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Leak Testing
The basic principals

What is a Leak?

A leak is a flow of gas (or liquid) through the wall of a vessel (via an imperfection such as a hole, crack or bad seal). Leaks require a pressure difference to generate the flow; they always go from higher pressure to lower pressure. Leaks are pictured as going from positive pressure (inside an object) to outside (at atmospheric pressure). This is not always the case (a leak could be from atmosphere to inside an evacuated object), but it helps to think about it this way because the units and terminology are based on this model.

What units do you use?

For leaks of air into atmosphere, units are usually expressed as mm3 or cm3 (cc) per second or minute. So 16.6 mm3/sec = 1 cm3/min. A bubble under water is about 30 - 50 mm3, so 1 bubble per second is about 30 mm3/sec or 2 cm3/min. A standard unit of leakage which takes account of air pressure is the mbarl/sec. (Millibar-litre per second). A leak into atmosphere of 1 mbarl/sec is equivalent to a volume leak of 1000 mm3/sec.

What technique should be used?

Key questions at the start of any leak test requirement are:-

What size is the component and what is itís internal volume?
Is there access to inside or is it a sealed unit?
What is the leak limit?
Is it rigid or flexible?
Does it have hidden internal volumes that may affect leak measurements?
Are parts at ambient temperature?
Are the parts clean and dry?
What is the surface finish of any sealing surfaces?

Based on the acceptable leak rate limit (shown in ml/sec) alone the following test method can be used


Tracer Gas


Flow Rate


Water Submersal / Dunk Tank

High Vacuum Helium


Air Decay


10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 5



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