A leak is the result of a fault or defect in the wall of an object through which gases or liquids can pass from one side of the wall to the other. This defect can be caused by a hole, a porosity, a permeable element, improper assembly or another manufacturing defect. Every mechanical object has such defects - there is no such thing as an "absolutely tight" object. Tightness refers to the operational requirements of a final assembly, a specific system or component where no liquid leakage is allowed or a defined maximum gas leakage is permitted.

Tightness criteria must be individually defined and quantified for each requirement. The transport of a medium through a wall can be represented by a "leak rate". It is defined as the flow rate of a given liquid or gas through a leak under specified conditions. The driving forces for medium transport can be pressure gradients or concentration differences across the wall.

Once it is clear that my system is leaking, the leak needs to be found. Unlike a bicycle tire, which you can dip in a water bath and see where the leak is by the rising bubbles (also known as the baleen test), this is difficult to do with a large system.

Other location methods include sniffing and spraying helium or other tracer gases. Which of these methods is used depends on the original purpose of the system. The golden rule of leak detection is to operate a system as close to actual conditions as possible. Therefore, if the pressure in the system is lower than the ambient pressure, air will be drawn into the system - helium injection is recommended. 

If the pressure in the system is higher than the ambient pressure, the gas would escape through a leak. In this case, helium purging should be used.

Velocity is determined by multiplying the pressure change by the volume. To calculate the leak rate (SCCM), this number is divided by the change over time multiplied by the ambient atmospheric pressure. Δt = decay time (min)

A modern helium and/or hydrogen test gas leak detector works on the principle of mass separation by different deflection radii of a charged (ionized) particle of different mass in a magnetic field. Therefore, a so-called sector field mass spectrometer is used to separate helium (or hydrogen) atoms from other gases.
To achieve the right working conditions for the sector field mass spectrometer, a vacuum system with a backing pump and a turbomolecular pump is required. This pump group is used to provide the operating pressure for the mass spectrometer, but the turbopump also protects the analyzer cell (mass spectrometer). For this purpose, the leak detector has different modes with different inlets on the turbopump. While the coarse leak mode is connected to the bottom of the turbopump, the high-sense mode is connected to the top of the pump. This ensures that the analyzer is protected from a large helium flow in the event of a large leak, but still has the sensitivity needed to detect small leaks. This so-called counterflow principle was developed by Pfeiffer Vacuum in the 1960s and shows why the turbopump is so important for a helium (or hydrogen) leak detector.

All of us have had to deal with a simple leak at one time or another. A leaking bicycle or car tire is a good example of how dramatically a leak can affect the performance of a product and how important it is to diagnose and repair it.

The demands of high-tech industries and modern manufacturing have led to a high degree of specialization in leak detection. Today, we must be able to find leaks on high-speed production lines, in harsh environments, or at microscopic sizes.

No two leaks are alike, so leak detection requires a wide range of solutions and years of practical experience. Pfeiffer Vacuum excels in the use of helium and hydrogen as tracer gases and offers various detection techniques for all applications.