--- title: Sensor poisoning — the failure your display won't show you description: A poisoned catalytic sensor still reads zero in clean air. How poisoning and inhibition happen, what causes them on site, and the only way to catch them. image: https://resources.jmsconsultants.co.uk/hubfs/8187f8ce-467a-43cb-9a77-01d0f8ba350e.png --- [Skip to content](https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you#main-content) [![](https://resources.jmsconsultants.co.uk/hs-fs/hubfs/JMS%20logo.png?width=368&height=63&name=JMS%20logo.png)](https://jmsconsultastg.wpengine.com/) Open main navigation Close main navigation - [View All Articles](https://resources.jmsconsultants.co.uk/news) [View All Articles](https://resources.jmsconsultants.co.uk/news) 19 Oct 2026, 10:30:01 # Sensor poisoning — the failure your display won't show you ![Picture of Karl Pearson](https://app.hubspot.com/settings/avatar/cc3c8f82dd1422ca87abee67ee363ff4) [Karl Pearson](https://resources.jmsconsultants.co.uk/news/author/karl-pearson) Share: [facebook-f icon](http://www.facebook.com/share.php?u=https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you) [linkedin-in icon](http://www.linkedin.com/shareArticle?mini=true&url=https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you) [Twitter icon](https://twitter.com/intent/tweet?url=https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you) [pinterest-p icon](http://pinterest.com/pin/create/link/?url=https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you) [envelope icon](mailto:?body=https://resources.jmsconsultants.co.uk/news/sensor-poisoning-the-failure-your-display-wont-show-you) A gas monitor that reads 0% LEL looks like a gas monitor that is working. With a poisoned catalytic sensor, that is exactly what you will see, right up to the moment it fails to see a flammable atmosphere. ### How a catalytic sensor works, and how it stops working The catalytic sensor (the pellistor) remains the most widely used flammable gas sensor in the world. It works by burning gas. A heated bead coated with catalyst oxidises any flammable gas that reaches it. The reaction releases heat, the bead's resistance changes, and the instrument converts that change into a reading. No combustion on the bead, no reading. Poisoning attacks the catalyst. Certain compounds coat it or react with it, and the active surface shrinks. Less surface means less oxidation, which means less heat, which means a lower reading for the same gas. [![  Configure your onsite training course now    ](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/25825845/interactive-72860954069.png)](https://resources.jmsconsultants.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLLaA3cjMos7VW8mye%2FS1lYZrHMnZdbbNuDsx8M2QXPRbPw632H5I7BeOyav2SGwXyqmArFADhjS1f%2Bta80E66Wdmw7flbGgU%2FVli8kabDOyUzfrwRQliawBlT%2FthLsqdg%2BLmzlywguFgLcduCg%2FO6AtSUZkXduJk02UfNZZBTcXdZrgkedJit7U5I4helGn8feKT7JOptnoQYgvKguwTMAIe3uIC4aY67VTFs03N8U%2Bcw%3D%3D&webInteractiveContentId=72860954069&portalId=25825845&hsLang=en-gb) ### Why the display won't tell you This is the dangerous part. In clean air there is no gas to oxidise, so a healthy sensor reads zero and a poisoned sensor reads zero. The zero point is unaffected. Only the sensitivity has gone. You cannot see poisoning by looking at the instrument. A sensor that has lost half its sensitivity looks identical to a new one until gas is applied. Then it reads half what it should. Or, if it is badly poisoned, nothing. ### Poisoned or inhibited The two terms get used interchangeably. They are not the same. - **Poisoning is permanent.** The catalyst is damaged and the lost sensitivity does not come back. Silicones, lead compounds and phosphate esters are the classic poisons. - **Inhibition is temporary.** The catalyst is suppressed while the compound is present and recovers, partly or fully, after time in clean air. Halogenated compounds such as chlorinated solvents and some refrigerants are typical inhibitors. Sulphur compounds, including hydrogen sulphide, can do either depending on concentration and exposure. The distinction matters because an inhibited sensor may recover overnight, while a poisoned one needs replacing. Either way, a reading taken while affected is wrong. [![Configure your on-site course now!](https://hubspot-no-cache-eu1-prod.s3.amazonaws.com/cta/default/25825845/interactive-73591523282.png)](https://resources.jmsconsultants.co.uk/hs/cta/wi/redirect?encryptedPayload=AVxigLKwy4UpN6iMbmWlrMyM%2BSW3I1L%2FP0LELyqwR4MoaBU9urbo1O93E7Rv1mM2qo0K76Oa%2FIDH7dQ%2FbgHw5H4pUqbWN2NZoVzrD87dv6Nq3AaAJ9Tu7%2Fen8RAXxs77DgHHRrAZ%2FapjKz4qgQ1JURhL6P8nFPvYSripbnW0rVVYffhGsXmKE8J70rwqf5y%2ByDDtEHAePm6jo%2Fz5p63nPgaqyZQ%2BlGT6DZ%2FiZC%2BkF9XOhCuLEg%3D%3D&webInteractiveContentId=73591523282&portalId=25825845&hsLang=en-gb) ### Where it comes from on site The poisons are rarely exotic. Silicone is the most common, and it's everywhere: - sealants, gasket compounds and greases - release agents and lubricant sprays - some cleaning products and polishes - some hand creams and barrier creams - siloxanes in biogas at wastewater treatment works Lead residues turn up in older fuel tanks and vessels. Phosphate esters are found in fire-resistant hydraulic fluids. Chlorinated solvents are used in degreasing. A monitor clipped to someone sealing a joint, or left on a bench beside a can of spray lubricant, can be poisoned without ever entering a confined space. ### The only defence is gas Because poisoning is invisible in clean air, the only way to detect it is to expose the sensor to a known concentration of gas and check the response. That is what a bump test does. It is why manufacturers and BS EN 60079-29-2 call for a functional check before use, and why a sensor that has been near a known poison should be checked before it is trusted again. Poison-resistant catalytic sensors slow the process down. They do not stop it. Switching to infrared avoids catalytic poisoning entirely, but IR cannot detect hydrogen and some other flammable gases. Every sensing technology trades one limitation for another. ### The competence gap Most operatives can tell you their monitor should be bump tested. Far fewer can tell you why. And fewer still know that the silicone sealant in their own toolbox can quietly disable the sensor they're relying on. Understanding the catalytic sensor is in our name. So is understanding how it fails. JMS GDU and Gas Monitor courses teach the people who wear monitors what their sensors can and cannot do. The [JMS competency assessment](https://jmsconsultants.co.uk/competency-assessment/) shows you where the gaps are across your team. 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