Which tracer gas?
At Pfeiffer Vacuum we specialize on leak detection with tracer gases due to the unrivalled sensitivity and selectivity of tracer gas methods compared to other leak detection procedures. Tracer gas methods are insensitive to temperature changes, elastic behavior of test specimens and provide the fastest test cycle times in a production environment.
The requirements a tracer gas has to fulfill are:
- Low ambient concentration for good sensitivity in air
- High sensitivity detector technology
- Highly selective sensor technology
- Environmentally friendly
- Inert and nontoxic
These requirements are perfectly met with the dominant tracer gas helium. Over the past few years a mixture of 5 % hydrogen and 95 % nitrogen (also known as forming gas 95/5) has also obtained some industrial acceptance. The table below indicates the pros and cons for each tracer gas.
|Low ambient concentration||5 ppm in air||≥ 0.5 ppm in air|
|High sensitivity detector technology||Mass spectrometry, smallest detectable|
< 5 · 10-13 Pa m3 s-1
|Mass spectrometry, higher background than with helium, can be used for vacuum and sniffing methods|
|Highly selective sensor technology||Mass spectrometry, no cross sensitivity on|
mass 4 u
|Mass spectrometry, background on|
mass 2 u detector sensitive to ambient hydrogen sources
|Environmentally friendly||Noble gas, naturally in air, no green house potential||Naturally in air, no green house potential|
|Affordable||Medium gas cost||Low gas cost|
|Safe||Inert, non flammable, non explosive||Only in gas mixtures below lower explosion limit|
Helium is the second most common element in the universe, representing about 23 % of the total matter. Helium is a 100 % green gas and has absolutely no environmental impact on the atmosphere.
Helium is “mined” from natural gas and oil wells and stored. The annual world wide production is several thousands of tons. Helium is constantly seeping up from the ground all around us, but it is so light that almost all of it escapes into space fairly rapidly. There is a constant flow of helium from space and the sun to earth. This gives a dynamic equilibrium and is the reason for the world wide constant concentration of 5 ppm helium in ambient air. Helium is a very light colorless element and it is one of the six noble gases which means it does not react with anything for all practical intents and purposes.
Helium is used for the following applications:
- Protective gas for welding and cutting
- Filling (resonator) gas in lasers
- Carrier gas in gas chromatography
- Diving gas mixtures
- Cooling, e. g. during production of fiber optics
- Balloon filling gas
- Superconducting magnets (in liquid form)
- Low temperature physics (liquid)
- Nuclear magnetic resonance (liquid)
- Tracer gas in leak detection (12 % of annual helium consumption)
Helium is commercialized in many different purity levels from balloon gas with roughly 90 % of helium up to a purity of 6.0, which is 99.9999 % of helium. So which purity levels should one use for leak detection? Highest purity is rarely used in leak detection. This only makes sense e.g. for nuclear applications in order to avoid unwanted fission products or in UHP (Ultra High Purity) grade gas supply lines which are pre-treated by purging with ultra-pure argon and which are also particle-tested. Do not forget the influences of tubing and pressure reducer in these applications! For standard industrial applications purity in the range of 97 % to 99 % is more than adequate. There is absolutely no risk of accuracy loss or contamination for the leak detector by using standard purity level of helium gas.
Hydrogen (H2) is the lightest element. It is a colorless, odorless, tasteless, flammable gas found at concentrations of about 0.5 ppm in the ambient air. Hydrogen is produced by several methods, including steam/methane reforming, dissociation of ammonia and recovery from by-product streams from chemical manufacturing and petroleum reforming. Hydrogen can be stored and transported either as a gas or a cryogenic liquid. Hydrogen is flammable in the concentration range 4 % to 77 % in air or oxygen. At low concentration of hydrogen (< 4 % in air) the generated heat is not enough to ignite the surrounding gas. It is a common misconception that hydrogen will explode as soon as the concentration exceeds 4 %. If you use the recommended tracer gas of 5 % H2 / 95 % N2 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.