Electrochemical gas sensors are gas detectors that measure the volume of a target gas by oxidizing or reducing the target gas at an electrode and measuring the resulting current. A gas detector is a device which detects the presence of various gases within an area usually as part of a system to warn about gases which might be harmful to Humans This page is about the physical properties of gas as a state of matter Redox (shorthand for reduction-oxidation reaction describes all Chemical reactions in which atoms have their Oxidation number ( Oxidation state An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e

Contents 1 Construction 2 Theory of operation 3 Diffusion controlled response 4 Cross sensitivity 5 See also 6 References

Construction

The sensors contain two or three electrodes, occasionally four, in contact with an electrolyte. An electrolyte is any substance containing free Ions that behaves as an electrically conductive medium The electrodes are typically fabricated by fixing a high surface area precious metal on to the porous hydrophobic membrane. In Chemistry, hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of The working electrode contacts both the electrolyte and the ambient air to be monitored usually via a porous membrane. The electrolyte most commonly used a mineral acid, but organic electrolytes are also used for some sensors. A mineral acid is an Acid derived by Chemical reaction from inorganic Minerals as opposed to Organic acids These have Hydrogen The electrodes and housing are usually in a plastic housing which contains a gas entry hole for the gas and electrical contacts.

Theory of operation

The gas diffuses into the sensor, through the back of the porous membrane to the working electrode where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. Electrochemistry is a branch of Chemistry that studies Chemical reactions which take place in a Solution at the interface of an electron conductor In addition to measuring, amplifying and performing other signal processing functions, the external circuit maintains the voltage across the sensor between the working and counter electrodes for a two electrode sensor or between the working and reference electrodes for a three electrode cell. At the counter electrode and equal an opposite reaction occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.

Diffusion controlled response

The magnitude of the current is controlled by how much of the target gas is oxidized at the working electrode. Sensors are usually designed so that the gas supply is limited by diffusion and thus the output from the sensor is linearly proportional to the gas concentration. The word linear comes from the Latin word linearis, which means created by lines. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance This linear output is one of the advantages of electrochemical sensors over other sensor technologies, (e. g. infrared), whose output must be linearized before they can be used. A linear output allows for more precise measurement of low concentrations and much simpler calibration (only baseline and one point are needed).

Diffusion control offers another advantage. Changing the diffusion barrier allows the sensor manufacturer to tailor the sensor to a particular target gas concentration range. In addition, since the diffusion barrier is primarily mechanical, the calibration of electrochemical sensors tends to be more stable over time and so electrochemical sensor based instruments require much less maintenance than some other detection technologies. In principle, the sensitivity can be calculated based on the diffusion properties of the gas path into the sensor, though experimental errors in the measurement of the diffusion properties make the calculation is less accurate than calibrating with test gas. ^[1]

Cross sensitivity

For some gases such as ethylene oxide, cross sensitivity can be a problem because ethylene oxide requires a very active working electrode catalyst and high operating potential for its oxidation. Catalysis is the process in which the rate of a Chemical reaction is increased by means of a Chemical substance known as a catalyst Therefore gases which are more easily oxidized such as alcohols and carbon monoxide will also give a response. In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon Carbon monoxide, with the chemical formula CO is a colorless odorless tasteless yet highly toxic Gas. Cross sensitivity problems can be eliminated though through the use of a chemical filter, for example filters that allows the target gas to pass through unimpeded, but which reacts with and removes common interferences.

While electrochemical sensors offer many advantages, they are not suitable for every gas. Since the detection mechanism involves the oxidation or reduction of the gas, electrochemical sensors are usually only suitable for gases which are electrochemically active, though it is possible to detect electrochemically inert gases indirectly if the gas interacts with another species in the sensor that then produces a response. ^[2] Sensors for carbon dioxide are an example of this approach and they have been commercially available for several years.

See also

• Carbon monoxide detector

References

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