Pfeiffer Vacuum

5.1.2 Leakage rate

Let us consider a bicycle tube having a volume V = 4 l. It has been inflated to a pressure of three bar, and without any additional inflation should have a maximum pressure loss of Δp = 1,000 mbar after time t (30 days).

The leakage rate has already been defined in 1.3.3: (Formel 1-27).
Or to illustrate: The leakage rate of a vessel having a volume of 1 liter is 1 mbar · l / s if the interior pressure increases or decreases by 1 mbar in 1 second. Please refer to Table 1.6 for conversion to other customary units. Inserting the values for our bicycle tube then yields the permissible leakage rate:
Ql = 1.5 · 10-3
and we find that the bicycle tube with this leakage rate is sufficiently tight.

These kinds of leakage rates can be found by means of the well-known bubble test method (Figure 5.1).

(Figure 5.1)
Figure 5.1: Bubble leak test for a bicycle tube

Now let us consider a refrigerator in which a loss of 10 g of refrigerant having a molecular weight of 102 g / mol, i.e. around 2.24 bar · l, is allowable over a ten-year period. This results in a permissible leakage rate of
Ql = 7.1 · 10-6 .

These kinds of leakage rates can only be localized and quantified by means of extremely sensitive measuring methods, for example with mass spectrometry and test gases that are not present in the atmosphere

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