The size of the leak rate signal after tn time constants in %
τ
1
2
3
4
5
6
%
63.2
86.46
95.02
98.16
99.3
99.75
The calibrated leak at position 2 is positioned before the pumping line divides between the additional pump and the leak detector. This is necessary to have a correct calibration. Part of the helium of the calibrated leak is pumped by the additional pump as is also with helium comimg from a leak in the heat exchanger.
The calibrated leak at position 2 has the following properties:
| Advantage: | The leak detector can be calibrated without helium coming into the heat exchanger, therefore not producing a background signal and calibration can be done in a short time. |
| Disadvantage: | The responsetime can only be calculated. |
The calibrated leak at position 1 has the following properties:
| Advantage: | The time constant can be measured by an extra experiment |
| Disadvantage: | The calibration of the leakdetector takes a long time, because for the increase of the helium signal as well as for the decrease (to pump away the background helium) at least 5 timeconstants are necessary. |
Remark:
Long response times, even longer than 18 minutes in this case, do not matter for the correct test result. But to localize a leak, response times of seconds are necessary. So this example shows the importance to watch the time constant during a search gas leak test