The smallest detectable leak rate is dictated by the natural background level for the gas to be detected. Even with the test connector at the leak detector closed, every gas will pass – counter to the pumping direction – through the exhaust and through the pumps (but will be reduced accordingly by their compression) through to the spectrometer and will be detected there if the electronic means are adequate to do so. The signal generated represents the detection limit. The high vacuum system used to evacuate the mass spectrometer will normally comprise a turbomolecular pump and an oil-sealed rotary vane pump. (Diffusion pumps were used earlier instead of the turbomolecular pumps.) Like every liquid, the sealing oil in the rotary vane pump has 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 warm (operating temperature) this equilibrium state represents the detection limit for the leak detector. The helium stored in the oil thus influences the detection limit for the leak detector. It is possible for test gas to enter not only through the test connection and into the leak detector; improper installation or inept handling of the test gas can allow test gas to enter through the exhaust and the airing or gas ballast valve and into the interior of the detector, to increase the helium level in the oil and the elastomer seals there and thus to induce a background signal in the mass spectrometer which is well above the normal detection limit. When the device is properly installed (see Fig. 5.7) the gas ballast valve and the airing valve will be connected to fresh air and the discharge line (oil filter!) should at least be routed to outside the room where the leak test takes place.
An increased test gas (helium) background level can be lowered by opening the gas ballast valve and introducing gas which is free of the test gas (helium-free gas, fresh air). The dissolved helium will be flushed out, so to speak. Since the effect always affects only the part of the oil present in the pump body at the particular moment, the flushing procedure will have to be continued until all the oil from the pump’s oil pan has been recirculated several times. This period of time will usually be 20 to 30 minutes.
In order to spare the user the trouble of always having to keep an eye on the background level, what has been dubbed floating zero-point suppression has been integrated into the automatic operating concepts of some detectors (See the section on 180° sector mass spectrometers on the calibration page). Here the background level measured after the inlet valve has been closed is placed in storage; when the valve is then opened again this value will automatically be deducted from subsequent measurements. Only at a relatively high threshold level will the display panel show a warning indicating that the background noise level is too high. Figure 5.8 is provided to illustrate the process followed in 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 again be clearly identified.
Independent of this floating zero-point suppression, all the 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.
Modern leak detectors are being more frequently equipped with oil-free vacuum systems, the so-called “dry leak detectors” (UL 200 dry, UL 500 dry). Here the problem of gas being dissolved in oil does not occur, but similar purging techniques will nonetheless be employed.
This equation shows that it is by no means advantageous to attain high sensitivity by throttling down the pumping speed. The maximum permissible throughput would otherwise be too small. The unit is not functional when – either due to one large leak or several smaller leaks – more gas flows into the unit than the maximum permissible throughput rate for the leak detector.