are hardwired to remote control and monitoring equipment which is particularly configured to control and interface with pellistors.

There are a number of known problems associated with pellistors. Because they are used in hazardous environments for the detection of explosive gases, they must be constructed so as not to contribute to any risk of explosion. The heated bead (and any electrical control system) which could potentially provide an ignition source must be contained in a flame- and explosion-proof housing while still allowing ingress and egress of the ambient atmosphere in order to detect gases therein. Characteristically, this makes them relatively bulky devices. Pellistors are also prone to calibration drift. The beads are easily poisoned by certain chemicals, such as hydrogenated halocarbons. More problematically, it may not be readily apparent when this poisoning occurs. In high gas concentrations, they may also provide false and unsafe readings.

As a result, pellistors need regular maintenance and frequent replacement which makes them costly to use and maintain.

As an alternative to pellistors, there are numerous infrared gas detection products already on the market. Many of these products rely on the provision of one or more optical sources (eg. in the visible or infra-red spectrum), one or more corresponding optical sensors and one or more optical paths therebetween traversing an ambient gas sampling volume. Typically, the optical paths are split to provide both reference and measurement beams, and these paths are generally of a significant length in order to produce measurable absorption by the gas(es) to be detected.

However, the control systems and interfaces required for optical gas detectors are very different from the control systems and interfaces of the existing installed base of pellistor type gas detectors. This makes it impractical simply to substitute optical gas detectors for pellistors in an existing installation. This is particularly true in the special conditions prevailing in, for example, offshore oil rigs.

It has been proposed to construct a gas detector which incorporates optical gas sensing technology, but with an electronic interface module which converts the inputs and outputs of the optical gas sensor so that they simulate pellistor behaviour. For example, a pellistor-style three-cable interface to the outside world may be used to communicate with an optical gas sensor by way of the electronic interface which ensures an existing control panel sees the same electrical characteristics as from a pellistor device. An existing design of optical gas detector having a pellistor command and control interface is provided in a part-cylindrical and part-rectangular housing. At a first end of the cylindrical housing is provided a power supply enclosure. Adjacent to this are focussing mirrors with an integral heater. At the second (opposite) end of the cylindrical housing is provided the electronic interface and mountings for the optical source, the optical sensor and other components of the optical pathway. Between the first and second ends (ie. between the focussing mirror and the other optical components) the cylindrical housing defines a large gas sampling chamber which the optical pathways traverse. This gas sampling volume occupies perhaps 20 - 25% of the housing.

Light from the infrared source is split by a beam splitter and directed onto the focussing mirrors via the gas sampling chamber and then reflected back onto the infrared detectors, also via the gas sampling chamber. This takes place within the gas sensor housing which is designed to allow the free passage of the surrounding air.

This particular configuration can lead to a number of possible problems.

Firstly, the optical system is exposed to the environment leaving it subject to potential contamination and subsequent degradation of the optical path. This can have the effect of causing zero drift, calibration errors and a limited operating life before maintenance is required. When certain environmental conditions occur, exposed polished surfaces are subject to condensation. Heating elements are therefore required to prevent condensation. This requires additional circuitry and power and increases both the complexity and cost of the detector.

The distance between optical source and sensors is critical in the determination of gas concentration. Any deviation or variation in this distance can be revealed as a change in the zero point and a shift in the calibration point. The mechanical arrangement that is necessary to implement such a design is inevitably complex, expensive and prone to compromise as a result of changing temperature or mechanical shock or vibration, particularly in a large and heavy housing otherwise required to contain the power supply, heater, optics and interface circuitry.

With any infrared gas detection system, the temperature of the component parts is critical. Any variation in temperature subsequent to calibration leads to zero drift and inaccuracies, unless each of the parts is either thermally linked or temperature controlled. Temperature control is costly and complex and increases power demand. Thermal linking is difficult when using a large gas sampling chamber. The additional power requirement may prevent the apparatus from being a direct substitute for existing pellistor detectors.

The physical constraints placed upon such a design may make it too large to be directly interchangeable with the industry standard pellistor based gas detector.

It is an object of the present invention to provide an optical gas detector that provides a pellistor control and command compatible interface that overcomes at least some or all of the above problems.

According to one aspect, the present invention provides a gas detector comprising: a housing; an optical gas sensor unit mounted within the housing; electrical connections external to the housing; an electrical interface module electrically connected between the external electrical connections and the optical gas sensor unit for providing compatibility of the optical gas sensor unit with an external pellistor command and control circuit when connected thereto; wherein the optical gas sensor unit comprises a self-contained component containing an optical source, an optical detector and an optical pathway extending therebetween integrated into a single casing.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a cross-sectional side view on an axis of the cylindrical housing of a gas detector according to the present invention; Figure 2 shows a perspective view of an optical gas sensor unit suitable for use in the detector unit of figure 1 ; Figure 3 shows a plan view of the sensor unit of figure 2; and Figure 4 shows a cross sectional view of the sensor unit of figure 2.

