Infrared emitters within the sensor each generate beams of IR light. Each beam is measured by a photo-receiver. The “measuring” beam, with a frequency of around 3.3μm, is absorbed by hydrocarbon gas molecules, so the beam intensity is reduced if there is an appropriate concentration of a gas with C-H bonds present. The “reference” beam (around 3.0μm) is not absorbed by gas, so arrives at the receiver at full strength. The %LEL of gas present is determined by the ratio of the beams measured by the photo-receiver.
Benefits of IR technology
IR sensors are reliable in some environments that can cause pellistor-based sensors to function incorrectly or in some cases fail. In some industrial environments, pellistors are at risk of being poisoned or inhibited. This would leave a worker on their shift unprotected. IR sensors are not susceptible to the catalyst poisons so significantly enhance safety in these conditions.
Pellistor technology is considerably less expensive than IR technology, reflecting the comparative simplicity of the detection technology. However, there are several advantages of IR over pellistors. These include 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 that no flammable gas is 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 by chemical toxins. In high concentrations of flammable gas, pellistor sensors can burn out. As with poisoning or inhibition, this would probably only be picked up by testing. Again, IR sensors are not affected by 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.
Factors that affect IR technology
Exposure to high levels of flammable gas can cause “sooting” of pellistors, reducing their sensitivity and potentially leading to failure. Pellistors require oxygen to function, however, IR sensors can be relied on in applications such as fuel storage tanks where there is little or no oxygen, due to flushing with inert gas prior to maintenance, or which still contain high levels of fuel vapours. The fail-safe nature of IR sensors, which automatically alert you to any fault, provides an additional layer of safety. Gas-Pro IR measures in %LEL and has been certified for use in hazardous areas as defined by both ATEX/IECEx and UL.
Knowing when the technology has failed
IR sensors are reliable in environments that can cause pellistor-based sensors to function incorrectly or in some cases fail. In some industrial environments, pellistors are at risk of being poisoned or inhibited. This leaves workers on their shifts unprotected. IR sensors are not susceptible to these conditions, so significantly enhance safety.
Problems with IR sensors
IR sensors do not measure hydrogen, and they usually don’t measure acetylene, ammonia of some complex solvents either except for some specialist sensor types.
If nothing is done to prevent it, moisture can build up inside IR sensors on the optics scattering the IR light and causing a fault.
The fail-safe nature of IR sensors, which automatically alert you to any fault, provides an additional layer of safety, and this results in a fault if there isn’t enough light getting through the system e.g., if the light is being scattered form the beam.
IR sensors have very high resistance to interference or inhibition by other gases and are suitable for both high gas concentrations and use in inert (oxygen free) backgrounds where catalytic pellistor sensors would perform poorly.
Our portable products such as Our Gas-Pro IR and Triple Plus+ help customers to detect potentially explosive gases where traditional, “pellistor,” catalytic sensors will struggle – especially in low oxygen or ‘poisoning’ environments. And allow for the measurement of hydrocarbons at both % LEL and % Volume ranges making this instrument ideal for tank and line purging applications.
To explore more, visit our technical page for more information.