Helium standard leak rate

Fig. 53 Examples for conversion into helium standard leak rates...

N cuum envelope tests are integral leak tests using helium as the test gas, in which the test specimen is enclosed either in a rigid (usually metal) enclosure or in a light plastic envelope. The helium which enters or leaves (depending on the nature of the test) the test specimen is passed to a helium leak detector, where it is measured. Envelope tests are made either with the test specimen pressurized w/ith helium (Fig. 5.4c) or with the test specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard leak rate. 

Leak <—> Hole Q... Leak rate, In short Leak Substance quantity trhough hole per unit of Ume Definition 0 = At Heliim standard leak rate p, = 1 bar. P2 < 1 mbar (Ap = 1 bar) Test gas s Helium... 

According to the definition formulated above it is easy fo understand that the size of a gas leak, i.e. movement through undesired passages or pipe elements, w/ill also be given in mbar I s" A leak rate is often measured or indicated w/ith atmospheric pressure prevailing on the one side of the barrier and a vacuum at the other side (p < 1 mbar). If helium (wrfiich may be used as a tracer gas, for example) is passed through the teak under exactly these conditions, then one refers to standard helium conditions . 

A standard leak is a small container that leaks helium at a very slow and specific amount. As mentioned before, helium diffuses through borosilicate glass (or through ceramic or capillary tube). This diffusion is very consistent at any given pressure and temperature. Within the standard leak is a thimble of borosilicate glass (see Fig. 7.62) around which is the stored helium. The helium leak rate var-... 

Standard leaks come from the factory with their calibrated leak rate imprinted on their sides. The approximate annual attenuation is also indicated. After a certain amount of time (every 5 to 10 years) they can be returned to the factory for either recharging and/or recalibration. Recalibration is not critical if the helium leak detector is being used only for leak location and not leak calibration. 

The response time for a helium leak detector is the time required for a known leak rate to be indicated on the leak detector from zero to 63% of its maximum equilibrium level. 87 Of course the helium leak detector must be calibrated with a standard leak so that the maximum reading can be properly set on the leak detector. The response time is dependent on the quality of vacuum, the size of the leak, physical barriers (such as constrictions, bends, or traps) between the leak and the leak detector, and the proximity of the leak to the leak detector. Because of so many factors, there can be no set specific response time. 

Leak rates down to 10 Torr-liters/sec of nitrogen can be detected using helium leak detection methods. For accurate measurement the leak detector must be calibrated with a standard leak. 

From the table one can see that only helium leak detection with a mass spectrometer leak detector allows a real quantitative statement of leakage rates compliant with the requirements of the ISO 9000 series of standards for quality management. Therefore this method and its variants will be described in more detail in the following. 

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