With reference to figure 1, a gas detector 50 comprises a housing 51 which is preferably of cylindrical form. At one end of the housing 51 is provided a mounting thread 52 which is preferably configured to be physically compatible with installations adapted to receive pellistor type gas detectors. To this end, three electrical connections 53 are preferably provided consistent with conventional pellistor command and control functionality. These three electrical connections 53 extend into the housing 51 preferably by encapsulation 55.

The other end of the housing 51 has a gas detection window 54 extending through the housing 51 wall. Preferably, the gas detection window 54 is provided with a gas permeable, flame arresting filter 57 which may be formed from suitable sintered material that allows diffusion gas into the cavity formed by the housing 51, but also forms a combustion barrier.

Printed circuit boards 60 and 61 which provide the interface functionality (and any other electrical function required) are mounted within the cavity of the housing 51, to which the electrical leads 62 emerging from the encapsulation 55 are connected. The circuit boards 60, 61 may be configured in mutually orthogonal relationship with board 60 extending along the cylinder axis, and board 61 being circular and parallel to end walls of the cylindrical housing 51.

An integrated optical gas sensor unit 70 is preferably mounted directly to the circuit board 61 using any appropriate surface mount technique. The optical gas sensor unit 70 comprises a self-contained component having an optical source, an optical detector and an optical pathway extending therebetween integrated into a single casing, as will be described hereinafter. In the preferred configuration, the integrated sensor unit 70 comprises a plurality of pins 71, 72 which are received into suitable receptacles 73, 74 in the circuit board 61. Other connection arrangements may be envisaged, but the preferred plug-in arrangement facilitates convenient replacement of the optical gas sensor unit 70 without difficulty.

In the preferred configuration, the housing 51 is fabricated in two parts, a cylindrical cup 51a, and an end plate or base 51b. The end plate 51b has an upstanding flange 58 received into the open end of the cylindrical cup 51a. The end plate 51b may be retained within the cup 51a using any convenient mechanism such as: co-operating screw threads on the flange 58 and inner cylindrical wall of the cup 51a; co-operating bayonet fittings on the flange 58 and inner cylindrical wall of the cup 51a; or any other mechanism. Preferably, the mating faces of the cylindrical cup and the end plate 5 lb will include a sealing surface, eg. an o-ring or similar to prevent ingress of hazardous gases and/or liquids into the cavity containing the printed circuit boards 60, 61.

As shown, the optical gas sensor unit 70 may be received into the cup 51a so that a gas detection window 76 in the sensor unit casing 77 is aligned with and adjacent to the gas detection window 54 of the housing 51. A top surface 78 of the optical sensor unit 70 around the detection window 76 may butt up against the end wall of the housing 51 and may be sealed against it by a peripheral seal 79. In this way, the internal chamber of the housing 51 containing the electrical components may be completely sealed against ingress of hazardous and/or corrosive gases. Only the gas sampling volume of the integrated gas sensor unit 70 and the detection window 54 are exposed to atmospheric ambient.

Preferably, the housing 51 is formed from an Atex certified Exd material.

The integrated optical sensor unit 70 is preferably, but not exclusively, of the type described in GB 2372099, and as shown in figures 2 to 4. The integrated optical sensor unit could also be, for example, of the type described in patent number GB 2316172 B.

With reference to figures 2 to 4, gas sensor unit 1 comprises a non-focussed optical source 2 for emitting radiation in the optical spectrum. The expression "optical" is intended to cover all parts of the electromagnetic spectrum that are useful for the function of gas detection by absorption and includes the infra-red, visible and ultra-violet ranges of the electromagnetic spectrum. The source is preferably of the incandescent variety, producing a broad range of frequencies with which to measure absorption characteristics, but may also be of the solid state variety such as diodes producing limited frequencies or frequency bands.

The gas sensor unit 1 further comprises an optical detector 3 for detection of optical radiation emitted by the source 2. The optical detector 3 may be of any suitable type for sensing variations in intensity of radiation received from the source and providing as output a voltage or current as a function thereof. In a preferred embodiment, operating in the infra-red spectrum, the optical detector 3 is a pyroelectric detector.

The source 2 and optical detector 3 are respectively located at opposite ends of an optical pathway 4 (figure 3) which pathway is defined by a circumferential chamber 5 and a central chamber 6 respectively defining a generally circumferential portion 4a of the optical pathway 4 and a generally radial portion 4b of the optical pathway.

