How to find leaks in your vacuum system

September 3, 2020

4 MIN READ

Anyone who has used a vacuum system will know the frustration involved when a leak develops. This can result in poor base pressure or a steadily rising pressure during a process. Such issues can result in a batch of material being scrapped or needing to be re-worked, with significant cost implications.

How to prevent these issues?

Assuming that pumps and gauges are routinely maintained and performing correctly, a detailed leak test of the system will be required.

There are many methods of leak detection, such as a bubble test or pressure rise test, but none can compete with the sensitivity and ability to pinpoint a leak that a helium leak checker can offer. Additionally, a helium instrument allows for both quantitative and qualitative measurements.

A helium, or more precisely, a Helium 4 leak detector is essentially a tuned mass spectrometer which detects this gas and has the further option of identifying Helium 3 (used in cryogenics) and hydrogen. The unit has appropriate pumps supplied, making it portable, with only mains electricity required. The unit is extremely sensitive to helium. For high vacuum applications a leak rate below 10^-6 mbar l/s is needed, and Helium leak checkers have an ultimate sensitivity of 10 mbar l/s.

Why we use helium as the search gas

There are several key benefits of using Helium as the search gas:

It is non-toxic
Relatively low cost
Only 5 ppm occurs naturally in air
It is one of the smallest atoms, so small leaks can be investigated
It is a low-density gas which disperses easily

Ways to use a helium leak detector

Generally the detector is employed in what is known as qualitative or “local” mode, where the user is trying to find the source or sources of leaks. In this configuration, helium, as illustrated, is sprayed carefully and slowly around the test object. This object is often a vacuum processing chamber employed in a number of sectors including heat treatment, metallurgy, and optical coating.

A typical testing arrangement is shown below:

Any leak present will allow Helium to pass through and be detected by the instrument. There are some important practical rules to be followed to allow accurate and reliable leak detection:

Helium should be sprayed slowly across the test piece
The flow rate of the gas should be small: I bubble per second in water is a good rule of thumb
Leak check from top to bottom to allow the gas to disperse vertically

The leak detector generally has a small backing pump ranging from 2.5 m3h-1 to 20 m3h-1. For large vacuum chamber applications, this pump would be incapable of pumping the item down to a level of 10-3 mbar needed to allow the unit to reach its most sensitive level. As in the illustration, the most effective method is to attach the leak checker to the backing line of the system above a forevacuum pump. This will inevitably have a higher pump speed than the leak checker, and the unit will not detect the complete He flow. To give a true leak rate, the observed leak rate needs to be factored by the pumping ratio of the competing pumps.

In other instances, such as hermetically sealed electronic packages, high vacuum valves require the total leak rate to be determined to ensure that the expected lifetime or specification of the component is met. The diagram below demonstrates how the unit operates in a quantitative manner. The component is placed in a suitable vacuum container which, depending on its volume, might require additional pumps to allow the correct inlet pressure to be achieved.

Again, a pumping ratio would be needed to achieve the actual leak rate, but the value determined will be the total leakage of the entire component. Depending on its value, the device could be re-tested to find the actual leak(s) by the sniffer method and then reworked to meet specification.