Gas Mixtures for Flammability Monitoring what levels do i use? - Noor Ali

The use of gas mixtures is widespread for the calibration and function checking of gas detection systems. Historically, the flammability levels have been determined by a single standard, ISO10156 (Gases and gas mixtures – Determination of fire potential and oxidizing ability for the selection of cylinder valve outlets). However, recent changes have resulted in new standards that are being applied on a territory basis, meaning that ambiguity can arise on specifying calibration gas mixtures.

Using LEL Levels for Calibration
The most common method for specifying gas mixtures has been to quote a percentage of the Lower Explosive Limit – the LEL. For example, the use of 50% LEL Methane in Air is widespread throughout many safety applications and the default is often to order gas mixtures based on that specification.

This has been the status quo for many years and has caused no problems whilst the accepted standard for flammability limits has been ISO10156. Users, equipment manufacturers and gas companies have become au fait with the terminology over the years and it has been the norm to reorder mixtures of 50% LEL Methane etc. It is noteworthy that the gas company will need to convert the LEL value taken at the point of order placement to a molar volume value for filling purposes. The mechanism here is that this is referenced versus ISO10156, calculated and then crosschecked to ensure no safety issues exist. Thus, using the example of 50% LEL Methane in Air, your gas supplier would have converted this to a concentration of 2.5% Methane in Air based on the fact that the accepted LEL value for methane was 5.0%.