Magazine article Occupational Hazards

Using Electrochemical Sensors for Toxic Gas Measurement: These Sensors Are Widely Used in Industry, but Often How They Work and What Limitations They Have Are Misunderstood. This Article Clears Up Some of the Confusion

Magazine article Occupational Hazards

Using Electrochemical Sensors for Toxic Gas Measurement: These Sensors Are Widely Used in Industry, but Often How They Work and What Limitations They Have Are Misunderstood. This Article Clears Up Some of the Confusion

Article excerpt

Electrochemical sensors are one of the most common types of sensors used in portable gas detectors. Multi-sensor confined space monitors generally contain an oxygen sensor, a flammable/combustible sensor and one to three additional electrochemical sensors for specific toxic gases. Single-sensor instruments equipped with electrochemical toxic sensors are also extremely popular for use in situations where a single toxic hazard is present. In spite of the large number of electrochemical toxic sensors in use, there still is a lot of misinformation and misunderstanding when it comes to the performance characteristics and limitations of this important type of sensor.

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HOW ELECTROCHEMICAL SENSORS DETECT GAS

Substance-specific electrochemical sensors are available for many of the most common toxic gases, including hydrogen sulfide, carbon monoxide, sulfur dioxide, chlorine, chlorine dioxide, ammonia, phosphine, ethylene oxide, nitrogen dioxide, ozone and others. "EC" sensors are compact, require very little power, exhibit excellent linearity and repeatability, and generally have a long life span. The detection technique is very straightforward in concept. Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present. EC sensors usually are designed to minimize the effects of interfering contaminants, making the readings as specific as possible for the gas being measured.

Figure 1 illustrates the major components included in a typical electrochemical sensor. The gas enters the sensor through an external diffusion barrier that is porous to gas but nonporous to liquid. Many sensor designs include a capillary diffusion barrier that limits and controls the amount of gas that enters the sensor. The sensing electrode is designed to catalyze a specific detection reaction. Depending on the sensor, the substance being measured is either oxidized or reduced at the surface of the sensing electrode. This reaction causes the potential of the sensing electrode to rise or fall relative to that of the counter electrode. Current collector wires or filaments connect the electrodes with the external pins of the sensor. The instrument supplies power to the sensor, and interprets the output of the sensor by readings obtained through the external pins.

Electrochemical sensors are stable and long-lasting, require very little power and are capable of resolution (depending on the sensor and contaminant) to [+ or -] 0.1 ppm or even lower. Electrochemical sensors are normally usable over a wide range of temperatures, in some cases from minus 40 to 50 degrees C (minus 40 to 120 degrees F). However, the uncorrected sensor output may be strongly influenced by changes in temperature. For this reason, instruments generally include temperature compensating software and/or hardware for the EC sensors installed.

The simplest sensor designs use a two-electrode system. In two-electrode designs, the potential of the sensing electrode is compared directly to that of the counter electrode. In three electrode designs, what actually is measured is the difference between the sensing electrode and reference electrode. Since the reference electrode is shielded from any reaction, it maintains a constant potential. This provides a true point of comparison. The change in potential of the sensing electrode is due solely to the concentration of gas. The current generated by the sensor is proportional to the amount of gas present. The amount of current generated per ppm (parts-per-million) of gas is constant over a wide concentration range. This consistency in output over a wide range explains the exceptional linearity of three-electrode electrochemical sensors.

WHY [H.sub.2]S SENSORS DON'T WEAR OUT EVEN WHEN EXPOSED TO HIGH CONCENTRATIONS OF GAS

Chemical equations can be a little daunting, but working through a typical detection reaction is well worth the effort. …

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