Question 1: Why is air in-leak so important?

Answer: The air that leaks into a vacuum system expands when under vacuum which uses part of the capacity of the vacuum source. Example: suppose 1 cubic foot of air at atmospheric pressure (760 torr) leaks into a vacuum system operating at 76 Torr. Since 76 Torr is 1/10 of an atmosphere, the cubic foot that leaked into such an operating vacuum system would expand to 10 cubic feet inside the vacuum system. In fact, you can calculate the percent of the vacuum capacity lost to leaks with: %capacity loss = (acfm of air in-leak divided by the acfm capacity of vacuum source) x 100.

Question 2: How can I measure the air in-leak into my vacuum system?

Answer: Traditionally a rate-of-rise test is used. This requires the system be pumped down to a vacuum level below the deepest operational pressure. Isolate the vacuum source and then measure the rate that the absolute pressure rises in the piping or vessel being tested. It should not rise more than 10% of the deepest operational vacuum level in 24 hours.

This traditional method requires that all the equipment being tested be clean, dry, and empty. The “dry” requirement is sometimes missed. The system must be completely dry, otherwise liquid in the system will vaporize which will appear as an air leak, when in fact, it really isn’t. It is the vaporization of the liquid that was already present.

Another way to measure air in-leak, while operating the system at normal pressure, is to use a Busch Piccolo to introduce enough air to double the absolute pressure. The amount of air added to double the absolute pressure, is the amount of air that was leaking in originally.

Question 3: How do I find the leaks?

Answer: There are several techniques that can be used.

The traditional method is to use a helium leak detector.

Another way to find leaks is to use an acoustic leak detector. While the system is operating, the leak detector is used to listen for leaks as the vacuum system is scanned. A small microphone in the leak detector is tuned to respond to the sound frequency of air in-leak.

Another method that is both simple and effective is to apply a small amount of volatile solvent over a suspected area. When the solvent is drawn into the leak, there is an instant jump in pressure.

Question 4: Doesn’t a hydrostatic pressure test show that a vessel is leak free?

Answer: Not necessarily. Hydrostatic pressure tests measure pressure in the opposite direction – inside out, not outside in. Many seals and other equipment are directional in the way they seal. The pressure test is just that, a measure of pressure above atmospheric pressure and not applicable to vacuum systems.