Your sensor is more sensitive than you think


We all know that pellistor sensors are one of the primary technologies for detecting hydrocarbons.  In most circumstances, they’re a reliable, cost-effective means of monitoring flammable levels of combustible gases.

As with any technology, there are some circumstances in which pellistors shouldn’t be relied on, and other sensors, like infrared (IR) technology, should be considered.

Problems with pellistors

Pellistors are generally extremely reliable at detecting flammable gases.  However, every type of technology has its limits, and there are a few occasions where pellistors shouldn’t be assumed to be most suitable.

Perhaps the biggest drawback of pellistors is that they’re susceptible to poisoning (irreversible loss of sensitivity) or inhibition (reversible loss of sensitivity) by many chemicals found in related industries.

What happens when a pellistor is poisoned?

Basically, a poisoned pellistor produces no output when exposed to flammable gas. This means a detector would not go into alarm, giving the impression that the environment was safe.

Compounds containing silicon, lead, sulphur, and phosphates at just a few parts per million (ppm) can impair pellistor performance.  So whether it’s something in your general working environment, or something as innocuous as cleaning equipment or hand cream, you could be compromising your sensor’s effectiveness without even realising it.

What’s so bad about silicons?

Silicons have their virtues, but they may be more prevalent than you think; including sealants, adhesives, lubricants, and thermal and electrical insulation. They can poison pellistor sensors at extremely low levels.  For example, there was an incident where a company replaced a window pane in a room where they stored their gas detection equipment.  A standard silicon-based sealant was used in the process, and as a result all of their pellistor sensors failed their subsequent testing.  Fortunately this company tested their equipment regularly; it would have been a very different and more tragic story had they not done so.

Situations like this ably demonstrate the importance of bump testing (we’re written about it previously – take a look), which highlights poisoned or inhibited sensors.

What can I do to avoid poisoning my sensor?

Be aware, in essence –bump-test your equipment regularly, and make sure your detectors are suited to the environment you’re working in.

Find out more about infra-red technology in our previous blog.


Pellistor sensors – all you need to know

We’ve written about pellistor sensors before, but the information still remains vital and useful.  Here’s all you need to know…

Pellistor sensors (or catalytic bead sensors) have been the primary technology for detecting flammable gases since the ‘60s. Despite having discussed a number of issues relating to the detection of flammable gases and VOC, we have not yet looked at how pellistors work. To make up for this, we are including a video explanation, which we hope you will download and use as part of any training you are conducting:

A pellistor is based on a Wheatstone bridge circuit, and includes two “beads”, both of which encase platinum coils.  One of the beads (the ‘active’ bead) is treated with a catalyst, which lowers the temperature at which the gas around it ignites. This bead becomes hot from the combustion, resulting in a temperature difference between this active and the other ‘reference’ bead.  This causes a difference in resistance, which is measured; the amount of gas present is directly proportional to it, so gas concentration as a percentage of its lower explosive limit (%LEL*) can be accurately determined.

The hot bead and electrical circuitry are contained in flameproof sensor housing, behind the sintered metal flame arrestor (or sinter) through which the gas passes. Confined within this sensor housing, which maintains an internal temperature of 500°C, controlled combustion can occur, isolated from the outside environment. In high gas concentrations, the combustion process can be incomplete, resulting in a layer of soot on the active bead. This will partially or completely impair performance. Care needs to be taken in environments where gas levels over 70% LEL may be encountered.

For more information about sensor technology for flammable gases, read our comparison article on pellistors vs Infrared sensor technology: Are silicone implants degrading your gas detection?.

*Lower Explosive Limit – Learn more

 Click in the top right hand corner of the video to access a downloadable file.