INFICON Sensistor ILS500F Leak Detection System - Manual (Page 47)

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This section is a guide for optimizing step 2, 3 and 5.
7.5 Optimizing the Pre Evacuation Step
Begin to determine how deep the pre evacuation needs to be, or if it can be skipped
altogether. To do this it is important to fully understand the role of pre evacuation.
When the test object is connected it holds one atmosphere of ambient air. It is often
necessary to remove some or most of this air before filling with tracer gas.
There are two effects of not removing the air (i.e. pre evacuating):
1 the actual tracer gas concentration will be reduced
2 tracer gas does not reach all parts of the object
7.5.1 Calculate Tracer Gas Concentration
Example:
The fill pressure is 0.05 MPa (7.2 psi) above atmosphere (gauge pressure). The object
has 1 atm = 0.1 MPa of air before filling.
Leaving this air in the object means the average tracer gas concentration will be:
A = Fill Pressure
B = 1 atm
C = Tracer Gas Fill Factor
The average tracer gas concentration in this example is only a third (33%) of what
expected.
When using a tracer gas mix of 5% the result will be:
0.33 x 5% = 1.7%
Pre evacuating down to -0.7 atm (-0.07 MPa) means there will be 0.3 atmospheres
(0.03 MPa) of residual air in the object before filling. This gives the following average
concentration:
Notice The fastest way to fill a pipe like object is to use push-through filling. That
does not require pre evacuation.
A
A B +
------- - C =
0.05
0.05 0.1 +
------------ - 0.33 =