QUESTION: We have changed the ventilation in our test chamber from air to nitrogen to reduce the background signal during leak detection. However the background signal has got worse rather than better. Surely this is impossible?
ANSWER: You are probably using nitrogen with a high helium content.
BACKGROUND: Gases are supplied at different purity rates and identified according to the percentage content of the specified gas, e.g. N2 4.8 indicates a nitrogen content of 99.998%. This is a very pure gas and is perfectly suited as a purge gas but still has 0.002% of gases other than N2.
Helium is generally not specified when it is a component of other gases because it is an inert gas and hence not a problem for the majority of applications. Only gases supplied with an analysis certificate will indicate the helium concentration amongst other things. However, the analysis is very expensive compared to the price of the gas. Low-purity, e.g. 2.0, or “purified” nitrogen actually contains substantially more helium than the ambient air. Concentrations can be more than 100 ppm, which is 20 times more than in the natural atmosphere (helium content 5 ppm). A simple method of measuring helium concentration, however, is to use an Adixen leak detector with a sniffer sensor. Simply hang the sniffer sensor out of the window and set the sensing factor to 5.0E-06 (= 5 ppm). (If the value is constant you can assume that the concentration is not higher.) You then hold the sniffer sensor in the nitrogen flow. In a “good” gas, the signal will be less than 1.0 E-06, i.e. the helium content is a least five times lower than in the ambient air. This is generally the case with nitrogen qualities of 4.8 and above.
Glossary
ppm: (parts per million) is a measurement of concentration 1 ppm = 1/1 000 000 = 1*10-6 = 0.001 %o = 0.0001 %
