Because the question is whether the atmosphere will burn, not what the concentration is in absolute terms.

The lower explosive limit is the concentration below which a gas will not ignite.

Expressing readings as a proportion of it makes the risk directly readable.

Alarms are set well below the limit to give time to act.

Different gases have different limits, so the sensor is calibrated for a specific gas.

A sensor calibrated for one gas reads a different gas with a correction factor rather than accurately.

It burns the gas on a heated catalytic element and measures the resulting temperature change.

It responds to a wide range of flammable gases, which is its main strength.

It is inexpensive and well proven over many decades.

Because it burns the gas, it requires oxygen to be present.

In an oxygen-deficient or inert atmosphere it under-reads or fails entirely.

That matters specifically in purged vessels and inerted spaces.

Silicones, sulphur compounds, lead and halogens - they coat the catalyst and reduce sensitivity.

A poisoned bead can read low or zero while appearing to work normally.

That is the most dangerous failure mode in gas detection because it is silent.

Silicones from sealants, lubricants and personal care products are a common and unsuspected source.

Regular calibration with test gas is what detects it.

In a contaminated environment, infrared sensors avoid the problem entirely.

They cannot be poisoned, work in inert atmospheres, fail detectably and generally last longer.

They measure absorption of infrared light by the gas rather than burning it.

No oxygen is required, so they work in purged and inerted spaces.

Failure is usually detectable as a fault rather than as a silent loss of sensitivity.

Longer service life offsets some of the higher purchase cost.

They also tolerate high gas concentrations without damage, which catalytic beads do not.

Hydrogen - it does not absorb in the relevant infrared band.

That is a genuine gap rather than a performance limitation.

Sites with a hydrogen risk cannot rely on infrared detection alone.

Catalytic bead or dedicated hydrogen sensing technologies are used instead.

Mixed installations using both technologies are common for this reason.

Establishing which gases are actually present is the first step in technology selection.

By the site's contaminants and gases rather than by price.

A silicone-rich environment will destroy catalytic beads repeatedly.

An inert or oxygen-deficient atmosphere rules catalytic sensing out.

A hydrogen risk rules out relying on infrared alone.

Cost differences look significant per detector and small against repeated sensor replacement.

The gases present and the contaminants present are both design inputs rather than afterthoughts.

Equipment installed in hazardous areas falls under the explosive atmospheres regime, and the schemes differ between markets.

Certification covers the equipment, its installation and its maintenance.

Area classification determines what level of protection is required.

Certification from one market is not automatically accepted in another.

Maintenance carried out incorrectly can invalidate the certification.

Establishing the applicable scheme is part of specifying rather than a commissioning detail.