Vacuum Technology Know How - Triumf

5
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Leak detection
5.1 General
5.1.1 Leaks and leak detection
What is a leak? A leak, also referred to as leakage, enables a substance to flow toward a
pressure gradient. Expressed in simpler terms, leaks are small holes through which gases or
liquids flow from the side of higher pressure to the side of lower pressure. This can involve
simple, harmless leaks, such as a dripping water faucet, or hazardous toxic substances that
escape through leaks.
Any number of technical products will not function, or will not function for an adequate period
of time, if they have leaks. Examples include: The refrigerant circulation system in refrigerators,
air conditioning systems in cars, automobile tires, automotive fuel tanks or home fuel
oil tanks, as well as distillation systems in the chemical or pharmaceutical industries. In many
cases, the leak-tightness of machines and systems in the production process is an indispensable
prerequisite for the quality of the manufactured products.
Returning to the original definition of a leak, we thus find that it is impossible to completely
prevent substances from flowing through a wall. The term ”tight“ therefore refers to the
requirements of the respective machine, plant or vessel, and must be quantified accordingly.
5.1.2 Leakage rate
Let us consider a bicycle tube having a volume V = 41. It has been inflated to a pressure of
three bar, and without any additional inflation should have a maximum pressure loss of
�p = 1,000 mbar after time t (30 days).
The leakage rate has already been defined in 1.3.3: (Formula 1-27).
Or to illustrate: The leakage rate of a vessel having a volume of 1 liter is 1 mbar . �p
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:
. V
Q = l �t
1,000 mbar . 4 l mbar . l
Q = = 1.5 l . - 3 10
30 . 24 . 3,600 s s
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).
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
2.24 l . 1,000 mbar mbar . l
Q = = 7.1 l . - 6 10
10 . 365 . 24 . 3,600 s s
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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