Dealing with Lurking Inhibitors and Poisons from Sensors

It is true that gas detector sensors will be in contact with toxic and explosive or corrosive gases. However, they may fail after such exposure. There are various types of chemicals and vapors which can poison or inhibit different classes of sensors. These range from ordinary cleaners or lubricants to custom chemicals or rarely used substances.

Among the most sensitive sensor type is the catalytic bead LEL sensor. This type is used as standard in a range of personal gas monitors, in many different applications. This makes it imperative to know which substances can poison the sensor, as this could make the difference between a functional gas monitor and one which fails to activate the alarm at the critical time, leading to a potentially serious buildup of combustible gas.

A catalytic bead LEL sensor detects gas as follows: as the bead surface meets the gas, it ignites it and the resulting temperature change is reflected as a readout on the monitor screen. However, poisons or inhibitors of the sensor prevent this combustion from continuing to completion or keep it from igniting altogether.

This can blank out the readout even when gas is present. Both inhibitors and sensors produce roughly the same type of result but through different mechanisms of action and they are found to occur in different materials.

The Source of Sensor Poisons

Frequently fond sensor poisons include lubricants and additives to gases, silicone-based compounds, or those which contain lead or sulfur. If any of these come into contact with a sensor that has been turned on, and which has a heated-up bead as a result, they melt at once and bond to the surface, forming a permanently attached capsule.

This means the sensor must be replaced as it has become useless, unable to burn any combustible gas to which it is exposed and therefore unable to give any readout. The point is that if the instrument has been calibrated and bump tested as per schedule, the user can tell if it is functioning properly prior to this exposure, but if not, the blank readout could be just the continuation of what was occurring earlier. Thus, the presence of sensor poisoning cannot be picked up.

The user is now in danger as the meter is not actually reliable in an environment containing flammable gas. If any suspicion of such a situation exists, the remedy is to calibrate the LEL sensor at once, as this will tell if the sensor is still working. Provided the sensor can give a readout after calibration, it is able to detect the calibration gas and is functioning. If not, however, it is poisoned, and the calibration test will fail.

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

Course of Recommended Action Following Potential Sensor Exposure to Inhibitors

Inhibitors also reduce sensor sensitivity to combustible gases if they are present in the gas mixture, though their action is not as irreversible as that of sensor poisons. They also reduce the working life of the sensor, but this may be partially reversed by prompt calibration and bump testing. Halogenated compounds are among the most common sensor inhibitors.

Following contact with inhibitors, the sensor should be calibrated and assessed to see if it is still working. The burning of the calibration gas at the bead surface will also burn off part of the inhibitor chemical adhering to the bead and thus refresh the surface. However, if large amounts of inhibitor came in contact with the bead or if repeated loss of sensitivity has occurred, the bead may not be salvageable, and the calibration test will fail.

Sometimes it is quickly known that the sensor was in contact with a poison or indicator, but sometimes this recognition does not occur. For instance, many silicone-based chemicals and lubricants are used in the form of sprays which scatter over a wide area and may even spatter a personal gas detector if it is too close to the surface of application.

Another way is when gas detectors come into contact with chemicals or vapors arising from the products as they dry over a long period of time. A third method is during storage of gas detectors, for instance in lockers or tool boxes, when stored along with such poisons or inhibitory chemicals, though during their active use they may have been kept away from exposure to these chemicals.  

If there is any thought that the LEL sensor may have been poisoned, it should be calibrated immediately to make sure it is in working order.

Dealing with Sensor Poisoning Events

There are a variety of sensor poisons and inhibitors, which is why each day’s usage of a gas detector should begin by calibration and bump testing of the device, along with a prompt repeat calibration at the first suspicion of exposure to such a substance.

This process will keep the detector in good working condition and able to detect gases effectively. On the other hand, if poisoning or inhibition has occurred, the failure of calibration will indicate this, and this means the device is non-functional and must be replaced or serviced.

This information has been sourced, reviewed and adapted from materials provided by Industrial Scientific.

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