The Fundamentals of Rough & Medium Vacuum
Screw pumps, which have a range of 103 to 10-2 mbar, operate using two counter-rotating screw rotors which are engineered so that they rotate “in on each other”, thereby trapping the gas in the volume between the “screws” of their rotors. As the screws rotate, this trapped volume (as it travels towards the exit port) decreases which not only compresses the gas but moves it towards the exit. Screw pumps are frequently used as fore pumps for Roots pumps.
Roots pumps, which have a pressure range of 10 to 10-4 mbar, are commonly employed as ‘booster’ pumps to improve ultimate pressure and pumping speeds. Roots pumps employ two counter-rotating interconnecting units rotating within a chamber. Gas enters through the intake flange and is “pinched” between the two rapidly rotating units and the chamber wall and is then expelled through the exit port.
The advantages of Roots booster pumps are that they are very quiet and compact, enjoy a long service life, have no contacting parts, and they provide clean pumping (i.e. there are no particles or oils to contaminate the vacuum system).
A multi-Roots fore pump working in conjunction with a HV, UHV or XHV pump is typically a more economical option for achieving high vacuum, compared to a bigger sized discrete fore pump due to its improved pumping speeds and ultimate pressures.
Rough and medium vacuums are usually measured by what are referred to as “direct gauges”, which measure pressure irrespective/independent to the composition of the gases involved.
Direct gauges fall into two categories: those that rely upon some form of mechanical deformation, such as the diaphragm, the Bourdon tube, the piezo resistance or electrical capacitance; and those that employ the height of a liquid column, which are known as “hydrostatic” gauges.
Mechanical gauges employ metallic internal workings which change their shape depending on pressure, with this deflection linked to a needle-gauge. A variation of these is the capacitance manometer, in which the diaphragm (which makes up part of the capacitor) flexes with pressure change, resulting in a (measurable) capacitance change.
The detection of leaks, as well as their elimination, management and/or accountability, are as important in vacuum as they are in pressurized systems. As gas is compressible, the pressure (or vacuum) influences the amount of the leak which is quoted in mbar.liter/sec, with the leak rate being the amount of gas that “out-flows” through a leak in a given pressure differential per unit-time.
There are several generalized ways of measuring leaks, each of which depends on the lowest detectable leak rate applicable: the bubble test; differential pressure measurement; pressure decay; pressure rise tests; the helium sniffer mode; and, the helium vacuum mode. These two latter test methods are also referred to as the “tracer gas detection” methods. All methods can be used in rough and medium vacuum.
The bubble test involves pressurizing the system, smearing a potential leak point with soap and seeing if it froths, whilst the differential pressure measurement involves gauging the loss of pressure over a set period of time.
However, the most interesting leak tests involve the helium “sniffer” and the helium “vacuum” test.
In simple terms the helium “sniffer” test involves a sniffer probe being passed around the unit under observation with the “sniffed” gas being passed through a mass spectrometer, for helium identification and measurement.
The sniffer test has the advantage that it shows where leaks actually occur. However, helium concentrations of 5ppm in ambient/atmospheric air mean that it is difficult to differentiate between a background signal and a very low leak rate.
The helium “vacuum” test is usually employed on units subjected to HV and UHV applications. In simple terms, the unit is placed inside a vessel, and pressurized with helium. Gas within the vessel is then subjected to a mass spectrometer test, and any helium detected will indicate a leak. The major disadvantage—though not the only one—is that the unit needs to be placed within a vessel of a suitable size. Alternatively, the vessel is evacuated by the leak detector and helium used to ‘sniff’ externally.