INFICON Sensistor ILS500 Leak Detection System - Manual (Page 54)

54 Recipes
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This section is a guide for optimizing step 2, 3 and 5.
9.5 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 hydrogen concentration will be reduced
2 tracer gas does not reach all parts of the object
9.5.1 Calculate Hydrogen 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 hydrogen concentration in this example is only a third (33%) of what
expected.
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 For optimizing the gas test step, refer to the application manual, the
Hydrogen Method.
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 =