GB 03302 - Edition 03 - July 12
11/11
A 200
About leak detection
Helium and leak detection:
which purity?
Helium is commercialized in many different purity levels, the highest
level of purity is requested from some laboratories for fundamental
researches or very accurate analyses.
The use of the Helium as a tracer gas into a mass spectrometer
doesn’t require such attention. A purity in the range of 97 % to 99 %
is enough.
There is absolutely no risk of accuracy lost or contamination for the
analyzer cell by using standard purity level of Helium gas.
Hydrogenated nitrogen
If Hydrogen (H
2
) and Oxygen (O
2
) are mixed and heated, they react
and create water vapor (H
2
O). During this process more heat is
generated which may (if the concentration is high enough) ignite the
surrounding gas. If this process propagates, the gas explodes.
At low concentration of Hydrogen (< 4 % in air) the generated heat is
not enough to ignite the surrounding gas.
At concentrations in the range 4 to 12 % the combustion may
spread only if actions are taken to prevent the generated heat from
dissipating. It is a common misconception that hydrogen will explode
as soon as the concentration exceeds 4 %. It may explode only if
conditions are favorable for spontaneous propagation of combustion.
Never use pure hydrogen or a mixture with a hydrogen concentration
higher than 10 % to realize a "hydrogen" test as tracer gas.
If you use the recommended tracer gas of 5 % H
2
/ 95 % N
2
and mix
it with air there will either be too little hydrogen or too little oxygen
to constitute a combustible gas mixture. Hence this gas mixture is
classified as non flammable. The actual limit is 5.7 %.
The mixture 10 % H
2
/ 90 % N
2
is commonly used in the industry
because it is flammable only under certain conditions. This mixture is
however classified according to ISO 10156 as Flammable Gas and
should only be used after due safety considerations and approvals.
adixen Vacuum Products - ASM 310 Operating instructions
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