GB2137356A

GB2137356A - Hydrogen sulphide detector

Info

Publication number: GB2137356A
Application number: GB08405259A
Authority: GB
Inventors: Thomas James Bird; Dr Alan Doncaster
Original assignee: DETECTION INSTR Ltd
Current assignee: DETECTION INSTR Ltd
Priority date: 1983-03-25
Filing date: 1984-02-29
Publication date: 1984-10-03
Also published as: GB8405259D0; GB2137356B

Abstract

A detector for detecting the presence of hydrogen sulphide gas comprises a support e.g. a metal filament, having a surface including a transition metal oxide the electrical conductivity of which is influenced by the presence of hydrogen sulphide gas in the atmosphere surrounding it. The support and coating are indicated by reference Ra, Rc in Figure 3. The detector also includes a heater for heating the support, and means, 3, 4, 6, 7, 8, 9, for monitoring the electrical conductivity of the surface 2 to detect the presence of hydrogen sulphide gas in the atmosphere surrounding the surface. Preferably the detector includes a reference element Rr having similar electrical properties to the support connected in opposition to the support to cancel out the effects of temperature changes. The oxide may be oxides of tungsten, molybdenum, tin and caesium. <IMAGE>

Description

SPECIFICATION Hydrogen sulphide detector This invention relates to a gas detector for monitoring a space to detect the presence of hydrogen sulphide and to detect any change in the concentration of hydrogen sulphide present in that space.

Hydrogen sulphide is a highly toxic gas with a strong odour. The human threshold of smell of hydrogen sulphide is well below the toxic limiting value of hydrogen sulphide but prolonged exposure to hydrogen sulphide temporarily destroys the human sense of smell. This enhances the dangers to humans of hydrogen sulphide gas since prolonged exposure to the gas causes the individual to think that the concentration has decreased whereas, it may in fact have increased but the individual's sense of smell has been destroyed. The toxic limiting value of hydrogen sulphide is 10 parts per million and exposure to higher concentrations than this rapidly results in nausea, unconsciousness and eventual death.

Due to the particular nature of hydrogen sulphide it is very desirable to protect areas where there is a possibility of any hydrogen sulphide being present with a specific detector whose sensitivity provides reliable detection of the gas at concentrations of the order of one part per million. It is also important that such a detector is not permanently destroyed, or temporarily inhibited by exposure to a high concentration of the gas such as may result from a burst seal, or broken vessel containing the gas.

According to this invention a detector for detecting the presence of hydrogen sulphide gas comprises a support having a surface including a transition metal oxide the electrical conductivity of which is influenced by the presence of hydrogen sulphide gas in the atmosphere surrounding it, a heater for heating the support, and means for monitoring the electrical conductivity of the surface to detect the presence of hydrogen sulphide gas in the atmosphere surrounding the surface.

It is believed that the transition metal oxide behaves as a semiconductor material and, in the presence of hydrogen sulphide, oxygen ions are displaced from the metal oxide lattice which results in an effective change in the doping level ofthe semiconductor material with a resulting change in its electrical conductivity. Ideally the transition metal oxide reacts only in the presence of hydrogen suphide and, in particular, is not influenced by the presence of other gases which may normally be expected to be present in the region where hydrogen sulphide monitoring is required. Naturally, the normal gases which are present are the normal gases present in air, namely oxygen and nitrogen with small amount of carbon dioxide and water vapour. An example of one of the areas in which hydrogen sulphide monitoring is required is in an oil field, particularly an offshore oil field.As oil is removed from an oil well, water is usually pumped into the well to help flush out the oil and this water can react with some of the components of the crude oil to produce hydrogen sulphide. In an oil field, other gases that may be present are typically the light hydrocarbon gases, carbon monoxide and carbon dioxide and therefore it is especially preferred that the transition metal oxide is not influenced by the presence of these gases.

Transition metal oxides such as molybdenum, tin and caesium oxide as well as mixed transition metal oxides have been found to produce good responses but many of them deteriorate over short periods of time.

The most suitable that has been found so far is tungsten trioxide, the electrical resistance of which is not only strongly influenced by the presence of hydrogen sulphide but which has been found to be stable over long time periods. Tungsten trioxide is only minimally influenced by the presence of carbon monoxide, carbon dioxide and the light hydrocarbons.

