City Technology Limited

Coal Industry (Patents) Limited

Anthony D Tantram

Yat S Chan

John Robert Finbow

Bryan S Hobbs

Michael J Kent

Alan G Palmer

Jonathon H Gilbey

An oxygen sensor (1) with an inlet (2) connects with a small chamber (3). An inlet provided with a diffusion barrier (4) permits flammable gas and oxygen to enter the chamber. The oxygen concentration within the chamber is low, and a catalytic element (5) within the chamber causes combustion of flammable gas, thus consuming oxygen within the chamber. The signal from the oxygen sensor is processed to calculate a concentration for the flammable gas.

Flammable gas detection

A gas calibration method providing an absolute standard for the determination of an active gas in a gas mixture, comprises passing the gas mixture into a chamber (1) of known volume. The chamber contains a galvanic sensor ( b 2) providing a signal current, proportional to the rate of reaction of the active gas at the sensor electrode, to a current measuring device (8). After sealing the chamber, a signal processing means (9) samples the signals from (8) as a function of time, and calculates the sensitivity of the sensor and the concentration of the active gas.

Gas calibration method and apparatus

The diffusibility or permeability of a sample of material (6) is determined by mounting the sample in the wall of a chamber (1), having an electrochemical gas sensor (4) fitted therein. If a differential pressure is applied across the sample, and output signals from the gas sensor are recorded at least two times, the signals and times can be processed to yield diffusibility or permeability values.

Diffusion measurement

The housing 1 of a gas sensor carries a diffusion barrier 2 for restricting the access of gas to the sensing element 4 and a filter 3 mounted between the diffusion barrier 2 and the sensing element 4. As a result, gas reaching the filter 3 must first pass through the diffusion barrier 2 so that its flux is substantially reduced and the life of the filter is increased correspondingly.

Gas sensor

An electro-chemical cell has porous sensing and counter electrodes S, C mounted on porous PTFE tapes 4, 5 and contacting the electrolyte of the cell which is drawn from a reservoir by a wick 12. Gas passes directly to the electrode S at 14 and any gas which does not react passes through the electrode S to the electrode C, passing across the electrolyte space by way of a porous insulating separator 10 which is partly hydrophilic and partly hydrophobic, thus providing a path for gas while preserving the electrolyte path between the electrodes. The current flowing between the electrodes is measured in the usual way. The electrode C will not see any particular gas which reacts fully at the electrode S since it will be removed by this electrode first. The electrode C will, however, see any other gas which only partially reacts at the electrode S, thus achieving partial or even complete elimination of the effect of an interfering gas.

An electro-chemical cell for quantitatively detecting an acidic or an alkaline gas, e.g. carbon dioxide, includes a sensing electrode 9 which is connected to a top cap 12 by a contact strip 8 and is accessible to the gas to be tested through a hole 13. The electrode 9 comprises an electro-chemical couple whose potential is pH-dependent (e.g. silver/silver oxide for carbon dioxide) the active material being supported by a nickel gauze to give good current collection. A counter electrode 2 without significant gas access is immersed in an electrolyte 3 whose pH is such that it will change following adsorption of the gas being tested for. A wicking separator 7 ensures an electrolytic path between the two electrodes. The components are contained within a metal can 1, the top cap 12 being held by folding over the rim 5. An insulating grommet 6 is fitted between the two electrodes. When using silver/silver oxide for the sensing electrode 9, the counter electrode may be of the same material and the electrolyte may be potassium carbonate. The current flowing between the electrodes is a measure of the concentration of gas being detected. A reference electrode 14 may also be included.

A gas sensor cell assembly has planar electrodes separated by a planar hydrophilic wick which extends through the electrode into an electrolyte expansion chamber where it contacts an electrolyte. The cell is extremely compact and operates in all orientations.

An electro-chemical sensor for the measurement of concentrations of gas or vapour in accordance with the limiting current principle comprises an electrolytic cell 1 having a sensing electrode 9, a counter electrode 2, an intervening body of electrolyte 4 and a restriction to the rate of access of gas or vapor to the sensing electrode in the form of a porous diffusion barrier 13. The improvement resides in the feature that substantially all the active pores of the diffusion barrier are sufficiently small as to cause diffusion through them to be in accordance with the Knudsen principle, i.e. the diffusion mechanism is effectively determined solely by collisons between the diffusion molecules and the walls of the pores. The barrier is constituted by a body of porous PTFE which may be in the form of a piece of unsintered PTFE tape or compressed PTFE powder formed by drying a dispersion of PTFE in water.

An electro-chemical sensor for the measurement of concentrations of gas or vapor in accordance with the limiting current principle and comprising an electrolytic cell having a sensing electrode, a counter electrode and an intervening body of electrolyte and also a restriction to the rate of access of gas or vapor to the sensing electrode. The improvement comprises said restriction being in the form of a gas-phase diffusion barrier including at least one narrow passage for the diffusion of gas. The gas-phase diffusion barrier may be in the form of a porous body, of which the permeability of the body may be more than twenty times greater than the diffusibility, but is preferably less than ten times the diffusibility and more preferably not more than 10% greater than the diffusibility. Alternatively the gas-phase diffusion barrier may be defined by a capillary. The capillary may be of plastics material and may be enclosed within an outer metal sheath of lesser co-efficient of expansion. The capillary may be connected in series with a porous body.

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Great Bookham, United Kingdom

Anthony D Tantram