Leak Testing: Choosing the Right Method for the Required Sensitivity

Leak testing establishes whether a pressure boundary holds and, where it does not, the location and size of the breach. The available methods span roughly six orders of magnitude in sensitivity, so selecting the right one matters more than performing it well: a test far more sensitive than the specification requires wastes time and money, and one less sensitive gives a pass that means nothing. itmad provides leak testing to ASME V Article 10 and ASTM standards. itmad operates across the Middle East, Asia and Europe. Last reviewed: October 2026.

Methods

In bubble emission by direct pressure, the item is pressurised and either immersed or coated with a surfactant solution, and bubbles form at a through-wall defect, showing both its presence and its exact location. It is fast, low cost and well suited to weld seams, fittings and flanges. Where an item cannot be pressurised, bubble emission by vacuum box places a box over a wetted surface and evacuates it, which suits tank floors, roof plates and weld seams accessible from one side only.

Pressure decay pressurises the system, isolates it and monitors it over a defined hold period, with temperature compensation, which establishes overall integrity but does not locate the leak. Vacuum decay is the reverse, used where pressurising is impractical or unsafe, and suits closed vessels, jacketed equipment and systems that normally operate under vacuum. In the fluorescent dye method, a fluorescent tracer is introduced into the system and the external surface is examined under UV-A, which is effective for small leaks in complex geometry and in hydraulic, fuel and lubrication systems where a slow weep would otherwise take days to appear. Tracer gas detection introduces helium or hydrogen mixtures on one side and detects them on the other, orders of magnitude more sensitive than bubble methods and used where the consequence of a small leak is severe.

Testing alongside hydrostatic and pneumatic tests

Leak testing is often carried out together with a hydrostatic or pneumatic strength test, but the two answer different questions. A hydrostatic test proves that the pressure boundary can withstand the applied load, but it does not reliably detect small leaks, because the surface tension of water holds back defects that will pass gas in service. Where a specification requires both, the leak test is normally performed at a lower pressure, with a method matched to the requirement.

Selecting the method

Selection depends on the acceptable leak rate in the specification, on whether the item can be pressurised or evacuated and whether it can be wetted, on access to both sides of the boundary, and on the consequence of the fluid escaping in service. Where the specification names a method, itmad applies it. Where it states only an acceptance criterion, itmad selects a technique capable of resolving it and records the justification, because a test that cannot detect a leak of the required size is worse than no test.

Standards applied

ASME Boiler and Pressure Vessel Code Section V, Article 10, covers leak testing, with acceptance to the relevant construction code section. ASTM E432 covers the selection of a leak testing method, ASTM E515 bubble emission techniques and ASTM E1003 hydrostatic leak testing, and ISO 20485 covers the tracer gas method. Client, project and manufacturer specifications, and in-house procedures, apply where they govern.

Where it applies

Pressure vessels, tanks and storage systems; pipework, pipelines and spools; valves, flanges and fittings; heat exchangers and condensers; hydraulic, fuel and lubrication systems; fabrication and pre-shipment verification; and post-repair confirmation after weld repair or component replacement.

Related services

Where the required sensitivity exceeds what bubble or decay methods can resolve, helium mass spectrometry is covered separately, as are tube-by-tube leak location in heat exchangers and vacuum box testing of tank floors and weld seams.

What the client receives

A report identifying the item tested, the method applied and the test parameters, with the hold period, pressure and temperature compensation where a decay method was used. The location of each indication found is recorded with photographic evidence, the findings are assessed against the acceptance criteria in the specification, and the report includes the technician's qualification and the instrument calibration traceability.

Frequently asked questions

What does leak testing establish?

Whether a pressure boundary holds and, where it does not, the location and size of the breach. The methods span roughly six orders of magnitude in sensitivity, so selecting the right one matters.

Which leak testing methods are available?

Bubble emission by direct pressure or vacuum box, pressure decay, vacuum decay, fluorescent dye and tracer gas detection. Helium or hydrogen tracer gas is orders of magnitude more sensitive than bubble methods.

Does a hydrostatic test find small leaks?

Not reliably. A hydrostatic test proves that the boundary can withstand the applied load, but the surface tension of water holds back defects that will pass gas in service, so a separate leak test is used.

How is the method selected?

From the acceptable leak rate, whether the item can be pressurised, evacuated or wetted, access to both sides of the boundary, and the consequence of a leak in service. Where only an acceptance criterion is given, itmad selects a capable technique and records the justification.

Which standards apply?

ASME Section V Article 10, ASTM E432, E515 and E1003, and ISO 20485 for tracer gas, with client and manufacturer specifications where they govern. Acceptance is to the relevant construction code section.

What is the difference between pressure decay and vacuum box testing?

Pressure decay establishes overall integrity over a hold period but does not locate the leak. Vacuum box testing finds the exact location of leaks in surfaces accessible from one side only, such as tank floors and weld seams.