Leak testing is a test for measuring how much the object leaks relative to the leak specification. The leak testing can be considered to answer the question ”How much is it leaking?” But most of all to answer the following question ”Does it leak more or less than the current leak specification?” A leak test measures the size of the total leakage and may not find out how many leaks there are or where these leaks are located.
What is a Leak?
A leak, also referred to as leakage, enables a substance to flow toward a pressure gradient. Expressed in simpler terms, leaks are small holes through which gases or liquids flow from the side of higher pressure to the side of lower pressure. This can involve simple, harmless leaks, such as a dripping water faucet, or hazardous toxic substances that escape through leaks.
Any number of technical products will not function, or will not function for an adequate period of time, if they have leaks. Examples include: The refrigerant circulation system in refrigerators, air conditioning systems in cars, automobile tires, automotive fuel tanks or home fuel oil tanks, as well as distillation systems in the chemical or pharmaceutical industries. In many cases, the leak-tightness of machines and systems in the production process is an indispensable prerequisite for the quality of the manufactured products.
Returning to the original definition of a leak, we thus find that it is impossible to completely prevent substances from flowing through a wall. The term „tight” therefore refers to the requirements of the respective machine, plant or vessel, and must be quantified accordingly.
Units used within leak testing
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.
Techniques used within leak testing
The key questions at the start of any leak test requirement are the following:
- What size is the component and what is its internal volume?
- What is the leak limit?
- Does it have hidden internal volumes that may affect leak measurements?
- Is there access to inside or is it a sealed unit?
- Is it rigid or flexible?
- Are the parts clean and dry?
- Are parts at ambient temperature?
- What is the surface finish of any sealing surfaces?
Based on the acceptable leak rate limit (shown in ml/sec) the following test methods can be used:
|Flow rate||5 up to and including 10-1 ml/sec|
|Air decay||5 up to and including 10-2 ml/sec|
|Tracer gas||10-3 up to and including 10-5 ml/sec|
|High vacuum Helium||10-4 up to and including 10-9 ml/sec|
General systems within leak testing
Pressure / Vacuum Systems
The test piece and the reference volume are simultaneously pressurised (or evacuated) to a pre-set pressure. The air in the system is then allowed to stabilise, with the supply valves all closed. The Differential Pressure Transducer is automatically zeroed.
After this stabilisation time, the pressure change in the test piece is compared to the pressure change in the reference volume, using the Transducer. If the test piece is leaking, the difference will increase and be measured, an alarm limit may be set for a pass/fail decision.
The sequence is fully automatic, the accuracy and sensitivity of the system is defined by the method of setting the pre-set pressure together with the quality and type of control valves and Differential Pressure Transducer.
A vacuum pump evacuates the test chamber and test piece simultaneously to a pre-set vacuum. At this pre-set level, the chamber and the test piece are isolated and the chamber evacuated further to a very low pressure. A positive pressure variation is therefore created between the test piece and the chamber.
Helium gas is then introduced into the test piece, often in a 10% concentration. A Mass Spectrometer analyses a sample from the chamber as the vacuum continues to be drawn. The Mass Spectrometer measures the helium leakage and sets the pass/fail decision.
The test piece pressure is often compared to the chamber pressure before dosing with helium, to avoid saturating the Mass Spectrometer in the event of a gross leak.