What it finds

  • Through-leaks at welds, seals, fittings and pinholes.
  • The location of a leak, with some techniques.
  • The total leak rate of a whole system, with others.

How it works

SNT-TC-1A's leak testing techniques give a useful map of the family:

  • Bubble testing. Soap solution on a pressurized joint, immersion in liquid, or a vacuum box over a weld, where escaping gas makes bubbles. Cheap and direct.
  • Pressure change testing. Pressurize or evacuate the system, seal it and watch the pressure over time. This measures the whole system, but it does not say where a leak is.
  • Halogen diode testing. A tracer gas containing halogens is put in the system, and a sniffer probe finds it escaping.
  • Mass spectrometer testing. Usually with helium, which is small, inert and rare in air. It can find extremely small leaks, either by sniffing or by placing the part in a vacuum chamber.

Whichever is used, the procedure should define the leak rate that matters, so that the test is sensitive enough without chasing leaks that do not matter.

Where it is used

Leak testing is used on pressure vessels, piping systems and tank floors, heat exchangers, refrigeration and vacuum systems, aerospace propulsion and fluid systems, valves, and sealed packages. See our guides to space, pharmaceutical manufacturing and food and beverage processing.

Limits

  • It shows leaks, not soundness. A deep crack that has not gone through the wall will pass a leak test.
  • Leaks can be plugged. Water, oil, paint or debris can block a path during the test and open up later, so parts need to be clean and dry.
  • Temperature affects pressure. Pressure change results must allow for temperature, or a change in the weather can look like a leak.
  • Tracer background. Trapped or nearby tracer gas can give false readings.
  • Stored energy. Pressurized gas is hazardous. Pressure limits and safe distances are part of the procedure.

Who is allowed to do this work is set by a certification scheme. See How to Become an NDT Level I, II or III.

Frequently asked questions

Is a leak test the same as a pressure test?

No. A hydrostatic or pneumatic pressure test is mainly about strength and is done under code rules. A leak test asks whether anything gets through, and it is usually run at a much lower pressure.

Why use helium?

Helium is inert and safe, its small atoms pass through tiny paths, and very little of it is in normal air, so a mass spectrometer can find small amounts.

Which technique is the most sensitive?

Mass spectrometer testing is usually the most sensitive, but it is also more involved. Pick the simplest technique that can detect the leak rate that matters.

How NDT LLC can help

Our team of Level IIIs helps with leak testing procedures and technique selection. Contact us.

This article is general information for a business audience, not engineering, legal or regulatory advice. Always work to the current edition of the codes, standards, regulations and manufacturer data that apply to your equipment and contracts.

NDT LLC is an independent consulting firm. It is not affiliated with, endorsed by or sponsored by ASNT, ASME, ASTM International, or any other organization named in this article; names are used only to identify the organizations and documents referred to.