Helium leak detection: local and integral testing explained

Helium leak testing uses helium as a tracer gas to find or measure leakage. A detector identifies helium that has passed through a leak path under controlled conditions.

First establish what the test needs to do. Locating a leaking joint and measuring the combined leakage of an assembly are different tasks.

Why helium is used

Helium is chemically inert and can be detected selectively with a suitable mass spectrometer. Its relatively low natural concentration in air helps distinguish a test signal from the background.

A meaningful leak-rate result depends on the arrangement, calibration and test conditions. The detector does not directly measure the physical size of a hole.

Local testing finds the position

In the vacuum spray method, the component is evacuated and connected to a suitable detector arrangement. A controlled helium flow is applied around suspected locations such as joints and welds. Helium passing into the evacuated space produces a response; working systematically helps locate the source.

A sniffer test reverses the direction. A component containing helium at a specified positive pressure is examined externally with a sampling probe. The equipment and fixtures must be rated for the intended pressure. A vessel designed for vacuum should not be assumed suitable for internal pressurisation.

Integral testing measures combined leakage

An integral test exposes a defined boundary to helium, or collects helium escaping from it, so combined leakage can be assessed. An evacuated component may be enclosed in a controlled helium atmosphere. Alternatively, gas escaping from a pressurised component may be collected in a surrounding enclosure.

The result can establish whether the tested assembly meets an overall requirement, but generally does not identify the individual joint responsible.

Give the system time to respond

Gas takes time to travel through the component and connecting pipework. Volume, conductance and pumping arrangements influence the delay. Moving too quickly can miss a leak or associate its response with the wrong location.

Helium already admitted must also clear sufficiently before later results can be interpreted. Account for the complete arrangement’s response, not just the detector specification.

More helium is not always helpful

Excess helium raises background and may linger in enclosures or be retained by materials. Controlled application helps preserve a clear relationship between the tested location and the response.

Check background before testing and verify detector response with an appropriate calibrated reference leak, following the test procedure. If auxiliary pumps remove some helium without it passing through the detector, account for that division when determining leakage quantitatively.

Agree the acceptance conditions

Specify the tested boundary, permitted leak rate and units, helium concentration, test pressures, method and response time. Establish how calibration and background will be checked.

A helium result obtained under one pressure difference is not automatically the leakage of another gas or liquid in service. Conversion needs an appropriate technical basis.

For an enquiry, send Girovac a drawing or photographs, assembly volume, normal operating conditions and leakage requirement. State whether the immediate need is to locate a fault or verify an acceptance limit.