What’s the difference between a pellistor and an IR sensor?

Sensors play a key role when it comes to monitoring flammable gases and vapours. Environment, response time and temperature range are just some of the things to consider when deciding which technology is best.

In this blog, we’re highlighting the differences between pellistor (catalytic) sensors and infrared (IR) sensors, why there are pros and cons to both technologies, and how to know which is best to suit different environments.

Pellistor sensor

A pellistor gas sensor is a device used to detect combustible gases or vapours that fall within the explosive range to warn of rising gas levels. The sensor is a coil of platinum wire with a catalyst inserted inside to form a small active bead which lowers the temperature at which gas ignites around it. When a combustible gas is present the temperature and resistance of the bead increases in relation to the resistance of the inert reference bead. The difference in resistance can be measured, allowing measurement of gas present. Because of the catalysts and beads, a pellistor sensor is also known as a catalytic or catalytic bead sensor.

Originally created in the 1960’s by British scientist and inventor, Alan Baker, pellistor sensors were initially designed as a solution to the long-running flame safety lamp and canary techniques. More recently, the devices are used in industrial and underground applications such as mines or tunnelling, oil refineries and oil rigs.

Pellistor sensors are relatively lower in cost due to differences in the level of technology in comparison to IR sensors, however they may be required to be replaced more frequently.

With a linear output corresponding to the gas concentration, correction factors can be used to calculate the approximate response of pellistors to other flammable gases, which can make pellistors a good choice when there are multiple flammable vapours present.

Not only this but pellistors within fixed detectors with mV bridge outputs such as the Xgard type 3 are highly suited to areas that are hard to reach as calibration adjustments can take place at the local control panel.

On the other hand, pellistors struggle in environments where there is low or little oxygen, as the combustion process by which they work, requires oxygen. For this reason, confined space instruments which contain catalytic pellistor type LEL sensors often include a sensor for measuring oxygen.

In environments where compounds contain silicon, lead, sulphur and phosphates the sensor is susceptible to poisoning (irreversible loss of sensitivity) or inhibition (reversible loss of sensitivity), which can be a hazard to people in the workplace.

If exposed to high gas concentrations, pellistor sensors can be damaged. In such situations, pellistors do not ‘fail safe’, meaning no notification is given when an instrument fault is detected. Any fault can only be identified through bump testing prior to each use to ensure that performance is not being degraded.

 

IR sensor

Infrared sensor technology is based on the principle that Infrared (IR) light of a particular wavelength will be absorbed by the target gas. Typically there are two emitters within a sensor generating beams of IR light: a measurement beam with a wavelength that will be absorbed by the target gas, and a reference beam which will not be absorbed. Each beam is of equal intensity and is deflected by a mirror inside the sensor onto a photo-receiver. The resulting difference in intensity, between the reference and measurement beam, in the presence of the target gas is used to measure the concentration of gas present.

In many cases, infrared (IR) sensor technology can have a number of advantages over pellistors or be more reliable in areas where pellistor-based sensor performance can be impaired- including low oxygen and inert environments. Just the beam of infrared interacts with the surrounding gas molecules, giving the sensor the advantage of not facing the threat of poisoning or inhibition.

IR technology provides fail-safe testing. This means that if the infrared beam was to fail, the user would be notified of this fault.

Gas-Pro TK uses a dual IR sensor – the best technology for the specialist environments where standard gas detectors just won’t work, whether tank purging or gas freeing.

An example of one of our IR based detectors is the Crowcon Gas-Pro IR, ideal for the oil and gas industry, with the availability to detect methane, pentane or propane in potentially explosive, low oxygen environments where pellistor sensors may struggle. We also use a dual range %LEL and %Volume sensor in our Gas-Pro TK, which is suitable for measuring and switching between both measurements so it’s always safely operating to the correct parameter.

However, IR sensors aren’t all perfect as they only have a linear output to target gas; the response of an IR sensor to other flammable vapours then the target gas will be non-linear.

Like pellistors are susceptible to poisoning, IR sensors are susceptible to severe mechanical and thermal shock and also strongly affected by gross pressure changes. Additionally, infrared sensors cannot be used to detect Hydrogen gas, therefore we suggest using pellistors or electromechanical sensors in this circumstance.

The prime objective for safety is to select the best detection technology to minimise hazards in the workplace. We hope that by clearly identifying the differences between these two sensors we can raise awareness on how various industrial and hazardous environments can remain safe.

For further guidance on pellistor and IR sensors, you can download our whitepaper which includes illustrations and diagrams to help determine the best technology for your application.

Changes to Workplace Exposure Limits (WELs)

What Are Work Place Exposure Limits?

Workplace exposure limits (WELs) provide a legal maximum level for harmful substances in order to control working conditions.

Directive and National Standards

The EU Directive 2017/164 establishes new ‘indicative occupational exposure limit values’ (IOELVs) for a number of toxic substances. The UK Health & Safety Executive (HSE) has decided to change UK statutory limits to reflect the new IOELVs. This decision by the HSE has been taken to comply with Articles 2 and 7 of the Directive requiring Member States to establish the new occupational exposure limit values within national standards by August 21st 2018.

Gas Detector Alarm Thresholds

The exposure limits defined in this Directive 2017/164 are based on the risks of personal exposure: a workers’ exposure to toxic substances over time. The limits (configured into gas detectors as ‘TWA alarm levels’) are expressed over two time periods:

  • STEL (short-term exposure limit): a 15 minute limit
  • LTEL (long-term exposure limit): an 8-hour limit

Portable (personal) monitors are intended to be worn by the user near to their breathing zone so that the instrument can measure their exposure to gas. The instruments TWA (time-weighted) alarms will therefore alert the user when their exposure exceeds the limits set within the national standards.

Portable monitors can also be configured with ‘instantaneous’ alarms which activate immediately when the gas concentration exceeds the threshold. There are no standards to define alarm levels for instantaneous alarms, and so we have these generally set at the same thresholds as the TWA alarms. Some of the new TWA thresholds are low enough to make frequent false alarms a significant problem if they were also adopted for the instantaneous alarm setting. Therefore, new portable instruments will retain the current instantaneous alarm thresholds.

Fixed gas detectors only utilise ‘instantaneous’ alarms as they are not worn by the user and therefore cannot measure an individuals’ exposure to gas over time. Alarm levels for fixed detectors are often based on the TWA alarms as these are the only published guidelines. HSE document RR973 (Review of alarm setting for toxic gas and oxygen detectors) provides guidance on setting appropriate alarm levels for fixed detectors in consideration of site conditions and risk assessment. In some applications where there may be a background of gas it may be appropriate for fixed detector alarm levels to be set higher than those listed in EH40 to prevent repeated false alarms.

Re-configuration of Gas Detector Alarm Thresholds

Users of portable gas detectors who choose to adjust their instrument alarm thresholds to align with the Directive can easily do-so using a variety of accessories available from Crowcon. For full details of calibration and configuration accessories visit the product pages at www.crowcon.com.

Other documents you may find useful:

http://www.hse.gov.uk/pubns/priced/eh40.pdf

http://www.hse.gov.uk/research/rrhtm/rr973.html

 

Carbon monoxide alarms now mandatory

As a founding member of CoGDEM (the Council of Gas Detection and Environmental Monitoring), we are really pleased that the Communities Minister, Penny Mordaunt, has made it mandatory for private landlords to install smoke and carbon monoxide (CO) alarms in rented properties.

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Chris’ quick guide to bump testing

Following on from last week’s article, ‘Why do I need to bump test my instrument?’, I thought I’d give you a little more detailed information about what is a bump test and how to carry one out.

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