As best seen in figure 4, the circumferential chamber 5 is defined by: a chamber base 7; an internal surface of an outer cylindrical wall 8 of the sensor casing; an external surface of an inner cylindrical wall 9 of the sensor casing; and a radial planar end wall 10. Preferably, the chamber base 7 provides a planar reflective surface.

The central chamber 6 is defined by an internal surface of the casing base 11 and an internal surface of the inner cylindrical wall 9 of the sensor casing. The casing base 11 provides a planar reflective surface, in the central chamber 6.

Optical communication between the circumferential chamber 5 and the central chamber 6 is by way of a gap 12 in the inner cylindrical wall 9. To enhance reflection of radiation from the circumferential chamber 5 to the central chamber 6, a deflector element 13 provides a reflecting surface 14 which generally extends from the outer cylindrical wall 8 to the inner cylindrical wall 9. The reflecting surface 14 is planar. The reflecting surface 14 is generally oblique to the tangent of the outer and inner circumferential walls 8, 9 at the position of the gap, but may also be radial.

The deflector 13 is preferably formed from a wedge shaped element which also forms the radial end wall 10. The wedge shaped element can be fixed into position by screw 15 which may allow for some adjustment in the angle of the wedge shaped element. Alternatively, a reflector, fabricated from sheet metal and located in position by a pin or spot welding, may be used.

The top 16 of the sensor casing includes the gas permeable window 17, as described earlier in connection with figure 1 (window 76). This allows controlled diffusion of gas under test from the external ambient of the sensor casing to the optical pathway 4 in the chambers 5 and 6.

Preferably, the disc element 17 has a radius that is greater than the radius of the inner cylindrical wall 9 and less than the radius of the outer cylindrical wall 8 so that the gas permeable window completely extends over the central chamber 6 and partially extends over the circumferential chamber 5. The remaining portion 18 of the top 16 of the sensor casing provides a reflective inner surface 19 partially covering the circumferential chamber 5 to enhance the optical transmission characteristics of the circumferential chamber.

The optical detector 3 is mounted in the base 11 of the sensor casing and preferably comprises a dual element pyroelectric detector. The optical detector elements 3 a, 3b are preferably arranged in a spaced relationship along a vertical axis V of the sensor housing, ie. an axis parallel to the central axis defined by the inner and outer cylindrical walls 8, 9. This axial spacing of the detector elements 3 a, 3b ensures that the characteristics of the optical pathways leading to each of the elements are substantially similar. Each element 3 a, 3b includes a filter (not shown) to allow the transmission of optical radiation at selected frequencies or frequency ranges. This dual element configuration enables the sensor to operate with one reference or compensation detector to increase accuracy of the measurements.

Electrical leads 20 to both the source 2 and the sensor 3 pass through the casing base 11 and through an encapsulant layer 21 that holds the base 11 in position. The encapsulant layer 21 also seals the casing so that it is gas tight except for the controlled diffusion window 17.

In a preferred design, the overall outside casing diameter may be as small as approximately 2 cm, and the casing height may be as small as approximately 2 cm and thus this easily fits within a conventional size and shape of pellistor housing.

In use of the preferred embodiment, the incandescent source 2 emits infra-red radiation over a broad spectrum of frequencies. The reflective surfaces formed by the inner and outer cylindrical walls 8, 9 and the radial end wall 10 guide the infra-red radiation around the circumferential chamber 5. The non- focussing nature of the reflector surfaces means that positioning of the source 2 within the circumferential chamber 5 is not critical. Once the radiation reaches the other end of the circumferential chamber 5, via optical pathway 4a, radiation is reflected off the reflecting surface 14 of deflector 13 onto the radial inward optical path 4b, towards the detector elements 3 a, 3b.

The preferred planar geometry of the reflecting surface 14 is such that the radiation incident upon the detector elements 3 a, 3b is principally normal to the elements' surfaces which provides optimum temperature characteristics for the sensor 1 and ensures that a substantially equal amount of radiation falls on both elements. This provides for better matching conditions between the two detector element outputs.

The circumferential optical path 4a also utilises the space within the sensor casing in a highly efficient manner, and allows the chamber walls 8, 9 to be formed from cylindrical elements that are easy to manufacture and also easy to assemble. The completion of the optical path 4 with the radial portion 4b enables easy positioning of the optical detector within a large central chamber 6.

