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Leak testing is employed todetect leaks and determine the rate at which a fluid i.e. a liquid or gas willpenetrate from inside a tight component or assembly to the outside or viceversa as a result of pressure differential between the two regions or ofpermeation of somewhat extended barrier. It has become conventional to use theterm âleakâ to refer to an actual discontinuity or passage through which afluid flows or permeates. âLeakageâ refers to the fluid that has flowed througha leak. âLeak rateâ refers to the rate of fluid flow per unit of time under agiven set of conditions, and is properly expressed in units of mss per unit oftime. Standard leak rate refers to the rate of flow o atmospheric air underconditions in which inlet pressure is 0.1 Mpa Â±5%; outlet pressure is leass tha1 Kpa; temperature is 298KÂ±5 and dew point is less than -298K.
The term âminimum detectableleakâ refers to the smallest hole or discrete passage that can be detected andâminimum detectable leak rateâ refers to the smallest detectable fluid flowrate. The amount of leakage required for a leak testing instrument to give aminimum detectable signal can be determined. This amount is generally used todenote the sensitivity of the instrument. Instrument sensitivity is independentof test conditions, but when an instrument is applied to a test, thesensitivity of the test depends on the existing conditions of pressure,temperature and fluid.
Mass Spectrometer based HeliumLeak Detection
For very sensitive leaktesting, Mass Spectrometer based Helium Leak Detector is used. The commercialhelium leak detector can detect the presence of less than one part of helium in10 million parts of air. The helium leak detector is portable mass spectrometerespecially designed to be highly sensitive to helium gas. It is an instrumentfor separating and sorting of atoms of different mass. Gas molecules enteringthe mass spectrometer are bombarded by electron emitted from a heated filament.The ion beam produced by the electron bombardment is accelerated in the form ofnarrow beam by means of an electric field. The ions then pass between thepoles= pieces of a permanent magnet. The magnetic field deflects the ions incircular path. The radius of curvature of the path depends upon the mass of theion. Ions having equal mass will all emerge from the magnetic field at acertain position. The helium leak detector is adjusted so that only the heliumions are collected. The flow of helium ions to the collector
inducesa minute electrical current which can be detected, amplified and used toactivate an electrical meter to control the pitch of an audio signal generator.
Helium is generally used forleak detection because it is an inert gas and does not react with other gasesand materials in the system. Helium is not present in any significant quantityin atmosphere, thus causing little interference in sensitive leak detectionwork. Helium, having light mass, passes through small leaks more readily ascompared to other heavier gases.
There are five differenttechniques in using helium leak detector. They are:-
2.Envelope Vacuum Technique
4.Envelope Pressure Technique
5. Pressurization Technique
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