INFICON Sensistor ILS500 Leak Detection System - Manual (Page 55)

Recipes 55
ninb63e1-f (1410)
A = Fill Pressure
B = 1 atm
C = Tracer Gas Fill Factor
D = Evacuation Pressure
The average hydrogen concentration in this example will be 0.8 (80%).
0.8 x 5% = 4%
This is almost twice of that achieved with no pre evacuation.
9.5.2 Example - Calculate Tracer Gas Filling
The air left in the object can not always be expected to mix evenly with the injected
tracer gas. This is especially so for tube shaped objects such as pipes etc. The flow
inside a regular “tube” is predominantly laminar. This means no or very little turbulence
occurs. Air left in the “tube” will therefore be pushed in front of the injected tracer gas
and end up in the remote end of the “tube”.
Example:
The test object is an aluminium pipe for a refrigerator with brazed copper ends. The
joints between copper and aluminium must both be tested.
Fill pressure is 0.5 MPa (72 psi). Length is 10 m (33 ft.). Skipping pre evacuation we
will have:
A = Fill Pressure
B = 1 atm
E = Air left in the object
of air left in the pipe. This is equivalent to 1.7 m (5.7 ft.) of the total length if no
turbulence occurs during filling. There is an evident risk that there will be only air inside
one of the joints, which means that a leak there will remain undetected.
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 pipe before filling.
A D +
A B +
------- - C =
0,05 0,07 +
0,05 0,1 +
-------------- - 0,8 =
B
A B +
------- - E =
0,1
0,5 0,1 +
----------- 0,17 =