Chemistry: analytical and immunological testing – Composition for standardization – calibration – simulation,...
Reexamination Certificate
1999-03-31
2003-10-14
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Composition for standardization, calibration, simulation,...
C436S009000, C436S181000, C073S001020, C073S001060
Reexamination Certificate
active
06632674
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention provides a method and related apparatus for checking the reliability of gas detection instruments through the generation of a test gas and its application to the gas detection instrument.
2. Description of the Prior Art
The reliability of toxic gas detectors is of great importance in many applications, especially when these instruments are used for ensuring the safety of personnel. Reliability is typically obtained by periodic checking of the instrument response to a test gas, however calibration test gases are typically supplied in large, bulky and expensive gas cylinders.
Potentially hazardous atmospheres are found in many locations, due to the presence of toxic gases, combustible gas mixtures or the excess or deficiency of oxygen concentration. Many types of gas detection instruments have been developed to provide a warning that the atmosphere contains potentially hazardous components, or to initiate remedial action. Examples of these gas detection instruments include the detection of combustible gases (primarily methane) in coal mines, hydrogen sulfide in oil fields and water treatment plants, carbon monoxide in places ranging from steel mills to bedrooms, and oxygen in confined spaces, such as sewers. Within each gas detection instrument there are one or more gas sensors, whose function is to provide an electrical signal, which varies in response to the gas concentration.
Many types of sensor technology are used for gas detection, including electrochemical, infrared, catalytic bead (heat of combustion), and tin oxide sensors. Details of these various sensor types are discussed in standard texts such as C. F. Cullis, J. G. Firth, “Detection and Measurement of Hazardous Gases”, Heinemann, London, 1981; P. T. Mosely, J. O. W. Norris, D. E. Williams (Eds.), “Techniques and Mechanisms in Gas Sensing”, Adam Hilger, Bristol, 1991. Each of these various technologies have different advantages and weaknesses, such that the method used will depend on the gas to be detected and the application requirements.
In general, most gas sensors provide a relative output signal, such that the output signal is not an absolute measure of the gas concentration. Instead the response is typically proportional to the gas concentration, with an empirically determined proportionality constant. Before the instrument can be used to measure the concentration of a gas, the instrument is first exposed to a known test gas concentration and the output signal correlated with the known gas concentration. This process is known as calibration.
Another role of calibration is to provide a function check to confirm that the gas detection instrument is operating correctly. Unfortunately, the output from many types of sensors can vary over time and in some cases sensors can fail to operate correctly without warning. It is therefore desirable to re-calibrate the sensor periodically. The interval between calibrations will depend on the sensor technology and on the accuracy requirements of the application. For example, electrochemical gas sensors, which are widely used for toxic and oxygen gas detection in industrial work place safety applications, are typically re-calibrated monthly; while many infrared combustible gas sensors may only require calibration every six months or every year.
Calibration is often a time consuming process, but for critical applications such as safety monitoring, a more frequent calibration interval may be used than is employed for a less critical application. One method commonly employed to reduce the burden of calibration is to perform a so-called “bump test”, in which the instrument is exposed to a test gas of sufficiently high concentration to activate the warning alarms for a short period of time. If the instrument alarms are actuated, then the instrument is deemed to be working correctly. However, if the instrument alarms do not actuate, then the instrument requires servicing. While calibration of the instrument is usually performed with test gases with concentrations known to a high degree of accuracy, bump tests are often performed with a more economical test gas mixture whose concentrations are known to a lower degree of accuracy.
Test gases are commonly available in compressed gas cylinders. For everyday use, small hand-held, disposable gas cylinders are widely used. Unfortunately, the use of disposable gas cylinders is both expensive and cumbersome, due to the requirements of safely containing and using compressed gases.
Alternative methods of test gas generation have been developed. Electrochemical gas generators, such as those disclosed in U.S. Pat. No. 5,395,501, are available for several gases including hydrogen sulfide, chlorine, and chlorine dioxide. Electrochemical gas generators are obviously limited to those gases that can be produced by an electrochemical reaction. As for any electrochemical reaction, the amount of product produced is linearly proportional to the current passing, as described by Faraday's law, and the diluent gas flow can be readily controlled, electrochemical generators in principle can provide good control over the gas concentration. In practice, the effects of absorption, changes in electrolyte composition, competitive electrode reactions and errors in the gas flow control limit the accuracy of these devices.
Calibration methods have also been devised in which the test gas is periodically generated, under the control of a microprocessor or other controller within the gas detection instrument. This approach allows the instrument to perform a gas test on the instrument without the need for a human operator. For example, electrochemical gas generators are used by Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical Information Sheet, entitled
A world of gases . . . A single transmitter
) to provide test gas to automatically check the performance of gas detection instruments, and ensure that the sensors are responding within their specified limits.
Automatic calibration methods have also been described in the prior art. For example, U.S. Pat. Nos. 4,384,925, 4,151,738, 5,239,492 and 4,116,612 describe methods for automatic calibration of a gas detection instrument in which calibration gas is automatically applied to the sensors under the control of a microprocessor. However, in most of these examples, the source of the test gas is still a compressed gas cylinder.
Electrochemical gas generators have also been incorporated into gas detection instruments. See, for example U.S. Pat. No. 5,668,302 and PCT International application WO 98 25139 which describe the incorporation of an electrochemical gas generator into an electrochemical gas sensor. These electrochemical gas generators have been used for carbon monoxide sensors, though the test gas produced is hydrogen from the electrolysis of the aqueous sulfuric acid electrolyte. In this latter example, while the incorporation of the gas generator into the sensor has clear advantages, it would be desirable to test the sensor with the intended analyte gas, in this case carbon monoxide, instead of the surrogate gas, hydrogen. The electrochemical properties of hydrogen are very different from carbon monoxide, and the oxidation of the latter gas is highly dependent upon the catalytic nature of the electrode surface. As a result, a good response of the sensor to hydrogen does not guarantee that the sensor will perform equally well to carbon monoxide.
Permeation-tubes are another commonly used device for producing calibration gases. These devices typically contain a polymeric tube containing the liquefied gas in an air stream. As the internal gas concentration is constant and the external concentration is near zero, the diffusion rate of gas through the polymeric material will be constant for a constant temperature. While permeation tubes are widely used to provide laboratory test gases, their use in field calibrators is limited due to the requirement for very tight control (e.g. +/−0.1° C.) of the temperature for accurate
Cross LaToya
Eckert Seamans Cherin & Mellott , LLC
Industrial Scientific Corporation
Jenkins David C.
Silverman Arnold B.
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