Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-12-28
2003-10-21
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S412000, C204S415000, C204S431000, C204S432000
Reexamination Certificate
active
06635160
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas sensors, and more particularly but not exclusively, to electrochemical gas sensors.
BACKGROUND OF THE INVENTION
Electrochemical gas sensors are in common use to warn of danger from toxic gases and may be used also to warn of fire outbreak. Existing electrochemical gas sensors have not always failed safe; that is they have not indicated that they have failed, or given a signal equivalent to that from a dangerous gas concentration, when they are no longer operational.
Reliability of existing gas sensors has generally therefore only been ascertained by regular tests involving exposure to a calibration or test gas. The concentration of the test gas in the vicinity of the sensor must be known accurately and this often involves use of a considerable quantity of gas and of special methods to ensure the concentration is constant and reproducible. Particularly in fixed installations, such testing can be awkward and expensive and can necessitate temporary disabling of the system, which itself is undesirable because there is a risk that the system may not be switched on again. An electrochemical gas sensor whose function can be checked while responding to a signal gas, or with a remotely-controllable self-test capability, is therefore highly desirable.
Gas sensors with self-test by generation of a gas have been described in UK Patent GB 1,552,535 (Bayer Ltd.). The self-test sensor arrangement described comprises two elements: a sensor and a gas generation means, for example an electrolysis cell, joined to the sensor by a delivery channel. Test gas, generated by the cell, is delivered to the input of a diffusion limited gas sensor, with a membrane between the point of gas delivery and the outside world. Delivery is by a piston, a pressure difference resulting from the generation of gas itself or other means. Signal gas approaches the gas sensor from the outside world via the membrane. It is not apparent whether the membrane is intended to be a diffusion barrier, whose permeability controls the response of the sensor, or whether the membrane has a high permeability and acts mainly to prevent contamination of the apparatus by, for example, airborne dust.
In the aforementioned UK Patent, the concentration of test gas detected by the sensing electrode depends on the balance of rate of generation of the gas and the rate of loss of gas through the membrane. The latter depends on a number of conditions (including: air flow, humidity and temperature) outside the membrane. Unless a large amount of test gas is generated, or the membrane has very low permeability, the response from the sensing electrode under test depends on air flow in an undesirable way. However, a very low permeability of the diffusion barrier causes the sensor to have low sensitivity to the signal gas and so might itself be undesirable. The arrangement described in UK Patent GB 1,552,538 therefore needs a comparatively large amount of gas to give reliable operation; in turn requiring significant energy, which limits its usefulness, in particular in situations where only low power is available.
A possible failure mode of fixed installation gas sensors, incorporating the aforementioned arrangement, is that gas access from the atmosphere might become partially or completely blocked. Delivery of test gas inside the cell might result in a rise in response above that expected, as the rate of escape is reduced. However, this only becomes apparent if the sensor response is dependent entirely on the concentration of test gas inside the sensor. As the test gas is delivered directly to the sensing electrode in the aforementioned arrangement, the electrode response to the test gas is likely to be limited partially by the activity of the electrode itself, and is therefore unlikely to give a reliable indication of a blocked outer barrier.
SUMMARY OF THE INVENTION
An aim of the invention is to provide an improved gas sensor which allows reliable checks on its performance to be made and overcomes at least some of the aforementioned problems.
According to the present invention there is provided a gas sensor comprising: a sensing cell and a test cell, the test cell being arranged, so that in use, a test gas is generated on demand and a pathway is provided for directing said test gas from the test cell to the sensing cell, there being at least first and second sensing electrodes in the sensing cell, characterised in that the test gas is directed so that at least two electric currents are generated and means is provided for comparing the currents in order to provide a value indicative of the status of at least one electrode.
The life of the test gas generator depends on the amount of test gas needed in each test; this could be prohibitive in a sensor intended for long life or for frequent test if the desired amount of test gas is large. The present invention reduces considerably the amount of test gas required and makes feasible the use of small test gas generators which store small amounts of consumable substance.
Advantageously electrochemical gas generation of, and subsequent detection of, a test gas is achieved in a manner which uses less energy than previous devices.
Inclusion of a test gas generator cell and sensing cell in the same housing reduces the volume of test gas needed and hence the energy needed to power the test gas generator. Furthermore delivery of test gas between two diffusion barriers allows an increase in sensitivity above a certain threshold. This enables a warning of a blocked gas access from outside the cell to be provided.
Substantially planar cell design allows sensing and generator electrodes to be fabricated on the same substrate at a single stage. This reduces cost and allows closer spacing of components. In practice the volume of test gas required is reduced and this provides a more responsive sensor.
According to a second aspect of the present invention there is provided a sensing cell with two sensing electrodes, with gas access to the second cell via the first cell, there being a known diffusion impedance between the said cells, each cell, in use, generates an electric current, the ratio of which currents, in response to a signal gas from the atmosphere, is used to provide an indication of activity of the electrodes.
The aforementioned arrangement may be used to derive a calibration coefficient for the cell, hence maintaining its calibration beyond the point at which a conventional cell would have failed. Most importantly if the ratio reaches a predetermined value a warning of impending failure of the cell is provided. This may be activated automatically by suitable referencing means such as an integrated circuit.
Combination of a test gas generator cell and a sensor cell allows decay of the sensing electrodes to be detected using test gas when no signal gas is present. This can be used to derive a calibration coefficient. Such an embodiment is less sensitive than a conventional arrangement, or of cells with sensor and generator in the same housing, but only one sensing electrode is required. The electrode senses variations in concentration of test gas resulting, for example, from variation in performance of the test gas generator, or from variation in external air velocity, thereby allowing smaller volumes of test gas to be used, and therefore requires less energy to generate the gas.
If a second gas generator is used, which delivers gas to the second sensing electrode, together with a first gas generator which delivers gas to the first sensing electrode, each gas subsequently being able to diffuse to the other sensing electrode, then comparison of the electrode response to gas from two gas generators allows measurement of the decay of each of the two electrodes to be made. This enables an even more accurate estimate of the calibration coefficient to be made than if one generator is used alone.
Preferably sensing and generator electrodes are in the same housing and arranged so that gas is delivered from the generator electrode into a space between two diffusio
Bianco Paul D.
Central Research Laboratories Limited
Fleit Martin
Fleit Kain Gibbons Gutman & Bongini P.L.
Tung T.
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