What You Need to Know about Sensor Poisons and Inhibitors

What You Need to Know about Sensor Poisons and Inhibitors

Ryan Thompson | Tuesday, November 5, 2019

The sensors inside your personal gas detector are meant to be exposed to toxic, corrosive, and explosive gases, but that doesn’t mean they are infallible. Chemicals and vapors from everyday cleaners and lubricants and other specialized chemicals can all act as sensor poisons or inhibitors to different sensor types.

The catalytic bead LEL sensor is one of the most susceptible to poisons and inhibitors. Catalytic bead LEL sensors are standard in many applications, so knowing what can act as a poison or inhibitor can be the difference between a fully-functioning monitor and one that might not alert you to the presence of a combustible gas.

The way catalytic bead LEL sensors detect gas is one of the main reasons they’re so susceptible to poisons and inhibitors. When a gas contacts the bead inside a catalytic bead sensor, the gas burns and the bead detects a change in temperature. This change in temperature is then translated into the reading you see on the screen of your gas monitor. When poisons and inhibitors come in contact with this bead, they can prevent the gas from burning, or may prevent it from burning as much as it would have, which can influence the readout on your gas monitor.

What are sensor poisons?

Sensor poisons are substances that adhere to a catalytic bead sensor and cause permanent damage that prevents it from functioning properly. Some of the most common poisons are silicone-based products (which can include common items like lotions and hair products), lubricants, gas additives, lead, and sulfur compounds. If any of these poisons come in contact with the heated catalytic bead, they instantly melt and adhere to the surface. Since this sensor now has a layer of the melted poison substance surrounding it, it is no longer able to burn and therefore detect any combustible gas hazards. Once these poisons encapsulate the catalytic bead, there is no way to remove them and the sensor must be replaced.

Some of the most common poisons are silicone-
based products, lubricants, gas additives, lead,
and sulfur compounds.

How do I know if a sensor is poisoned?

It’s important to regularly calibrate and bump test gas detectors to determine if the sensors are functioning properly. If you don’t regularly bump test or calibrate your gas detectors, you could find yourself in a dangerous situation. You could enter combustible environment, but not see any reading on your gas detector because the bead has been poisoned.

If there is any chance that the LEL sensor may have been poisoned, calibrate it immediately to make sure it is working properly. If the sensor can read the calibration gas and provide a readout during this process, then it will be able to detect gas in the field. If the sensor is poisoned, it will not be able to read the calibration gas applied and will show a failed calibration.

What are sensor inhibitors?

Inhibitors, while not as destructive as poisons, still have a negative effect on the sensor. Inhibitors can desensitize the bead and may shorten its life span, but after a calibration and bump test, the sensor may still be usable. Some of the most common inhibitors are halogenated compounds as well as anything containing astatine, bromine, fluorine, chlorine, and iodine. If combustible gases and inhibitors are present at the same time, the catalytic bead sensor may not detect the combustible gas.

What should I do if a sensor has been exposed to inhibitors?

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If an instrument has been exposed to any inhibitors, calibrate it to be sure the instrument is still working. When the catalytic bead burns the gas from the calibration cylinder, it will also burn off some of the inhibiting material that may have been attached. If the bead is repeatedly desensitized by inhibitors, or comes in contact with a large quantity, it may not recover, resulting in a failed calibration.

In some cases, it may be obvious that the sensor was exposed to a poison or inhibitor—but not always. Many lubricants or silicone-based products come in spray form and can bounce off the surface they are applied to. If you are standing nearby with a gas detector, these products can easily come in contact with your personal gas detector. Many of these products also take some time to dry and release chemicals and vapors in the process. You must also consider storage. Just because a poison or inhibitor wasn’t applied directly to the instrument doesn’t mean that it may not have been exposed to something while in a tool box, storage locker, or anywhere else gas detection equipment may be stored.

The most common poisons and inhibitors are listed here, but there are more. Your best bet is to start each day with a calibration or bump test and calibrate your instrument any time you think it has come in contact with a potential poison or inhibitor.

How can I reduce the impact of poisons and inhibitors?

Catalytic bead LEL sensors are particularly susceptible to poisons and inhibitors, but other sensors are less so. Infrared sensors (IR) are not influenced by poisons or inhibitors, so in certain cases, switching to an IR sensor could simplify your gas detection. Hydrocarbon infrared (HC IR) sensors can detect a broad range of hydrocarbons and can be used in inert atmospheres, unlike catalytic bead LEL sensors. HC IR sensors are often used in confined space applications, areas where silicone particles are common (oil and gas), chemical refining, and any application with low oxygen levels. HC IR sensors are available for MX6 iBrid® six-gas monitors and Ventis® Pro5 Multi-Gas Monitors.

 

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