GB2135780A - Carbon monoxide detection - Google Patents

Abstract

Carbon monoxide in an atmosphere is determined by pumping a sample of the atmosphere to an electrochemical sensor, treating the atmosphere to remove carbon monoxide and pumping the treated sample to the sensor and determining carbon monoxide from the difference in the sensor outputs. Pump 3 passes atmosphere through adsorbent column 4 (for interfering gases) and over electrochemical cell 8 to obtain a first signal. Reversing the direction of pumping causes atmosphere to pass through adsorbent column 4, over catalyst bed 6 to cause oxidation of carbon monoxide and over cell 8 to obtain a second signal. Processor 9 processes the signals and a corrected value for carbon monoxide concentration is transmitted to digital display 10. The value for carbon monoxide is thus corrected for the presence of interfering gases especially for the presence of hydrogen which may be present in underground coal mines, which interferes with electrochemical cells and cannot easily be removed from the atmosphere. <IMAGE>

Description

SPECIFICATION Improvements in gas detection This invention concerns improvements in gas detection, more particularly it concerns improvements in the detection of carbon monoxide using an electrochemical cell.

In underground coal mines, the level of carbon monoxide in the atmosphere, and its rate of increase or decrease, is of great importance. Carbon monoxide is generated by "heatings", which are relatively small areas of spontaneous combustion, as well as by fires, and these or the carbon monoxide generated may endanger lives or the continuing operation of the mine. The most accurate method of assessing carbon monoxide which is in relatively widescale use involves a measure of the absorption of specific radiations in the Infra-Red region by instrumentation. Such instruments are, however, bulky and are essentially expensive laboratory instruments which cannot easily and reliably be taken underground.Nonetheless, samples of mine air can be withdrawn through long tubes and taken to the surface for analysis using infra-red techniques, but the function of such a system is necessarily limited to one of general surveillance and early warning and not to the precise location of trouble spots for which a hand held instrument is a much preferred alternative to the chemical stain tubes currently in use. In recent years, a number of electrochemical cells have been developed and brought on the market, which measure carbon monoxide in hand-held or at least portable instruments. These cells and the instruments are fairly robust, stable as regards output and relatively cheap, but do not yield data which directly and unambiguously relate to that derived from infra red instruments.It now appears the deviation of CO levels measured by electrochemical cells from that measured by infra red instruments is progressively more evident during the critical stages of fire development, and that this is due to the generation of quantities of other oxidisable gases including hydrogen. Hydrogen in particular can cause erroneous measurement and cannot be simply or readily separated from CO by conventional means.

The present invention provides a method of obtaining a corrected value for carbon monoxide concentration in an atmosphere, comprising obtaining a first signal indicative of carbon monoxide content from electrochemical cell means capable of detecting carbon monoxide by passing an atmosphere over said cell, treating the atmosphere to remove carbon monoxide and passing the resulting gas over said cell means to obtain a second signal apparently indicative of carbon monoxide content and processing said signals to give a corrected value for carbon monoxide concentration.

The invention also provides an apparatus for obtaining a corrected value for carbon monoxide concentration comprising electrochemical cell means capable of detecting carbon monoxide, means for removing carbon monoxide, pump means and signal processing means, whereby an atmosphere may be passed over said cell means to derive a first signal indicative of carbon monoxide content, the atmosphere may be treated to remove carbon monoxide and the resulting gas passed over said cell means to derive a second signal apparently indicative of carbon monoxide content, and the signals processed to give a corrected value for carbon monoxide concentration.

A preferred embodiment of the invention uses first and second quantities of adsorbents capable of removing easily adsorbed and potentially interfering components of the atmosphere before the atmosphere is passed to the cell means or the means for removing carbon monoxide.

Preferably, the two adsorbents are in the form of columns connected in series with an electrochemical cell positioned between them.

The pump means is preferably capable of providing alternating flow through the apparatus, first in one direction and then in the other direction. In addition to providing the electrochemical cell with the appropriate streams of gas, this configuration results in the gases having passed the electrochemical cell eluting from the downstream column the adsorbed components, including moisture. A timer or controller/timer is preferably used to control the action of the pump means, and this may be microprocessor-based.

