What is an electrochemical gas sensor?

What is an electrochemical gas sensor?

Electrochemical gas sensors, also known as electrochemical gas analyzers or electrochemical gas detectors, are devices used to detect the concentration of a specific gas like Carbon monoxide or oxygen in the air or an external circuit, mostly for safety reasons.

This article will go over the basic working of an electrochemical gas sensor giving you all the information you need to know about it as simply as possible.

Major components

All electrochemical sensors have a few components common to them, including

i. A sensing counter

ii. Reference electrode

iii. Gas-permeable membrane

iv. Electrolytic liquid

The sensing counter and reference electrode are contained in the same housing, which has a gas-permeable membrane. When it is working (sensing), it is submerged in an electrolytic fluid. All of these components together function as a unit and perform as an electrochemical sensor.

Working

The sensor works on the principle of diffusion. The housing membrane of the sensor allows the gas to pass through but stops the liquid. An electrochemical reaction, i.e., oxidation or reduction occurs when the gas reaches the electrode, the type of reaction depends on the type of gas involved. This chemical reaction initiates a flow of electrons, also called current, between the working and counter electrodes (some sensors have four electrodes).

An oxidation reaction is when a flow of electrons happens from the working to the counter electrode. A reduction reaction is opposite to this, causing the electrodes to move from counter to working electrode. The generated current is directly proportional to the concentration of the gas being detected. Then the calibrator that is installed in the sensor is used to amplify the current. The amplified current is then processed to give the user a reading in volume percentage or PPM (Parts per Million).

Cross sensitivity

Most electrochemical sensors are designed to detect a specific gas, but they show some degree of cross-sensitivity. This means that the non-target gas also produces the same or similar degree of the result by the sensor. It mainly happens because of the similar but higher degree of chemical reactivity by the non-target gas compared to the target gas. Filters are used to reduce this phenomenon to avoid getting false detections by the sensor.

Temperature

Temperature plays an important role in the sensor's working; temperature variations can affect the sensing capability of the sensor. The manufacturers tend to provide some temperature compensation using different calibrations against a wide temperature range.

Variety

Electrochemical sensors can detect a wide range of gases, but their main purpose is to detect harmful gases such as,

i. Oxygen

ii. Carbon monoxide

iii. Nitrogen dioxide

iv. Sulphur dioxide

Electrochemical sensors offer many advantages, but their main drawback is that they can not be used to detect every gas. For the mechanism of an electrochemical sensor to work, the gas that it is designed to detect has to be electrochemically active. The inert gases can be detected If they react with another species in the sensor that produces a response that a sensor can detect. Carbon dioxide electrochemical sensors work on the same principle.

Although a lot more goes into the chemical side of working with an electrochemical sensor, this is all you need to know unless you're a student of electro-chemistry or a nerd.