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    Leak testing method is needed to check fabricated components and systems for nuclear reactors, pressure vessels, electronic valves, vacuum   equipments, gas containers at pressures etc. Leak testing is employed to detect leaks and determine the rate at which a liquid or gas will penetrate from  inside a tight component, or assembly to the outside, or vice versa, as a result of pressure differential between the two regions.

Leak test methods are employed either as 'proving tests' i.e. to determine the job leaks or not and/or as 'location tests' to pin point the location of leak.

Depending upon the range of leak detection capability, a number of test methods are available.

Leak detection may be carried out by visual inspection using soap bubbles or mass spectrometers etc. Among the various methods of detection available, bubble testing is widely used for less sensitive applications and mass spectrometer is used for high sensitive detection.


 Applications - The technique can most advantageously be applied for checking the integrity of pressure vessels, valves, instruments, piping circuits,  containments, condensers, heat exchanges, pumps, cylinders etc.

  Bubble forming solution is applied to the surface of a pressurized vessel if it is too large or not possible to submerge. The ideal liquid for bubble testing    should have low surface tension and low viscosity. The bubble size depends on the viscosity of the liquid, pressure and diameter of leak. Sometimes,     vibration or hammering of the pressurized component under test is applied for enhanced detection sensitivity.

     The best combination of pressurizing medium is hydrogen (has a fast flow through small leaks) & ether (has a low surface tension). Methanol & helium       are better than hydrogen, and can be used for detection of smaller leaks.

       Advantages - The advantages of bubble testing are that it is inexpensive, can be carried out by relatively less experienced personnel, is rapid, gives         accurate location of leak, and the whole specimen is inspected simultaneously.

          Limitations - The technique cannot locate very small leaks. In some cases, leaks have been known to pass gas in one direction only, and if this is           inward, the bubble technique will not locate such leaks.


         For very sensitive leak testing, mass spectrometer based helium leak detector is used. The commercial helium leak detector can detect the presence          of less than one part of helium in 10 million parts of air. The helium leak detector is a portable mass-spectrometer especially designed to be highly          sensitive to helium gas. A mass spectrometer is an instrument for separating or sorting atoms of different mass. The helium leak detector is adjusted          so  that only helium ions are collected. Helium is generally used for leak detection because it is an inert gas and does not react with other gases and          materials in the system. Helium, having a light mass, passes through small leaks more readily as compared to heavier gases.

        There are five different techniques in using helium leak detector. They are -

  1. Probe technique
  2. Envelope Vacuum technique
  3. Sniffer technique
  4. Envelope pressure technique
  5. Pressurization technique

In all these techniques, it is necessary to have clean test specimens/components since dirt, moisture, scale and oil may easily seal comparatively large leaks.

  1. Probe technique - In this technique, a fine jet of helium such as that obtained from a hypodermic needle, is passed over the exterior surface of the specimen. Helium gas will be drawn into any opening through the walls of the specimen and register on the leak detector as a visible or audible indication.
  2. Envelope vacuum technique - Sometimes, it is desired to determine only the presence of leaks or the total magnitude of all the leaks. In such a case, the specimen containing helium air mixture is put into an evacuated chamber. The chamber is evacuated using an auxiliary pump and the pump outlet is connected to the leak detector for measuring the leak.
  3. Sniffer technique - In this technique, the specimen to be tested is filled with helium or a mixture of helium and air to a pressure greater than atmosphere. The surface of the test object is then scanned with a "sniffer" connected to the leak detector. Helium flowing out through any opening will be sucked into the leak detector system by the sniffer and the leak rate is indicated.
  4. Envelope Pressure technique - In this technique, the test system is surrounded by a hood containing helium. The test system is then evacuated. Helium will flow through any leaks into the evacuated test system and then to the leak detector. This technique gives the overall leak rate of the component.
  5. Pressurization technique - In applying this technique, the component is first placed in a helium pressurization vessel and exposed to a helium atmosphere. The component is removed from the pressurization vessel and transferred to a second chamber which is connected to a vacuum pump and helium leak detector.


  • ASTM E 493 - Test methods for leaks using mass spectrometer leak detector in the inside-outside testing mode.
  • ASTM E 1003 - Methods of testing for hydrostatic leak testing
  • ASTM E 515 - Methods of testing for leaks using bubble emission technique
  • ASTM E 499 - Methods of testing for Leaks using the mass spectrometer leak detector in the detector probe method
  • ASTM E 498 - Methods of testing for leaks using the mass spectrometer or residual gas analyzer in the tracer probe mode.
  • ASTM E 1002 - Methods of testing for leaks using ultrasonics
  • ASTM E 908 - Practices for calibrating leaks, gaseous reference
  • ASTM E 432 - Practices for guide for selection of leak testing methods
  • ASTM E 479 - Practices for guide for preparation of a leak testing specification.
  • ASTM E 425 - Definitions of terms relating to leak testing


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