Such degassing by lowering the pressure or by heating the chamber surface is not unusual, but even then does not totally eliminate all the gas at the surfaces. This desorption starts at a pressure of about 10 -1 mbar. The removal of this surface helium is called “degassing” and starts when all of the gas has been pumped out, once the molecules have been “desorbing” from the inside surface of the metal. The majority of this background helium is contained in between 100 and 150 micro-layers of gas molecules and is permanent gas (contained in the air) that is in the leak detector, pumps, test part etc. This reference reading provides the “background noise” for helium, which can be thought of as the ambient level of helium. The reference (or background) reading for helium is an important part of the process.
Based on the ionization current the leak rate is then calculated. The spectrometer itself works in the following way: any helium molecules sucked into the spectrometer will be ionized, and these helium ions will then “fly” into the ion trap where the ion current is analysed and recorded. The gas from any potential leaks is collected and pumped into the mass spectrometer for analysing, and any value above the background trace of helium is evidence of a leak. Helium detection works in the following way: the unit being checked is either pressurised from within or else pressured from without with helium. Furthermore, helium is chosen as a tracer gas because it is light, very quick and absolutely harmless. This association with helium is one of the reasons why one of the most accurate and rapid leak detection methods employs helium as the tracer gas, and a mass spectrometer for the analysing/measuring. Helium is also non-flammable and generally widely available and low cost. Helium has also relatively low mass so that it is ‘mobile’ and is completely inert/non-reactive. These include the fact that it constitutes only ~ 5 ppm in air so that background levels are very low. Helium is used as a tracer gas to detect leaks for several reasons.
A leak diameter for 1x10 -12 mbar*l/s (which equates to 1Å) is also the diameter of a helium molecule, and is the smallest leak rate that can be detected. The only credible method to detect leaks smaller than 1x10 -6 mbar*l/s is with a helium leak detector. Indeed, what is acceptable at a lower vacuum would be considered utterly unacceptable (and certainly highly dangerous) at a higher vacuum level. Leak detectionĭifferent vacuum processes and applications call for different leak rate requirements. There are two aspects of leak technology worth examining: leak detection and leak measurement. Tightness (or “the absence of leaks”) is required for numerous reasons, including: to ensure and maintain the pressure/vacuum for product safety for environmental standards and for process efficiency. Of course, this is an extreme case, but as vacuum pressures get lower and lower, even the most seemingly secure and pristine of systems will soon show themselves to be less than tight.