The Right Sensor Technology for the Job

How do you know that your gas detector is the right tool for the job? The different sensors found inside gas detection instruments can use several different technologies. Understanding the basics of these technologies can help users to make sure they are getting the best performance. Most importantly it’s vital to know the limitations of each technology so that personal safety is not compromised. In this article Gordon Haddow, Technical Specialist at e2v, looks at four of the main technologies used in gas sensing: Electrochemical Cells, Pellistors, Infrared, and Metal Oxide Semiconductors. The applications of each are highlighted together with some of the pitfalls that users need to look out for.

Electrochemical Cell Sensors
Since the 1970s, electrochemical cells have been used for the detection of toxic gases such as carbon monoxide, nitrogen dioxide, chlorine and many others. In these devices the gases diffuse into the sensor and undertake a chemical reaction to generate an electrical current proportional to the concentration of target gas. Electrochemical cells use very little power which makes them easy to incorporate into intrinsically safe instruments and also helps to provide long battery life. However, for certain gases, electrochemical cells require a constant voltage bias to be applied which is needed even when the instrument is turned off otherwise there will be a significant stabilisation time whenever the power is re-applied. This often means that an instrument is never fully off and will always draw a small current from the battery.

Electrochemical cells are basically chemical devices and in order for the chemical reaction to occur they must operate within the right temperature range and also with sufficient oxygen concentration. Their use is normally
restricted to temperatures up to about 50°C beyond which they start to dry out as water evaporates from the electrolyte. They will also not function correctly in an area or system with very low oxygen levels as oxygen is required as part of the chemical reaction.

Another issue to be careful with is cross sensitivity. Users need to be aware that many electrochemical sensors respond to more than just the ‘headline’ gas type. For example a
carbon monoxide sensor would typically have a noticable response to hydrogen. Careful reading of the datasheets and application notes is needed to see what other gases the chosen
sensor might respond to and whether this will be suitable for the application.

A lifetime of only three years is common with lead based electrochemical O2 sensors as the internal chemistry becomes depleted, just like
in an alkaline or zinc-carbon primary cell. However e2v recently introduced to its range a new electrochemical technology for oxygen detection which overcomes this problem. A much longer lifetime is possible because it is not limited by the sacrificial mass of a lead anode which is typically used in these devices. Also by removing the lead content, this technology is now compliant with the updated ‘RoHS2’ directive (Restriction of Hazardous Substances) which will soon apply to industrial gas sensing.