In gas detection terms, pellistors have been the primary technology for detecting flammable gases since the 60s. In most circumstances, with correct maintenance, pellistors are a reliable, cost-effective means of monitoring for combustible levels of flammable gases. However, there circumstances under which this technology may not be the best choice, and infrared (IR) technology should be considered instead.
Under circumstances, IR technology can have a number of advantages over pellistors.
- IR technology provides fail-safe testing. The mode of operation means that if the infrared beam failed, this would register as a fault. In normal pellistor operation, conversely, a lack of output is ordinarily an indication of no gas present, but this could also be the result of a fault.
- Pellistors are susceptible to poisoning or inhibition; a particular concern in environments where compounds containing silicon, lead, sulphur and phosphates, even at low levels. IR instruments don’t, themselves, interact with the gas. Only the IR beam interacts with the gas molecules, so, IR technology is immune to poisoning or inhibition.
- In high concentrations of flammable gas, pellistor sensors can burn out. As with poisoning or inhibition, this would only be picked up by testing. Again, IR sensors are not affected under these conditions.
- Low levels of oxygen mean that pellistor sensors won’t work. This can be the case in recently purged tanks, but also in confined spaces generally, where pellistors may be ineffective. IR technology is effective in areas where oxygen may be reduced or absent.
An incident of pellistor poisoning occurred at one company when it replaced a window pane if the room in which they stored gas detection equipment. Silicon-based sealant of a standard type was used, and as result, all their pellistor sensors failed their subsequent testing. Happily, this company routinely tested all it sensors, or this incident would be well-known and infamous. As it was, the problem was picked up, and no one came to any harm.
It is clear that, if relying on pellistor detectors in environments where poisons or inhibitors may be encountered, regular and frequent testing is the only way to ensure that performance is not being degraded, unless you choose a different technology.
However, it cannot be assumed that IR technology is always the better choice, either. There are circumstances when pellistors are preferred over IR technology.
- Because by hydrogen doesn’t absorb infrared light, IR detection does not detect it. Hydrogen is highly flammable, and pellistors are a better choice if this is a potential hazard in the local working environment. Other technologies exist to measure hydrogen specifically, including electrochemical sensors.
- IR technology is strongly affected by gross pressure changes, so its use in processes likely to undergo wide pressure fluctuations is usually avoided and pellistor based detection considered, assuming there is no risk of poisoning etc.
- Pellistor technology is considerably less expensive than IR technology. Many working environments present little to no risk of pellistor poisoning, etc. Here, routine, daily bump testing and sensor replacement as required may be a reasonable management strategy. However, to compare costs properly, on-going testing and maintenance needs to be included in the calculation.
When assessing which sensor technology to use in your gas monitoring. There are many factors to consider. Risks to assess include (but may not be restricted to) pellistor poisoning, inhibition or burn-out; low oxygen environments; need to detect hydrogen; and lifetime ownership costs. Sites may have diverse environments requiring different sensors technologies, or a combination of both pellistor and IR sensor in one device.
If you want to more, watch out for a Crowcon Whitepaper, coming soon, which reviews this subject in more detail.