It is especially preferred that the transition metal oxide is provided as a surface coating covering the external surface of the support. The support may be electrically insulating and, in this case, it may be formed by a block of ceramic material and carry means to contact a surface coating of the transition metal oxide.

Alternatively, the support may be electrically conducting and, in this case, it is preferably formed as a metal filament which is coated with the transition metal oxide. Afurther arrangement is for the support to be formed as a conventional pellistor type of gas detector element with a metal filament surrounded by a ceramic bead. In this case the ceramic bead is, itself, coated with a transition metal oxide surface film which is, in turn, electrically connected to the metal filament of the pellistor.

When the support is in the form of, or includes, a metal filament, the metal filament may double as the heater. The metal filament may be made from a substantially inert material such as platinum or, alternatively, the metal filament may be made from a transition metal for example tungsten, and in this case its surface is preferably oxidised to provide the transition metal oxide coating.

When the support is electrically conducting and is in electrical contact with the transition metal oxide, the detector preferably also includes a reference element which is substantially identical to the support but which does not include a coating of a transition metal oxide, and means to connect the reference element in opposition to the support to cancel out the electrical effect of the support. This means may be a Wheatstone bridge circuit and in this case the out of balance current of the bridge circuit is used to give some indication of the resistance or conductivity of the surface coating of the transition metal oxide. Preferably however the means includes a pair of high current voltage reference devices connected across the support and across the reference element together with means to compare the difference in current taken by the support and reference element.The reference element may include a coating of an inert oxide such as aliminium oxide to make its thermal and electrical properties as similar as possible to those of the support. When the support is formed by a platinum filament the tungsten trioxide coating can be conveniently prepared by dropping a dilute solution of ammonium tungstate onto the heated filament until an even coating is deposited along the filament. It is preferred that the filament is in the form of a coil and in this case the surface coating should be sufficiently thin so that the coating on adjacent turns of the coil is not in contact. This naturally ensures that the surface area to length ratio of the filament is not reduced.

A particular example of a detector in accordance with this invention will now be described with reference to the accompanying drawings, in which Figure 1 is a perspective view of a detector element, and its equivalent circuit diagram; Figure2 is a simplified circuit diagram; Figure 3 is a typical circuit diagram of the complete detector; Figure 4 is a sectional elevation of the detector head; and, Figure 5 is a partly sectional elevation of a modified detector head.

The basis of the detector is the detector element shown in Figure 1. This detector element comprises a filament formed by a helical coil of platinum wire 1 carrying a surface coating of tungsten trioxide 2. When the detector head is heated by passing a current through the filament 1 the coating 2 behaves as a semiconductor material and under these circumstances the electrical resistance of the detector head is, as shown by its equivalent circuit diagram in Figure 1, the same as if a variable resistor Rc is connected in parallel with the filament 1 whose resistance is Rf. The presence of hydrogen suphide in the atmosphere surrounding the detector head causes the resistance Rc of the surface coating 2 to change and this in turn changes the resistance of the detector head.

If a constant voltage V is applied to the filament the current flow I is given by: V V Rf Rc therefore I = V (conductivityf + conductivity,).

At a constant temperature with a constant applied voltage V the filament current If is constant and therefore any change in the total current I results from a change in the coating current IC. Naturally temperature changes occur in practice and accordingly to compensate for these, the detector includes a reference filament Rr which is substantially the same but includes an inert reference coating of alumina. The filament 1 with a coating 2 will hereafter be referred to as the active element, whilst the reference filament coated with alumina will be referred to as the reference element.

The reference and active elements are connected in series with a high current voltage reference device 3 connected to one end of the reference element and a further high current voltage reference device 4 connected to the junction between the two elements. This arrangement is shown in Figure 2 and the high current voltage regulator devices provide a predetermined voltage output with variable current of up to, say, 300 ma. Suitable high current voltage regulator devices are the LM337 series of elements made by National Semiconductor of Santa Clara, California, United States of America.

