The theory of leak detection derives from the physical gas laws. Important parameters are the pressure in the vessel, the temperature and changes in temperature, the volume of the vessel and ultimately the vacuum pumping on the system. Types of leaks include:

• Leaks in detachable connections
• Leaks in permanent connections such a brazed or welded joints
• Porosity leaks of the material
• Leaks due to thermal changes
• Virtual Leaks due to gas liberated from the hollows cavities inside cast parts

For a full technical theoretical explanation of the physical science aspects of leak detection, below is a link to the leak detection section from the Fundamentals of Vacuum Technology, Leybold Vacuum 2001/2002

Leak Detection Fundamentals from Fundamentals of Vacuum Technology

The span of leak detection methods are as wide as the sizes of leaks out there. A large leak you can hear with your ear or with listening devices. The smallest leak a human fingerprint could cover or mask for a period of time. One common type of leak detection method is pressurizing a part then placing the part underwater to see if a bubble forms. Even then the size leak that can be detected  relies on the pressure inside the object and the patience of the person watching for the bubbles (for a very large leak) or a single bubble for a  large leak. Bubble testing becomes impractical for anything other than a large leak., The advantage is that the process is relatively simple and cheap.

For production operations, vacuum decay or pressure decay become effective and cost effective. These methods involve measurement over time the loss of vacuum in rise in pressure or pressure decay where pressure is lost over time. Using a Pressure Decay System, a differential pressure sensor can be used to measure small amounts of pressure change.

The method for finding much smaller leaks require investment in equipment. These methods then move into either sniffing or vacuum techniques. Sniffing is a inside-out technology. measuring the leak rate emitting from the tested object. Sniffing uses a tracer gas either helium or hydrogen gas. In recent years, there has been a move to a 5% hydrogen/95% nitrogen tracer gas. Benefit of using only 5% hydrogen is that 5% is below the combustible or flammability level of hydrogen in nitrogen. Thus the tracer gas is a green label non-combustible gas. Helium has become more expensive and helium leak detectors more complex than their hydrogen counterparts.

Vacuum leak detection is an inside-out process where helium is sprayed outside the vessel and helium sucked into the vessel is detected and measured by the helium leak detector. Vacuum leak detection can find the smallest leaks. Draw back is the cost of the system compared to other technologies.

The chart below details the most common leak detection methods, the smallest leak the technology can detect, the pressure range and whether the method quantitative. Keep in mind that detecting a leak is one thing but locating the leak with some technologies is not easy.