A method by which an oxygen measuring instrument can test the
functionality of the oxygen sensor. Oxygen sensors of the galvanic type
operate by consumption of an internal easily oxidizable anode, such as
lead or cadmium. Failure of the sensor due to complete consumption of
all of the anode material, such that the oxygen sensor is no longer able
to detect oxygen, is often rapid with little warning. This invention
describes an electrical test, which may be performed in-situ on the
oxygen sensor by the instrument, which provides a means for detection of
an imminent failure, while the sensor is still operational.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of
electrochemical gas sensors having
a consumable electrode, and particularly to a method for testing an
operational sensor to determine if it is near to its end of useful life.
2. Description of Related Art
Over the last thirty years, instruments have become available for
monitoring workplace atmospheres for hazardous gases. Atmospheres may be
hazardous because of the presence of toxic gases, or combustible
because of a deficiency or excess of oxygen. These gas detection
instruments typically contain a gas sensor producing an electrical
output signal which varies as a function of the gas concentration, and
electronics to drive the sensor and to amplify and manipulate the output
signal to give an auditory or visual warning or both in the event of a
potentially dangerous atmosphere. Many of the present day instruments
have digital displays and give a continuous output showing the gas
concentrations of interest and often incorporate microprocessor
controls, thus allowing more advanced features such as data logging,
calculation of time weighted average exposures.
The concentration of oxygen is especially important, since if the
concentration falls significantly below normal atmosphere (21% v/v at 1
atm. pressure) then insufficient oxygen will be absorbed by the blood in
the lungs, resulting in decreasing oxygen concentration and impairment
of judgement, nausea, vomiting, inability to move freely or cry out, and
eventually convulsive movements and death (L. R. Cooper, Oxygen
Deficiency in Detection and Measurement of Hazardous Gases, Ed. C. F.
Cullis, J. G. Firth, Heinemann, London, 1981). If the oxygen
concentration is too high, then the combustion of many flammable
materials is facilitated, which also presents a possible hazard. Oxygen
detecting instruments often have both an upper and lower concentration
alarm level, typically at about 25 and 19% volume respectively.
The most common type of oxygen sensor used in instruments for monitoring
workplace safety is an electrochemical 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, Talanta,
Amperometric Gas Sensors (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).)
Amperometric electrochemical sensors contain at least two electrodes in
contact with an electrolyte. Oxygen diffuses into the sensor through a
diffusion barrier to one of the electrodes, known as the cathode. The
electrons required for the reduction of the oxygen flow through the
external circuit from the anode, where an equal magnitude oxidation
reaction occurs. This flow of electrons constitutes an electric current,
which provides the output signal. The potential of the cathode is
selected such that all the oxygen which reaches the cathode is
electrochemically reduced. This potential may be established by
application of an external potential, thus operating the sensor in
so-called polarographic mode, or by use of an anode material which is
sufficiently electronegative in is the electrochemical series, such as
lead or cadmium. A sensor of this latter type is known as a galvanic
oxygen sensor, examples of which have been described, for example, in
Lawson, U.S. Pat. No. 4,085,024, Tantram et al, U.S. Pat. Nos. 4,132,616
and 4,324,632, Culliname in U.S. Pat. No. 4,446,000, Bone et al, U.S.
Pat. No. 4,810,352 and Fujita et al, U.S. Pat. No. 4,495,051.
A polarographic sensor requires an external circuit to control the
potential of the sensor electrodes at a fixed value, whereas the
galvanic sensor can be operated by simply placing a load resistor
between the two electrodes and measuring the potential difference across
this resistor, which is proportional to the current flowing through the
resistor. Galvanic sensors may also be operated with a potentiostat
circuit, which fixes the potential between the two electrodes. For most
galvanic sensors operated in this mode, the applied potential will be
zero, but other potentials may also be used.
Oxygen sensors are well known in the prior art, and polarographic and
galvanic sensors have both been widely used for measuring the oxygen
concentration in both gases, especially air, and in liquids (M. L.
Hitchman, Measurement of Dissolved Oxygen, John Wiley & Sons, N.Y.
1978; I. Fatt, Polarographic Oxygen Sensors, Its theory of Operation and
its Application in Biology, Medicine and Technology, Robert E. Krieger
Publishing Company, Malabar, Fla. 1982).
In a typical galvanic sensor, the flow of electrons from the anode is
generated by the oxidation of the anode material. For a lead anode, the
reaction is believed to be oxidation of the lead to form lead oxide
(PbO). The rate of oxidation depends on the amount of oxygen being
reduced, which in turn depends on the rate of diffusion of oxygen into
the sensor through a diffusion barrier. Since the rate of diffusion
depends on the concentration of the oxygen outside the sensor, external
oxygen concentration.
Since the anode is consumed in a galvanic sensor during the detection
process, the sensor has a finite lifetime. Once all of the anode
material has been consumed, the sensor will no longer detect oxygen. The
output current of a working sensor is limited by the rate of diffusion
of the oxygen into the sensor via the diffusion barrier and so the
output current is independent of the state of the anode. Once the anode
is consumed, then the sensor will fail and this failure often occurs
rapidly, with little or no warning. When the sensor fails, the output
current decreases. However, a fall in output current can be due either
to a failed sensor or to the gas detection instrument being in an
environment with a reduced oxygen concentration. Thus, there may be
confusion about whether the sensor has failed or the oxygen
concentration has decreased; this confusion is at the least very
annoying and potentially dangerous.
Therefore, a method is needed to predict when a sensor will fail, before
it actually does, so that a warning can be provided to the user in
advance. Early warning of imminent sensor failure will allow the sensor
to be replaced before it fails.
The ability to determine whether the sensor is working correctly, or to
predict imminent failure is an important advantage for an instrument
used for safety applications. These various problems outlined above have
been addressed in the prior art to various levels of satisfaction. The
most common method of ensuring that gas sensors are working correctly is
frequent and periodic calibration.
Calibration is usually performed manually, by the application of
calibration gases of known composition, or by exposure of the gas
detection instrument to clean air. Automatic calibration methods have
been described in the prior art, for example, Stetter et al in U.S. Pat.
No. 4,384,925, Hyer and Roberts in U.S. Pat. No. 4,151,738, Hartwig and
Habibi 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 gas are automatically applied to the
sensors under the microprocessor control.
Calibration methods have also been devised in which the test gas is
generated as needed, such as the electrochemical gas generators used by
Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical
Information Sheet, titled 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. Finbow et al. in U.S. Pat. No. 5,668,302
discloses incorporating an electrochemical gas generator within an
electrochemical gas sensor, behind the diffusion barrier, to provide a
means for automatic function testing of the gas detection instrument.
Other methods have been devised which can achieve calibration without
prior knowledge of the gas concentration, based on application of
Faraday's law of electrolysis to a known volume of gas, described by
Tantram and Gilbey in U.S. Pat. No. 4,829,809 and by Matthiesen in U.S.
Pat. No. 4,833,909; these methods do not require a known test gas
concentration.
Calibration is a very important process in gas detection, but does not
provide any warning of imminent failure of a galvanic oxygen sensor.
Since the failure can occur rapidly, the sensor can be successfully
calibrated, only to fail a short time thereafter. Clearly a better
method of determining the status of the galvanic sensor is required.
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