The leak detection technique depends on the separation of helium from other gases in a vacuum. It is accomplished by ionizing a gas sample containing helium, pushing the sample through a magnetic field, and collecting the helium ions as they emerge. Since helium ions exit along a different path from all other ions, collection of helium is reasonably simple. The current produced by the helium ion flow is used to drive an ammeter. Often audio alarms, and visual display on leak detection system to give information and warnings about leak levels. Ionization, separation and collection takes place within a spectrometer tube, which is the heart of the system.

Helium is used as a tracer because it penetrates small leaks rapidly. Helium has also the property of being non-toxic, chemically inert, inexpensive to produce, and present in the atmosphere only in minute quantities (5 ppm). Typically a helium leak detector will be used to measure leaks in the range of 10⁻⁵ to 10⁻¹² Pa·m³·s⁻¹.

• A flow of 10⁻⁵ Pa·m³·s⁻¹ is slightly less than 1 ml per minute at Standard conditions for temperature and pressure (STP).
• A flow of 10⁻¹³ Pa·m³·s⁻¹ is slightly less than 3 ml per century at STP.

Typically there are two types of leaks in the detection of helium as a tracer for leak detection.

• Residual leak: A residual leak is a real leak that may be gross, or small, according to the sensitivity setting of the leak detector.
• Virtual leak: A virtual leak is the semblance of a leak in a vacuum system caused by slow release of trapped gases, as gases may adhere as pockets to the interior sides of a chamber. This can cause confusion to the operator as it may be a false indication of a present leak.