Four ways of finding vacuum leaks using helium
February 6th, 2019
Table of content
- Detecting vacuum leaks at low vacuum levels
- Why is helium used to detect leaks?
- How does helium leak detection work?
- How do you calculate vacuum leak rates?
- Vacuum leak detection methods
- Which helium leak detection (HLD) methods are there?
- Standard leak rates of helium leak detectors
- Challenges when using helium leak detectors
In the “integral testing (sample under vacuum)” method, the chamber is again placed within a container but in this instance the container is pressurized with helium – and the test chamber is directly connected to the leak detector. A sample of the gas within the chamber is drawn off and passes through a mass spectrometer where again, any increase in helium from the background reading is recorded.
The second pair of procedures are sometimes referred to as the “sniffer” and “spraying” tests. In the “local-sniffer (sample under pressure)” method, the chamber is pressured up with helium and a sniffer device passed around the chamber’s likely leak points (welds, flanges, portals, instrument ducts etc) to suck up any escaping gas. This “sniffed” gas is passed to a mass spectrometer to record any elevated (i.e. above background) helium levels.
In the “local-spraying (sample under vacuum)” method, the chamber is vacuum pumped and helium gas is liberally sprayed/directed towards likely leak points, with the intention that some of this pure helium will be sucked into the chamber. The gas, from within the chamber, is passed into a spectrometer to record any elevated helium levels.
There are several standards relating to leak detectors and leak detection. One of these, DIN EN 1330-8, designates the ‘helium standard leak-rate’ for use where a leak-test is carried out with helium at a pressure differential of 1 bar external atmospheric pressure to < 1 mbar internal pressure (which in practice is common conditions).
The SI unit of a measured leak rate is Pa.m3.s-1. The SI unit of pressure is the Pascal (Pa) where 100 Pa = 1 mbar = 1 hPa. A commonly used unit for leak rate is mbar.l.s-1.
It must be appreciated that there are certain challenges associated with detecting small leaks using helium. HLDs are exceptionally sensitive, and surrounding or entrapped helium can easily impact the accuracy of leak detection and leak measurement. A leak detector is itself not a leak-proof unit, which is why a helium-clean environment is essential, if accurate readings are to be obtained. In addition, for very small leaks, it is important to control external factors as these can easily alter results. Finally, ambient helium can enter the system via exhaust and venting ports, as well as permeate through O-rings.