What is the detection limit of leak detectors with mass spectrometers

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Detection limit, background, gas storage in oil (gas ballast), floating zero-point suppression

The smallest detectable leak rate is dictated by the natural background level for the test gas to be detected.

Even with the inlet at the leak detector closed, test gas will enter the mass spectrometer and will be detected there if the electronic means are adequate to do so.

The background signal generated in the mass spectrometer determines the detection level of the leak detector.

The high-vacuum pump system used to evacuate the mass spectrometer will normally comprise a turbomolecular pump and an oil-sealed rotary vane pump. Like every liquid, the oil in the rotary vane pump possesses the capability of dissolving gases until equilibrium is reached between the gas dissolved in the oil and the gas outside the oil.

When the pump is warmed up, this equilibrium state represents the detection limit for the leak detector.

However, it is possible for test gas to enter the leak detector not only via the inlet. Improper installation or inept handling of the test gas can allow test gas to enter the interior of the leak detector via the airing and/or gas ballast valve.

This inevitably results in a higher test gas concentration in the oil and the elastomer seals and thus to an increased background signal.

Conclusion: The more test gas is present in the oil the higher the background signal of the leak detector will be.

In case of the nowadays common installation of the leak detector (see Fig. 7 below), gas ballast valve and airing valve are connected to fresh air. The outlet of the leak detector should - if possible - be routed to outside the room where the leak test takes place. An increased background signal can be lowered

again by opening the gas ballast valve and introducing gas which is free of the test gas (e.g. fresh air). The helium which is stored in the oil will be flushed out, so to speak. Since the effect always affects only the part of the oil present in the pump body, the flushing procedure will have to be continued until the entire oil supply of the pump has been recirculated several times. This period of time will usually be 20 to 30 minutes.

Fig. 7: Vacuum diagram of a counterflow leak detector

In case of so-called “dry leak detectors”, which are leak detectors without oil-sealed vacuum pumps, the problem of gas storage in the oil does not exist. However, dry leak detectors must still be flushed with gas which is free of test gas since, over time, test gas will accumulate in these devices as well.

In order to spare the user the trouble of always having to keep an eye on the background level and simplify the operation of the leak detector, the so-called floating zero-point suppression has been integrated into the automatic operating concepts of all Leybold leak detectors. Here the

background level measured after the inlet valve has been closed is stored and then automatically deducted from subsequent measurements when the valve is then opened again. Only at a relatively high threshold level will the display panel show a corresponding warning.

Independent of the floating zero-point suppression, Leybold leak detectors offer the capability for manual zero point shifting. Here the display for the leak detector at the particular moment will be reset to zero so that only rises in the leak rate from that point on will be shown. This serves only to facilitate the evaluation of a display but can, of course, not influence its accuracy.

Fig. 8: Example of zero-point suppression

Above is provided to illustrate the zero-point suppression:

Chart on the left: The signal is clearly larger than the background.
Center chart: The background has risen considerably; the signal can hardly be discerned.
Chart on the right: The background is suppressed electrically; the signal can be clearly identified again.

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