A first optical detector element, eg. 3 a, incorporates an optical filter (not shown) that allows past only radiation in a bandwidth associated with the absorption spectra of the selected gas for detection, eg. carbon monoxide. The second detector element incorporates an optical filter that allows a broader spread of frequencies, or preferably a selected bandwidth different from that of the first filter and relatively immune from undesirable attenuation from other common gases, to provide a reference signal. The reference signal is used to provide compensation of the attenuation measured by the first sensor that arises from temperature, humidity, degradation of the source intensity and other obscuration factors, rather than from the presence of the selected gas in the optical pathway 4. The ratio of the reference and selected gas signals will therefore be substantially unaffected by these other factors.

The gas permeable window 17 ensures that any changes in gas concentrations external to the sensor housing are rapidly communicated to the optical pathway 4 particularly in the circumferential chamber 5, to be sensed by the detector elements 3a, 3b, providing good real time output of sensed gas conditions. The preferred design of gas permeable window 17 as shown ensures that natural diffusion of gas into the circumferential chamber 5 is sufficient so that no pumping of gas through the chamber is required.

The detector 50 provides backward compatibility both physically and electrically with the industry standard form of pellistor based detector heads, using existing pellistor three-wire connection and input / output characteristic that mimics that of a pellistor circuit.

The use of a self-contained, integrated infrared sensor unit 70 is very important. This optical sensor unit 70 contains all the necessary optical elements within a plug- in, miniature housing. The entire optical arrangement of the gas detector is contained within the sensor unit casing, and only exposed to the environment via a gas detection window 54, which preferably comprises a stainless steel sintered filter. This forms an effective barrier against contamination. Despite a power requirement of less than that of a pellistor, the sensor has a self-heating characteristic (from the optical source contained therein) that prevents condensation from occurring at any time throughout the entire optical path, without the need for a separate heater.

As a result of the integrated design of the sensor unit and its very small size, changes due to shock, vibration or ambient temperature are minimised. Housing the sensor and its associated electronics is straightforward and cost- effective.

The optical sensor unit 70 preferably utilises the described dual element pyroelectric device which provides the best possible thermal matching of the detection parts. A temperature sensor may also be integrated into the optical sensor unit casing that allows extremely accurate temperature compensation to be applied by firmware on the circuit boards 60, 61.

The small size of the integrated optical sensor 70, and the economy of the associated electronics makes it possible for the entire device to occupy a housing that is identical in size and shape to the most commonly used pellistor heads.

Other embodiments are within the accompanying claims.

Claims

1. A gas detector comprising: a housing; an optical gas sensor unit mounted within the housing; electrical connections external to the housing; an electrical interface module electrically connected between the external electrical connections and the optical gas sensor unit for providing compatibility of the optical gas sensor unit with an external pellistor command and control circuit when connected thereto; wherein the optical gas sensor unit comprises a self-contained component containing an optical source, an optical detector and an optical pathway extending therebetween integrated into a single casing.

2. The gas detector of claim 1 in which the optical gas sensor unit is a plug-in type.

3. The gas detector of claim 1 or claim 2 in which the optical gas sensor unit is adapted to be received directly into a printed circuit board.

4. The gas detector of claim 1 in which a gas sampling volume through which the optical pathway extends is entirely contained within the optical gas sensor unit casing.

5. The gas detector of claim 1 in which the optical gas sensor unit has a gas permeable window in at least one face, and is mounted towards one end of the detector housing, the gas permeable window being adjacent to a corresponding gas permeable window in the gas detector housing.

6. The gas detector of claim 5 in which the corresponding gas permeable window is provided at one axial end of the gas detector housing, the remainder of the gas detector housing being substantially impermeable.

7. The gas detector of claim 5 or claim 6 in which the corresponding gas permeable window of the gas detector housing comprises a flame arresting component.

8. The gas detector of claim 1 in which the gas sensor unit comprises a chamber having optically reflective surfaces defining a substantially circular portion of the optical pathway and a substantially radial portion of the optical pathway and at least one reflector oriented generally at an oblique angle to the substantially circular portion of the optical pathway to separate the substantially circular portion of the optical pathway and the substantially radial portion of the optical pathway.

9. The gas detector of claim 1 in which the housing comprises a substantially cylindrical cup portion and an end cap adapted for sealing engagement therewith, the end cap including electrical connections to a printed circuit board assembly mounted thereon, the optical gas sensor unit being mounted to a distal end of the assembly such that it abuts an axial end wall of the cylindrical cup portion, the axial end wall of the cup portion including a gas detection window therein.

10. The gas detector of claim 1 in which no separate heating source is provided within the detector housing, all necessary heating for the optical pathway being provided by the optical source within the integral optical gas sensor unit.

11. The gas detector of any one of claims 5 to 7 in which the portion of internal chamber defined by the gas detector housing which houses the electrical interface module is substantially hermetically sealed.

12. A gas detector substantially as described herein with reference to the accompanying drawings.