Helium mass spectrometer
From Wikipedia, the free encyclopedia
A helium mass spectrometer (often called a leak detector) or sniffer, is a scientific instrument, used to detect very small leaks, typically using a vacuum and injecting helium around a chamber or cavity.
The helium mass spectrometer was initially developed in the Manhattan Project during World War II to find extremely small leaks in the gas diffusion process.
The leak detection method uses helium (the lightest inert gas) as a tracer and detects it in concentrations as small as one part in 10 million. The helium is selected primarily because it penetrates small leaks readily. Helium is also non-toxic, non-hazardous, plentiful, inexpensive, and present in the atmosphere only in minute quantities (5 ppm).
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[edit] Process
The helium mass spectrometer leak detection technique depends on the separation of helium from other gasses in a vacuum, which is a relatively simple procedure. It is accomplished by imparting an electrical charge to a sample of gas, 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; and because they have a charge of electrical energy, helium ions can be counted. The current is used to drive a meter, actuate an optional audio alarm, and illuminate a display. Ionization, separation and collection takes place within the spectrometer tube, which is the heart of the system.
[edit] Internal spectrometer tube operation
In the spectrometer tube, the heart of the helium mass spectrometer, the electrons produced by a hot filament enter an ion chamber under vacuum, and collide with gas molecules, creating within the chamber ions quantitively proportional to the pressure in the ion chamber. These ions are repelled out of the ion chamber, under vacuum, through the exit slit, by a repeller field. The combined electrostatic effect of the repeller, exit slit, focus plates, and ground slit collimates the ion beam so that it enters the magnetic field as a straight "ribbon" of ions.
[edit] Types of leaks
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 gasses, as gasses 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.
[edit] Uses
Helium mass spectrometer leak detectors are used in production line industries such as refrigeration and air conditioning, automotive parts, carbonated beverage, and aerosol packaging, as well as in the manufacture of steam products, gas bottles, fire extinguishers, tire valves, and numerous other products including all vacuum systems.
