Custom Sensor Solutions, Inc.

A Miniglossary of Gas Sensor Terms

Like any other discipline, gas sensors has its own vocabulary. More confusing, we sometimes use familiar terms in an unusual way. Still more confusing, different people in the gas sensor trade may use the terms differently. What follows is our interpretation of the special vocabulary of gas sensors:

Analyte - The "analyte" is the gas that you are setting out to measure.

Interference - These are gases that interfere with the measurement of the analyte gas. On certain reactive sensors, for example, ethanol and formaldehyde are interferences in the measurement of carbon monoxide. There are at least five types of interference: (a) positive interference, in which the interfering gas adds to the signal, giving a high analyte measurement; (b) negative interference, where the sensor response to the interference generates a signal that opposes the analyte signal; (c) reactive interference, where the interfering gas reacts chemically with the analyte in the gas phase, either reducing or enhancing the analyte response; (d) activation, in which the sensor is made more sensitive to the analyte, even after the interfering gas has gone; and (e) poisoning, where the response of the sensor to the analyte is permanently or temporarily reduced by exposure to the interfering gas.

Sample - The gas being tested or analyzed for the presence of the analyte. It may also contain interferences.

Standard - An artificial sample containing a known amount of an analyte gas, used for calibrating gas analyzers or conducting experiments at a known concentration.

Baseline - The electrical signal from a sensor when no analyte is present, but under otherwise "standard" conditions.

Zero - When used as an adjective, zero refers to a condition where the sensor is exposed to neither analyte nor interferences. It is a relative term; for example, 'zero air' usually refers to air that is clean enough that it does not give a response under the conditions of your measurement. A 'zero filter' is a chemical reactor, or absorber, that removes analyte and interfering gases from ambient air. It may be used to set the baseline of a sensor.

Selectivity - A selective sensor will respond only to the analyte gas. It will not respond to other gases that it is likely to encounter. Selectivity is therefore a situational term. A carbon monoxide sensor may be capable of responding to formaldehyde, but it may be considered selective if it is there is no likelihood of encountering formaldehyde in the expected application. Gas sensors have an inherent selectivity (the range of gases to which they will respond), but this may be enhanced by auxiliary devices that eliminate or avoid interfering gases.

Response time - This is the time required for a sensor to respond to a step increase in the analyte gas. Since most sensors approach the final signal asymptotically, response time is usually defined as the time needed to reach 90% of the final signal (for a given concentration of gas).

Recovery time - The time required for a sensor to return to baseline after a response to an analyte. It is usually expressed as the time for the signal to fall below 10% of the maximum response.

Specific Response (or Sensitivity) - The slope of the signal vs. analyte concentration curve, as measured, for example, in microamps per part per million (uA/PPM).

Sensitivity Limit - The lowest concentration of analyte gas that can be unambiguously detected. This is often chosen to be three times the sensor noise.

Noise - This is the variation in sensor signal over short periods of time (less than 10 seconds) due to causes other than changes in the analyte gas. Noise can originate from the nature of the chemical reaction, the electronics, external electromagnetic fields, or unknown sources.

Drift - This is the change in the signal over longer periods of time than those used to estimate noise. Baseline drift is a gradual change in the baseline signal from the sensor, and span drift (or sensitivity drift) is a change in the specific response to the analyte. In most sensors, baseline drift is a more serious problem than span drift, and limits the sensitivity of long-term measurements.

VOC - (Volatile organic compounds.) These generally refer to vapors of organic solvents such as methanol, hexane, chloroform, and other compounds that have sufficient vapor pressure to reach significant concentrations in an atmosphere. In other words, the term is not very tightly-defined. Many definitions of VOC boil down to the response of a given type of sensor to whatever volatile organics it happens to encounter. For example, a flame ionization detector and a photoionization detector will give different responses to the same sample of VOCs.

Part Per Million (PPM or ppm) - An expression of gas concentration, meaning the number of volumes of a gas per one million volumes of sample. Example: 100 PPM of carbon monoxide in air means 100 microliters, or 0.1 milliliter, of carbon monoxide in every liter of air. Analogous units are used for PPT (parts per trillion, 10¹²), PPB (parts per billion, 10⁹), or percent (parts per hundred). To avoid confusion with parts per trillion, PPT is rarely used to mean parts per thousand. A unit sometimes used in air pollution measurement is PPHT (parts per hundred thousand). It is worth noting that, because we are dealing with gases, these units are proportional to molar concentrations.

Current March 22, 1998

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