Leak signal response time

The signal response time is important for the safe localization of leaks. It states the time period after spraying helium on the leak until the leak detector indication reaches a defined level (63% of final equilibrium value). Figure 8.3 shows the typical exponential trace of the signal rise. The response time can be calculated from the parameters of the system under test. The critical factors are the volume of the system and the total pumping speed of the installed vacuum pumps. Typical response times of chemical plant systems are in the range of several seconds up to a few minutes. 

In leak detection, as rapid a response as possible is required from the test object-leak detector arrangement. It is generally assumed that the signal from the leak detector in the presence of He builds up exponentially (and decays in a similar manner after He investigation stops). In such a case, the response time ( ) is expressed in terms of a time constant (t). For an increase in signal to 63% of its equilibrium value ... 

The chamber in Example 4.11 has a volume of 5 m3. The volumes of the diffusion pump and the connecting line to the chamber are 100L and 660 L, respectively. The volume of the connection between the Roots and diffusion pumps is 10 L. Calculate the response time for 95% of the final signal on the leak detector when it is connected in positions 1 and 2. 

The results displayed in Figure 9.39 have been obtained dirring the operation of the Vulcain engine and during an accidental leak. These results are significant to the performances of the sensor indeed, the signal is very close to the sensitivity and the response time observed with the mass spectrometer. 

The connections of the capillary to the detector, injector and the insert are additional and frequent trouble spots. Injeetion of 2 mL of dichloromethane into the closed oven is a useful overall leak test. A rapid response of the detector (10-20 s) indicates a leak near the detector, a signal delayed by the residence time in the column (eg., 60s for a 30m column length) indicates a leak near the injector. The more specific signal test is then applied as discussed above. Ferrules for the capillary are frequent sources of problems with some equipment. Graphite or vespel ferrules must have a 0.5 mm bore for a 0.32 mm i.d. column. 

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