Leak detection is the way to find out where a leak is located. The common feature of methods used to detect leaks is that they are almost always operator-dependent, require strict supervision and are often very messy. The methods listed below will indicate a leak condition, but most of them will not accurately quantify the degree of leakage.
Components are filled with pressurised gas and immersed in a liquid. Typically they are filled with air and immersed in water, but nitrogen under acetone is also used. The most common observation technique is to look for a bubble stream. A variation of the bubble-stream method is to use sealed components submerged in a liquid within an enclosed volume; a vacuum is created at the surface of the liquid to draw the fluid into the closed device. This method is often used to soak-test parts over a prolonged period and then to visually check for condensation inside transparent parts. It is also used to check the weight of parts to ensure that product, that is, the powder, gas or liquid inside the device, has not been forced out and vapour/liquid has not been forced inside.
Chemicals are added to the working media within an assembly prior to some form of functional test. If they leak onto the surface of the component, they can be readily seen when viewed using ultraviolet light.
Chemical is sprayed onto one side of a component and by capillary action emerges on the opposite surface. This technique is also referred to as dye penetration. The difference between chemical trace and dye penetration is that the former is a trace substance in the fluid whereas the latter is coated onto the surface of the container.
Components or assemblies are filled or injected with an easily identifiable gas to create a pressure differential. Helium, hydrogen and sulphur hexafluoride (SF6) gases are employed and an operator searches for concentrations of gas at the surface of the piece being tested. Helium is the most commonly used. SF6 is generally avoided whenever possible because of its toxicity, but it is mandatory for some electrical equipment because of its flame-suppression properties. For sealed assemblies such as blister packs or swallowable cameras, the item must be closed in an environment containing the tracer gas.
Because they are noisy, large gas flows can be found using an ultrasonic detector. This method is suited to finding large leaks, but it is not recommended for the fine leaks in a production environment. When using ultrasonic testing, sources of misleading indications and other problems can include low test pressures and capillary action sealing, high-pressure situations and component damage, contaminated liquids, and preferential leeching and rogue surface tensions.