Although the present invention will be described hereinafter as operating with a single electro-chemical cell, over which both the COcontaining gas and the CO-free gas passes, an alternative apparatus and method utilises two cells, one deriving the first signal and the other deriving the second signal. The electro chemical cell may be any of a number which are commercially available, although it is preferred to use a CO detector cell manufactured and marketed by City Technology Ltd., 1 7/1 9 Sebastian Street, Northampton Square, London, EC1V OHB, which has been found to exhibit excellent stability and accu racy, as well as having a small retention volume and quick response times. The cell is preferably positioned in a chamber of rela tively small volume, positioned between the first and second quantities of adsorbents.

The adsorbent used is suitably an adsorbent which is effective to preferentially adsorb un saturated hydrocarbons and the oxides and hydrides of sulphur and nitrogen relative to carbon monoxide, hydrogen and the perma nent gases. Those known as chromatographic adsorbents, are to be considered for use, and include active carbon and alumina although molecular sieves are preferred, especially a combination of a molecular sieve and a polymeric adsorbent such as Poropak", which is the trade name for polystyrene beads marketed as a chromatographic support.

Preferably, the means for removing carbon monoxide is any catalyst effective to oxidise carbon monoxide to carbon dioxide without causing the oxidation of hydrogen also present in the gas treated, and is preferably of the metal oxide type, for example based on manganese dioxide. Such catalysts are sensitive to moisture, and the adsorbents adsorb water and protect the catalyst. If a catalyst is used which is not sensitive to moisture or other likely adsorbable contaminants of the atmosphere, adsorbents need not be used.

The apparatus of the invention may be in the form of a hand held portable or a fixed instrument. A hand held instrument is suitable for general purpose applications in underground coal mines and may be fitted with a hollow probe to enable samples to be taken from relatively inaccessible areas. In the preferred apparatus, cycle times are of the order of 30-40 seconds, which is perfectly adequate for such use. The portable instrument may be positioned, with or without a remote sampling probe, in an appropriate site underground and may be connected to signal transmission facilities to the surface or elsewhere. Although the invention is described herein with especial reference to use in underground coal mines, the invention is not to be limited to such use.Carbon monoxide detectors find use in a number of other fields, including the monitoring of areas with high motor vehicle concentrations and the monitoring of combustion efficiency in the interests of energy saving.

The invention will now be described by way of example only with reference to the accompanying drawing which is a schematic diagram, not to scale, of a preferred embodiment.

The apparatus incorporates two entries 1, 2 for an atmosphere to be drawn into the apparatus, and a pump, 3, which is capable of acting to draw atmosphere in one entry and to cause it to exit through the other, and capable of reversing the direction of flow. A gas flow rate of about 100 ml/minute has been found acceptable. Connected with the pump is a first column, 4, containing about 0.5g of "Poropak" followed by 1 -2g of molecular sieve adsorbent and connected to entry 2 is a second column, 5. The second column contains a similar amount of the same adsorbents as the first column, but also contains a layer, 6, of about 0.2-0.3g of a manganese dioxide-based catalyst. Connected between the columns is a small chamber, 7, containing an electrochemical cell, 8. The cell is a CO detector cell from City Technology Ltd.The output from the cell is fed to a processor, 9, which converts outputs into concentrations and substracts the smaller from the larger to give a corrected value, and outputs a signal to a digital display, 1 0. A controller timer, 11, acts to control the pump to initiate flow in one direction and after an appropriate period, to reverse the flow.

At the start of a cycle, the pump draws atmosphere into entry 1 and directs it through first adsorbent column, 4. Easily adsorbable gases, including moisture and all the gases except hydrogen and carbon monoxide to which the cell is sensitive, are adsorbed in the first part of the column, and essentially only the major air components, methane, carbon monoxide and hydrogen leave the column and pass into chamber 7 where they contact the cell. The cell produces a first signal which is supplied to the processor which retains a value corresponding to the plateau in detected carbon monoxide concentration. The gas is vented to atmosphere through entry 2.

After about 20-30 seconds, at a time precisely determined by the processor which detects the plateau in the cell output signal, the pump is caused to reverse the direction of flqw of gas. The atmosphere enters through entry 2, and again easily adsorbable gases are adsorbed in the initial section of the adsorber column 5. At the top of the column the remaining gases pass through the catalyst which destroys all the carbon monoxide by oxidation. The resulting gas is then drawn through chamber 7, and the cell produces a second signal corresponding to the amount of oxidisable gas (essentially only hydrogen) in the resulting gas, and the second signal is supplied to the processor. The gases leaving the chamber pass through column 4 and elute the adsorbed components, leaving the column regenerated for the next cycle of operation.

