Gas Sensor
Gas sensing is a critical component of monitoring safety and process performance in a multitude of applications ranging from building automation and control, oil and gas process industries, food safety, portable toxic gas detectors(coal mining) etc.
Gases can be detected by many different sense mechanisms. One such class of sensors is based on electrochemical sensing. Such gas sensors are called electrochemical or amperometric gas sensors. An electrochemical gas sensor produces a linear current output proportionate to the amount of gas concentration it is exposed to.
The Electrochemical Cell
The Electrochemical cell shown below is a 3 lead electrochemical cell. It consists of three electrodes stacked parallel to each other and separated by an electrolyte that provides ionic electrical contact between the electrodes.
WE (Working electrode): Gas contacting WE causes oxidation or reduction. This reaction generates a current proportional to gas concentration. (Over time the performance of the WE can deteriorate or change introducing error)
RE (Reference electrode) maintains a constant potential. It is immersed in the electrolyte and does not come in contact with the gas. (Presence of RE allows potentiostatic circuit to correct for deterioration of WE)
CE (Counter electrode) conducts current into or out of the cell. The CE current exactly balances the current generated at the WE.
A 2 electrode electrochemical cell has only the WE and the CE and is used more commonly in residential applications.
Enlarge
Typical 3 electrode electrochemical cell
Potentiostatic circuit
- The potentiostat measures the current flow between the WE (Working electrode) and CE (Counter electrodes)
- The potentiostat maintains the WE at a constant potential with respect to the RE (Reference electrode) by adjusting the current in the CE.
- The current flowing in the CE is linearly proportional to gas concentration.
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