2.5%vol CH4 or 2.2%vol CH4 when calibrating gas detectors?

 
Natural gas and mine gas were probably the first drivers for the development of gas detectors. It will not surprise us that as time moved these detectors became more sophisticated and we learned more about lower explosive limits (LEL). We quickly learned that mixtures of 5% natural gas and Air were explosive and called that 100% of the LEL. At a later stage we started to smarten even up more and realised that natural gas is composed of more than just methane (CH4). It has minor components of other alkanes too such as ethane, propane etc. It even has nitrogen in it. About only 80% of natural gas is actually methane, 6% minor components and 14% is Nitrogen. It is 5% of this mixture that has been found to be explosive in air(100%LEL).
 
Pure Methane however has been found to be explosive at 4.4% by volume, in air.
 
The main confusion has become that many people have thought that natural gas is methane, which clearly it is not.
Hence you see that there are many people buying calibration gas at 50% LEL and filling it with methane gas at 2.5% in synthetic air. You cannot fill a bottle with 2.5% natural gas. As the composition of natural gas is never exact and constant, let be that each country uses different qualities. Hence the standard of natural gas as a calibration gas is pretty much non-existent. Therefore the bottles get filled with 2.5% methane and just accept the error of inaccuracy involved with above assumption.
 
The shipping industry and continental USA are still the ones to use 2.5% by volume in air the most.
 
Europe has adopted the 2.2% methane(CH4) is 50% LEL as calibration gas in air. This is by far more accurate. The have moved away from the natural gas standard and used the pure chemical methane as the standard.
Hence you still find many calibration bottles claiming 2.5% methane is 50% LEL and others 2.2% is 50% LEL and that is confusing. Of course, it can be suggested to set your gas detector’s span level to correct values where 2.5% CH4 would be accurate at 57% LEL. Hence when not paying attention to these details the margin of error is 7% LEL. That may be a bit much.
 
Also, it might be worth noting that it has become common to offer calibration gas at 2.2% in 18% O2 with the balance of nitrogen, as opposed to synthetic air composed of 20.9% O2 balance N2.
 
 
 
 
 
 
 
 
 
 
 
 
Back to top
Cookies make it easier for us to provide you with our services. With the usage of our services you permit us to use cookies.
More information Ok