leaks and leak testing


Unfortunately, you will often find that the vacuum in your system does not reach the expected low pressure. The reason could be a leak. It is very important to understand the difference between real leaks, which are treated here, and virtual leaks. Real leaks are places in the vacuum system where air from outside gets into the system, through a poorly mounted gasket, bad welding or something similar. Virtual leaks can be small trapped gas volumes inside the chamber, with the result that you do not achieve low pressure and your mass spectrometer indicates the presence of a leak. But in reality, you often have only a design problem rather than a leak.

Leaks can be hard to find and sometimes it is not easy to decide if there is any leak at all. Here are few strategies.

It is very useful to know the “normal” behaviour of your system as you pump it down. How does the pressure normally develop as a function of time? How many minutes of pumping with the roughing pump are normally needed before the pressure is sufficiently low to turn on the second pumping stage, like a turbomolecular pump?

Using the mass spectrometer in the system, you can decide if you have an air leak or not (see here).

Small leaks in the UHV range or a little above are usually found by using a mass spectrometer inside the system or by a leak-tester (a pumping station with an included mass spectrometer) attached to the system. The idea is set the mass spectrometer to measure only the partial pressure of helium (mass 4) in the system. Usually, this is implemented as a leak test mode. Then, from outside the vacuum system, you spray He carefully on the various parts with special attention to “suspicious” places, such as flanges which you have recently closed. Start by spraying the upper parts of the chamber and move down. When you get close to the leak, the partial pressure of He inside the system will increase and you will be able to detect this. Be careful close to viton seals (e.g. near differentially pumped rotary feedthroughs). Such seals are not tight against helium and the gas will diffuse through them, sometimes with a long time delay.

If the leak is too big to turn on the electron multiplier of the mass spectrometer, you can try the same strategy using the Faraday cup.

If the leak is even bigger than that, you might not be able to turn on the mass spectrometer at all. You have, at best, the reading of an hot cathode ion gauge or a cold cathode ion gauge or maybe only from a pirani gauge. So all you can do is to use this pressure reading to find the leak. A good test is then to spray solvents with small molecules on the suspected part and watch for a rapid change in pressure. You can use ethanol, isopropanol or acetone, with the last choice having the disadvantage of being harmful to viton o-rings, LaB6 filaments and your health. When you hit the leak, the pressure will often rise because the molecules can go through the leak but it could also drop because the freezing of the evaporating liquid temporarily seals the leak.

If you have a very big leak, so big that you cannot even pump down to the base pressure of the roughing pump, you will often simply hear where the air gets into the system!


index               home