Elimination of Sensor Maintenance By Sensor Interrogation and Correction
Authors: Scheffler Towner 
Published: February 2020

Amperometric electrochemical gas sensors are virtually ubiquitous and well accepted in industrial hygiene and personal safety applications. This sensor type, used initially for detection of carbon monoxide (CO) and oxygen (O2), first appeared in the 1970s. Since that time, sensors were developed to detect many other gases of interest, including hydrogen sulfide (H2S), sulfur dioxide (SO2) and chlorine (Cl2), just to name a few.

Electrochemical sensors are highly accurate, have good reproducibility and linearity and are long-lived. That said, best industry practice dictates that gas detection instruments be tested regularly (every day that the instrument is used) to ensure proper functionality. Testing can take several forms, including full calibration using the test gas of interest or a simulant gas, or a calibration check or bump test. Each test requires considerable monetary investment, including a test gas source (usually a compressed gas cylinder) and delivery system that typically includes a pressure regulator and tubing. It is quite common at large industrial installations that all personnel are equipped with personal gas detectors. Clearly, providing test gas and delivery systems is a significant investment in equipment, consumables and non-productive time.

This paper discusses a new gas-free bump test and correction system that eliminates the need for test gas and delivery systems, while providing on-demand exercising of all sensor and instrument functions, real time sensor signal correction for sensor aging and response to environmental conditions.[1],[2] This new sensor technology is based upon a modification dual channel amperometric gas sensor. Dual channel amperometric gas sensors have two separate working electrodes include one that is designed for carbon monoxide detection and the other intended for hydrogen sulfide detection. These two independent electrodes share a common reference electrode1,2 (RE) and counter1,2 electrode (CE). In the present application, bias potentials applied to the two working electrodes are controlled by an on-board Application Specific Integrated Circuit (ASIC). This ASIC chip also amplifies and measures currents that flow between the two working electrodes and common counter electrode.

The present technology, first introduces as the H2S Pulse Sensor, also has two separate working electrodes, one to sense hydrogen sulfide, (as with a two-tox sensor) and a second electrode that responds to oxygen. Pulse Technology functions in the following manner: first, a small potential pulse is applied to the H2S working electrode; this potential pulse causes electrical current to flow between the H2S working electrode and the common counter electrode. This current is measured and analyzed by the ASIC. The current that flows within the sensor can be used to monitor or measure several items:

Is the sensor present?

Is the sensor conducting current?

Has sensor sensitivity changed from the last pulse test?

Has sensor sensitivity sensor changed enough to require re-calibration (an actual re-calibration using test gas), or

Can sensor output be corrected slightly in order to maintain maximum sensor accuracy?

The electrical pulse test can also determine if the sensor’s working electrode was poisoned or inhibited by an interferant gas.

Immediately following the pulse test, the second oxygen-responsive working electrode is activated by the ASIC and allowed to briefly equilibrate. Once appropriate current is established for oxygen reduction in ambient air, users are prompted to exhale into the sensor inlet. Exhaled breath contains approximately 15 vol-% oxygen as compared to ambient air that contains 20.8 vol-% oxygen. Decrease in oxygen due to exhaled breath pulse supplied by users causes decrease in current that flows between the oxygen-responsive working electrode and the common counter electrode. This current change is also measured and analyzed by the ASIC. The change in current due to exhaled breath is used to determine if gaseous flow path into the sensor is free of obstruction as compared to the last successful pulse test. The exhaled breath pulse test can discriminate between open, partially-blocked and fully-blocked flow paths.

Pulse Technology combines these two independent sensor function tests to provide a convenient and highly accurate and effective substitute for bump testing using calibration gas. Extensive testing of Pulse Technology indicates greater than 98% accuracy. Pulse Technology effectively eliminates daily bump testing using calibration gas and extends the interval between required full calibrations.

[1]Scheffler, T. B., Devices, systems and methods for testing gas sensors and correcting gas sensor output, U.S. Patent 7,413,645 B2, August 19, 2008.

[2]Scheffler, T. B., Devices, systems and methods for testing gas sensors and correcting gas sensor output, U. S. Patent 7,959,777 B2, June 14, 2011.

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