With this arrangement the voltage Vr across the reference element is constant and regulated by the regulator 3. Equally, the voltage V5 across the active element is also fixed, this time by the regulator 4. In general, the current required by the active element will be greater than that required by the reference element by an amount related to the conductivity of the active coating 2. This additional current is supplied by the active regulator 4 and can be measured by the inclusion of a sense resistor R5 connected between the inputs of the regulators 3 and 4.Thus, the voltage V5 dropped across the sense resistor R5 provides an indication of the conductivity of the coating 2 on the active element and hence this voltage can be used to provide an indication of the presence of hydrogen sulphide in the atmosphere surrounding the active element. Naturally, the voltage V5 developed across the sense resistor R5 can be processed by being backed off and amplified to provide a signal level which corresponds to the quantitative concentration of hydrogen sulphide in the atmosphere surrounding the active element.

Ideally there is a zero resistance between the filaments and their corresponding regulators 3 and 4 since any resistance introduces a current dependent voltage change across the filaments. Thus, the regulators 3 and 4 are preferably included as close as possible to the active and reference elements and included in the sensing head of the detector.

In a practical embodiment of the detector in accordance with this invention the detector head, which is shown in chain dotted lines in Figures 3 and 4 includes the reference element Rr and the active element P5 with the associates regulators 3 and 4 formed by voltage regulators type LM31 7T and LM31 7L respectively, both made by National Semiconductor. The detector head also includes capacitors C1 and C2.

The detector head is connected by a three wire line 5 to a control unit consisting of a power supply line V+ and a ground line GND. A line voltage regulator 6 again formed by a voltage regulator type LM 317T is connected in series with the voltage supply line to provide an output voltage to drive the regulator 3 in the detector head. A sense resistor P5 is connected between this line and a line driving the regulator 4 in the detector head. The voltage developed at opposite sides of the sense resistor R5 is operated on in operational amplifiers 7 and 8 and then the magnitude of the difference between these two voltage levels is provided by operational amplifier 9. The operational amplifier 9 provides an output which is proportional to the voltage developed across the sense resistor R5 and, in turn, this is related to the concentration of hydrogen sulphide present in the atmosphere surrounding the active element of the detector head.

The reference Rrand active R5 elements are typically housed in a brass or stainless steel housing assembly 10 having an inlet opening covered by a sintered metal disc 11 so that the elements Rr and R5 are protected from draughts and dirt. The elements Rrand R5 are held in place in the housing by a clamping plate 12 and the regulators 3 and 4 and capacitors C1 and C2 are mounted on a printed circuit board 13 located immediately behind the clamping plate 12 with the housing 10 acting as a heat sink for the regulators 3 and 4. The three wire line 5 emerges from the housing 10 via a top cap and gland 14. The construction of the housing 10 preferably conforms with international standards for an explosion proof enclosure so that it is suitable for use in hazardous areas.

Alternatively the regulators 3 and 4, the capacitors C1 and C2 and the printed circuit board 13 may be located in a junction box 15 into which the housing assembly 10 is screwed as shown in Figure 5. The wires leading from the reference and active elements being sealed into a gland 16 located between the detector head 10 and the junction box 15. In this configuration the junction box 15 and circuitry would normally conform to explosion-proof increased safety, standard.

Claims

1. A detector for detecting the presence of hydrogen sulphide gas comprising a support having a surface including a transition metal oxide the electrical conductivity of which is influenced by the presence of hydrogen sulphide gas in the atmosphere surrounding it, a heater for heating the support, and means for monitoring the electrical conductivity of the surface to detect the presence of hydrogen sulphide gas in the atmosphere surrounding the surface.

2. A detector according to claim 1, in which the transition metal oxide is tungsten trioxide.

3. A detector according to claim 1 or claim 2, in which the support is formed as a metal filament which is coated with the transition metal oxide, the metal filament doubling as the heater.

4. A detector according to claim 3, in which the detector also includes a reference element which is substantially identical to the support but which does not include a coating of a transition metal oxide, and means to connect the reference element in opposition to the support, to cancel out the electrical effect of the support.

5. A detector according to claim 4, in which the means to connect the reference element in opposition includes a pair of high current voltage reference devices connected across the support and across the reference element together with means to compare the difference in current taken by the support and reference element.

6. A detector according to claim 4 or claim 5, in which the reference element includes a coating of an inert oxide such as aluminium oxide, to make its thermal and electrical properties similar to those of the support.

7. A detector according to any one of the preceding claims, in which the support has the form of a coiled filament with the transition metal oxide forming a surface coating on the filament, the surface coating being sufficiently thin so that the coating on adjacent turns of the coil is not in contact.

8. A detector substantially as described with reference to the accompanying drawings.