The elution of adsorbed components from column 5 also occurs, of course, in the initial direction of flow.

The processor derives from the two signals, a corrected value for carbon moxoide concentration, which is then displayed in digital form on the display.

The present invention also permits, in principle, the determination of hydrogen, which does however require that the base line or instrument zero remains fixed. To assume that it does so is acceptabie instrument practice, although regular calibration is often resorted to where accuracy is at a premium. Zero checks are, however, difficult if there is no easy access to air of near absolute purity, as is the case in underground coal mines. In the case of derermining carbon monoxide, the present invention at least in the described embodiment caters for minor changes in zero.

This is particularly valuable at concentrations of carbon monoxide below 10 ppm where minor drifts in zero have greater effect on accuracy (per cent error) than the small deviations in span to which the electrochemical cell may be prone.

Claims (16)

1. An apparatus for obtaining a corrected value for carbon monoxide concentration in an atmosphere, comprising electrochemical cell means capable of detecting carbon monoxide, means for removing carbon monoxide, pump means and signal processing means, whereby an atmosphere may be passed over said cell means to derive a first signal indicative of carbon monoxide content, the atmosphere may be treated to remove carbon monoxide and the resulting gas passed over said cell means to derive a second signal apparently indicative of carbon monoxide content, and the signals processed by the processor to give a corrected value for carbon monoxide concentration.

2. An apparatus according to claim 1 wherein the means for removing carbon monoxide is an oxidation catalyst.

3. An apparatus according to claim 2, wherein the catalyst is a manganese dioxidebased catalyst.

4. An apparatus according to any one of claims 1 to 3, comprising first and second quantities of an adsorbent capable of preferentially adsorbing water vapour, unsaturated hydrocarbons, oxides and hydrides of sulphur and nitrogen, relative to carbon monoxide, hydrogen and the permanent gases, positioned to remove potentially interfering components of the atmosphere before it contacts the cell means.

5. An apparatus according to claim 4, wherein the adsorbent is combination of a polymeric adsorbent and a molecular sieve.

6. An apparatus according to claim 4 or 5, wherein the two quantities of adsorbent are in the form of columns connected in series, with an electrochemical cell positioned therebetween.

7. An apparatus according to claim 6, wherein the pump means is capable of providing alternating flow of atmosphere through the apparatus.

8. An apparatus according to claim 7, arranged so that in a first direction of flow, the atmosphere passes in sequence through a first column of adsorbent, passes over the cell, through a catalyst bed and through a second column of adsorbent, and in a second and reverse direction of flow through the second column, through the catalyst bed, over the cell and through the first column, whereby adsorbed material is elyted from the column downstream of the cell in each direction of flow.

9. An apparatus according to any one of the preceding claims, in the form of a handheld or portable instrument.

10. An apparatus according to claim 1, substantially as hereinbefore described with reference to the drawing.

11. A method of obtaining a corrected value for the carbon monoxide in an atmosphere, comprising obtaining a first signal inidicative of carbon monoxide content from electrochemical cell means capable of detecting carbon monoxide by passing an atmosphere over said cell, means treating the atmosphere to remove carbon monoxide and passing the resulting gas over said cell means to obtain a second signal apparently indicative of carbon monoxide content, and processing said signals to give a corrected value for carbon monoxide concentration.

1 2. A method according to claim 11, wherein the atmosphere is passed over a carbon monoxide oxidation catalyst to remove carbon monoxide.

13. A method according to claim 11 or 12, wherein the atmosphere is passed over an adsorbent to remove potentially interfering components before being passed over the cell means.

14. A method according to claim 13, comprising obtaining the first signal by passing the atmosphere through a first adsorbent, over an electrochemical cell, through a cata lyst bed and through a second adsorbent, reversing the direction of flow and obtaining the second signal by passing the atmosphere through the second adsorbent, through the catalyst bed to remove carbon monoxide, across the cell and through the first adsorbent.

15. A method according to claim 14, wherein the reversal of flow is actuated upon detecting a plateau in the first signal.

16. A method according to claim 11, substantially as hereinbefore described.