Production Leak Detection Systems
Amount of Leak Methods
Except for the Mass Flow Method, all Amount of Leak methods involve the use of Mass Spectrometry.
This section will overview Mass Spectrometry, and explore the five different mass spectrometry related leak testing methods offered by FTI: Hard Vacuum Inside/Out, Hard Vacuum Outside/In, Helium Accumulation, Carrier Gas and RGA .
Mass Spectrometry Overview:
The "heart" of methods using Mass Spectrometry is the mass spec unit itself, which is capable of detecting extremely small amounts of Helium (or other tracer gases). To do so, the gas entering the mass spectrometer tube is ionized and accelerated. Exposing charged particles moving at high speed to a magnetic field perpendicular to the velocity results in a force enacting on the particles which is perpendicular to both the velocity and the magnetic field, forcing the particles on a curved track.
The radius of this track depends on the mass of the particle in question, thus allowing the separation of different ions. By placing a collector plate (ion detector) in the right location, the concentration of any gas can be very accurately measured.
Helium, when released through a leaking part is offered a "torturous path" to reach the mass spec unit. The helium actually passes into the mass spec tube backwards, or against the flow of a vacuum pump. This process is called counter-flow or contra-flow. Vacuum pumps are very efficient at pumping large atoms and molecules (water and hydrocarbon vapors, oxygen, carbon dioxide, etc.), but inefficient at pumping helium. This allows a greater proportion of Helium than the larger atoms or molecules to reach the mass spec unit for measurement.
Upon reaching the mass spec unit, the gases are exposed to a filament that ionizes them (gives them an electrical charge). A repeller then accelerates all the ions. The ions travel through a magnetic field that deflects them according to their mass, separating them into a spectrum. This allows the Helium ions to be isolated and detected. A collector plate is located in the position where the Helium ions are calculated to arrive. For every electron given up by the collector plate, one Helium ion is present. The amount of Helium collected is then calibrated to correspond with the desired leak rate.
A standard leak, calibrated to the desired leak rate, is placed on the test chamber. The standard leak emits Helium, through a membrane, into the test chamber. The mass spec is calibrated to this leak rate as the reject level for the test part. To ensure that the machine is not giving false readings due to a problem with the machine (such as a plumbing blockage or faulty valves), an amount of Helium is introduced into the system (called background) at a level equal to 20% of the desired leak rate. The mass spec unit must measure this background on every test, or the machine faults for a low background level. This provides a means of checking the system on every cycle.
Mass spec leak testing requires that the mass spec unit is exposed to the Helium leaking from the test part for approximately 3-4 seconds. Complete cycle time also includes pump down time, machine cycle time (including sealing and other mechanical movements), and loading and unloading of the test part.
Mass spec leak testing machines from FTI offer flexibility in that, the leak rate being tested for can be changed simply by changing the calibrated standard leak, and re-calibrating the mass spec for the new leak rate. However, this is NOT applicable for all leak rates.
Click on the links below to learn more about each technique: