Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-06-23
2002-09-24
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S401000, C204S412000, C205S783000
Reexamination Certificate
active
06454923
ABSTRACT:
The present invention relates to gas sensors, and more particularly, but not exclusively to gas sensors incorporating a self-test capability. These types of gas sensors are hereinafter referred to as self-test gas sensors. The invention is suitable for use in electrochemical gas sensors, such as, for example, carbon monoxide (CO) sensors.
Conventional electrochemical gas sensors for use in CO sensors, operate by oxidizing a gas to be detected at a sensing electrode, thereby generating an electric current. The rate of access to the sensing electrode may be determined by a diffusion barrier, and the rate at which the electrode is capable of oxidizing the gas is arranged to be very much greater than the rate at which the gas can diffuse through the barrier. Generally the rate of oxidation, and hence the electric current generated, is controlled mainly by diffusion. This diffusion rate has a value (for a given gas concentration) and the sensor can be calibrated when it is manufactured. If the actividty of the sensing electrode falls with time, e.g., through poisoning, then the level of current generated eventually becomes limited by a lower oxidation rate. This results in a decrease in sensitivity of the sensor. There is then no way of determining, from the sensor output alone, whether the gas concentration is low, or whether the gas concentration is high and the electrode has lost activity become less sensitive.
Previously, so as to overcome this problem, the sensitivity of sensors was ascertained by regular tests involving exposure to a calibration gas of known concentration. In many situations, for example in a domestic carbon monoxide safety monitor, this was undesirable.
In the Applicant's published International Patent Application WO-A-9703372, the contents of which are incorporated herein by way of reference, there is described a self-test gas sensor having electrolytic gas generation electrodes and sensor electrodes being located in close proximity one to the other within the same housing. The aforementioned gas sensor has several advantages over previous gas sensors, including lower operating power, as the amount of gas needed to be generated for the self test procedure is reduced; and lower assembly costs, as the test and sensing cells are effectively assembled in the same operation. The self-test gas sensor operates by generating locally a small volume of gas (hereinafter referred to as a “test gas”) and detecting the test gas in the sensor, thereby confirming the status of the sensor, i.e. functioning or faulty.
It is desirable that the response of the gas sensor, during the self test procedure, should result only from the response of the sensor to the test gas generated during that procedure, and not arise from electrical interference which may occur between the sensor and gas generator circuits as a result of electrochemical reactions.
According to the present invention there is provided a self-test gas sensor comprising: a sensor for sensing a gas and a test cell, the test cell being arranged to generate a test gas on demand, and a test gas pathway for directing the test gas to the gas sensor so that generated test gas is detected by the gas sensor, thereby verifying that the gas sensor is functioning, characterised in that a baffle is disposed between the gas sensor and the test cell so as to prevent electrical interference therebetween.
Sensing electrodes are electrically isolated from test cell electrodes by way of the baffle. Preferably the baffle is situated in a fluid pathway between the sensing circuit and the test circuit. The baffle prevents flow of ions (and therefore electric current) between what is effectively a test cell and a gas sensor cell when the test cell is operating. The baffle may comprise two or more portions. The first portion preferably comprises a gas impermeable substrate, and the second portion comprises a gas permeable substrate. The gas permeable substrate of the baffle permits the passage of water vapour between the test cell and the gas sensor. The gas permeable substrate helps to maintain electrolyte, present in the gas sensor, at a substantially constant pH and/or concentration.
A further requirement is that the complexity of the sensor and gas generator cells, and their respective operating circuits, should be minimised in order to reduce unit costs.
According to another aspect of the invention a self test gas sensor includes first, second and third electrodes, the first electrode being a common electrode, the second electrode in operation with the first electrode, acting as a pair to generate a test gas; the third electrode in operation with the first electrode acting as gas sensor, characterized in that means is provided to isolate test and sensing electrodes one from another, so that only one pair of electrodes is operational at any instant.
It is also envisaged that an embodiment of the sensor having three electrodes is within the scope of the present invention, a counter electrode being common to a test and a sensing electrode. In this embodiment electrical interference or cross-talk between components is avoided by ensuring the sensing and test components operate at different instants. Cross talk between the two sets of electrodes is avoided by a switch which ensures that the test electrode and reference electrode are only energized when the sensing electrode is switched out of circuit. Clearly in an embodiment where there is a common electrolyte there is a risk of cross talk between two (or more) pairs of electrodes and switching one pair out of circuit whilst a separate pair are switched in circuit is one way of avoiding this.
Preferably a capacitive element is provided for storing energy from an energy source which supplies electric current to a pair of sensing electrodes during operation of the sensor, there being a switch arranged to disconnect the supply of electric current to the sensing electrodes and connect the capacitive element to the test electrodes thereby providing an independent source of current thereto.
In normal sensing use, the capacitive element is changed from the sensing circuit power supply. In test mode, the capacitor is isolated from the power supply by a switch, and connected to the test electrode circuit, so as to discharge through the test circuit and generate gas at the test electrode. The capacitor provides a supply isolated from the sensor circuit supply, and so the generation current will not flow through the sensor electrodes, provided also that a baffle as aforementioned is in place between the test electrode and sensing electrode.
Preferably the sensor and the test cell are disposed within a housing, which permits gas from the environment to pass to the gas sensor via a gas pathway. The gas pathway is preferably separated from a test gas pathway.
Preferably the electrically conductive pathways by-pass the or each baffle thereby further reducing the risk of interference between a test circuit and a sensing circuit.
There may be a counter electrode common to the sensor and the test cell. The sensor electrodes and the test cell electrodes may be formed on a gas permeable membrane, such as PTFE.
Strengthening ribs may be incorporated into the body of the housing. A diffusion barrier may be provided to limit the rate of arrival of a gas at the gas sensor.
UK Patent Application GB-A-2323171 (City Technology Limited) discloses a sensor in which electrical contact to an internal electrode is made, via an electrical connector, from an external terminal. The electrical connection is made between the electrical connector and the external terminal by forcing electrically conductive components together under pressure and maintaining the pressure throughout the working life of the sensor. It is believed that such connections eventually fail, not as a result of relaxation of the compression of components, but rather as a result of relaxation or perishing of either intervening gaskets or O-ring seals;
A further advantage of the present invention, over the arrangement described in GB-A2323171, is the
Austen Malcolm
Backes Monica
Dodgson John
Shaw John Edward Andrew
Bollman William H.
Central Research Laboratories Limited
Tung T.
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