Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing
Reexamination Certificate
2000-08-02
2002-08-06
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
C204S401000, C204S406000
Reexamination Certificate
active
06428684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for the detection of a fault condition in a gas detecting apparatus containing one or more electrochemical gas sensors.
2. Description of Related Art
Potentially dangerous gas mixtures may be found in many work place environments. These dangers include the risk of fire or explosion from combustible gases, exposure to toxic gases and excessively high or low concentrations of oxygen.
These dangers are well known and gas detection instruments are available to detect a wide range of gases. These instruments typically contain one or more gas sensors, which give a proportional electrical response dependent upon the concentration of the gas to be detected. If the concentration exceeds allowed concentration limits, then the instrument will provide an alarm to warn nearby personnel, or it may activate other remedial actions, such as to increase the ventilation.
Gas detection instruments for safety applications are broadly divided into two groups. In the first group are portable instruments, which are designed to be hand held or worn by the user and provide personal monitoring. This group also includes transportable instruments which although not handheld, are easily moved from one location to the next. In the other group are fixed instruments, which are typically wall mounted, to provide area monitoring.
Oxygen and many of the commonly encountered toxic gases are usually detected with amperometric electrochemical gas sensors. A typical electrochemical sensor is usually constructed with two or more electrodes in contact with an electrolyte. The electrode is usually separated from the outside environment by a gas porous membrane, and other diffusion barriers. The gas to be detected enters the sensor and passes through the membrane to the working electrode, where it is either oxidized or reduced, or the rate of oxidation or reduction of the electrode or another species in electrolyte may be limited depending on the availability of the toxic gas. The resulting electrical current is proportional to the rate at which the gas is being consumed by the electrode; this type of electrochemical sensor is therefore known as an amperometric sensor. The output current is usually linearly proportional to the gas concentration, since the response is limited by the rate at which the gas to be detected can diffuse into the sensor.
The theory of operation and practical usage of electrochemical gas sensors has been discussed in detail by Chang et al (S. C. Chang, J. R. Stetter, C. S. Cha, “Amperometric Gas Sensors”
Talanta
, (1993), 40, 461) and by Hobbs et al (B. S. Hobbs, A. D. S. Tantram, R. Chan-Henry in “Techniques and Mechanisms in Gas Sensing”, Ed. P. T. Mosely, J. Norris, D. E. Williams, (1991). In these sensors, the analyte gas diffuses into the sensor through a diffusion barrier to one of the electrodes, known as the working electrode. The electrons required for the oxidation or reduction of the gas flow through the external circuit to/from the counter electrode, where an equal magnitude reduction or oxidation reaction respectively occurs, and this flow of electrons constitutes an electric current, which provides the output signal. The potential of the working electrode is selected such that all the analyte gas which reaches the electrode is electrochemically oxidized or reduced. The nature of the response of the sensor to a toxic gas depends on both the design of the sensor and the nature of the gas. Some gases such as carbon monoxide (CO) and hydrogen (H
2
) are oxidized at the electrode, whereas other gases such as chlorine (Cl
2
), oxygen (O
2
) and nitrogen dioxide (NO
2
) are usually reduced in the sensor.
Oswin et al in U.S. Pat. Nos. 3,909,386, 3,992,267 and 3,824,167 describe a sensor for carbon monoxide and many variations of this basic design are known in the prior art. For most sensors, an external circuit (a potentiostat) controls the potential of the working electrode. In some sensors, such as galvanic oxygen sensors, the potential is generated by the oxidation of the counter electrode. A sensor of this latter type is known as a galvanic oxygen sensor, and descriptions have been provided by Lawson in U.S. Pat. No. 4,085,024, Tantram et al in U.S. Pat. Nos. 4,132,616 and 4,324,632, Culliname in U.S. Pat. No. 4,446,000, Bone et al in U.S. Pat. No. 4,810,352 and by Fujita et al in U.S. Pat. No. 4,495,051.
The output of most amperometric sensors is proportional to the gas concentration, and is described by the following equation:
I=nFCD&Dgr;
where I is the current (A), n is the number of electrons, F is the Faraday constant (9.648×10
4
C/mol), C is the gas concentration (mol/cm
3
), D is diffusion coefficient (cm
2
/s) and &Dgr; represents the cumulative diffusion barrier that the gas must pass through to reach the working electrode. In principle, it is possible to measure all the diffusion barriers comprising the sensor and thereby calculate &Dgr;, and hence calculate the sensitivity of the sensor; for example, see P. R. Warburton, M. P. Pagano, R. Hoover, M. Logman, K. Crytzer, Y. J. Warburton,
Analytical Chemistry
(1998), 70, 998. However, this calculation is not practical in common practice, and instead the gas detection instrument is calibrated by exposure to a test gas of known concentration and the output of the instrument is adjusted to match the nominal concentration of the gas. This calibration is usually performed manually and it is typically a tedious process, especially if there are a larger number of instruments. Calibration is also an expensive procedure, both in terms of the cost of the test gases with certified compositions and in terms of the labor time, and associated record keeping.
Automatic calibration methods have been described in the prior art, for example, Stetter et al in U.S. Pat. No. 4,384,925, Hyer et al in U.S. Pat. No. 4,151,738, Hartwig et al in U.S. Pat. No. 5,239,492 and Melgaard in U.S. Pat. No. 4,116,612 describe methods for automatic calibration of a gas detection instrument in which calibration gases are automatically applied to the sensors under microprocessor control. For portable instruments, so called docking stations are now available, such as the DS1000 Docking station from Industrial Scientific Corporation, Oakdale Pa. 15107, which perform the calibration and record keeping automatically.
Though most sensor technologies are very reliable, as required for a safety application, electrochemical sensors do sometimes fail in service. While most electrochemical sensors do not have a fixed service life, some sensors, such as galvanic oxygen sensors are consumed during the oxygen detection reaction and so have a limited lifetime. Whereas calibration is usually only performed at fixed time intervals, for many safety applications it is common practice to “bump test” gas detection instruments more frequently to ensure that they are working correctly. The bump test typically involves application of a test gas mixture for enough time to activate the warning alarms and/or other modes of display that indicate that the instrument responded correctly to the gas. The bump test gas procedure is commonly quicker than a calibration, but it still involves the expense of both time and test gas mixtures.
The cost and time required for manually performing calibration or bump tests on gas detection instruments have provided an incentive for the development of test methods which can be performed automatically by the instrument without human intervention. The optimum function test for a sensor is exposure of the sensor to an analyte gas of known concentration and measurement of the sensor's response. However, cost, size and complexity of the apparatus limit the ability to achieve this goal. Many simpler methods have been devised to measure the functional status of the sensor.
Electrochemical gas generators for testing gas sensors are well known in the prior art, and are described, for example, by Wolcott in U.S. Pat. No. 4,460,448 and by
Dennison, Schultz & Dougherty
Industrial Scientific Corporation
Olsen Kaj K.
Warden, Sr. Robert J.
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