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Mechanical vacuum gauges measure the pressure directly by recording the force which the particles (molecules and atoms) in a gas-filled space exert on a surface by virtue of their thermal velocity.
The interior of a tube bent into a circular arc (so-called Bourdon tube) (3) is connected to the vessel to be evacuated (Fig. 3.2). Through the effect of the external air pressure the end of the tube is deflected to a greater or lesser extent during evacuation and the attached pointer mechanism (4) and (2) is actuated. Since the pressure reading depends on the external atmospheric pressure, it is accurate only to approximately 10 mbar, provided that the change in the ambient atmospheric pressure is not corrected.
The best-known design of a diaphragm vacuum gauge is a barometer with an aneroid capsule as the measuring system. It contains a hermetically sealed, evacuated, thin-walled diaphragm capsule made of a copper-beryllium alloy. As the pressure drops, the capsule diaphragm expands. This movement is transmitted to a point by a lever system. The capsule vacuum gauge, designed according to this principle, indicates the pressure on a linear scale, independent of the external atmospheric pressure.
The most accurate pressure reading possible is frequently required for levels below 50 mbar. In this case, a different diaphragm vacuum gauge is more suitable, i.e. the DIAVAC, whose pressure scale is considerably extended between 1 and 100 mbar. The section of the interior in which the lever system (2) of the gauge head is located (see Fig. 3.3) is evacuated to a reference pressure pref of less than 10-3 mbar. The closure to the vessel is in the form of a corrugated diaphragm (4) of special steel. As long as the vessel is not evacuated, this diaphragm is pressed firmly against the wall (1). As evacuation increases, the difference between the pressure to be measured px and the reference pressure decreases. The diaphragm bends only slightly at first, but then below 100 mbar to a greater degree. With the DIAVAC the diaphragm deflection is again transmitted to a pointer (9). In particular the measuring range between 1 and 20 mbar is considerably extended so that the pressure can be read quite accurately (to about 0.3 mbar). The sensitivity to vibration of this instrument is somewhat higher than for the capsule vacuum gauge.
Capsule vacuum gauges measure pressure accurately to 10 mbar (due to the linear scale, they are least accurate at the low pressure end of the scale). If only pressures below 30 mbar are to be measured, the DIAVAC is recommended because its reading (see above) is considerably more accurate. For extremely precise measuring accuracy requirements precision diaphragm vacuum gauges should be used. If low pressures have to be measured accurately and for this reason a measuring range of, for example, up to 100 mbar is selected, higher pressures can no longer be measured since these gauges have a linear scale. All mechanical vacuum gauges are sensitive to vibration to some extent. Small vibrations, such as those that arise in the case of direct connection to a backing pump, are generally not detrimental.
Deflection of a diaphragm can also be electrically measured as “strain” or as a change in capacitance. In the past, four strain gauges, which change their resistance when the diaphragm is deflected, i.e. under tensile load, were mounted on a metallic diaphragm in a bridge circuit. At Leybold such instruments have been given a special designation, i.e. MEMBRANOVAC. Later, silicon diaphragms that contained four such “strain resistances” directly on their surface were used. The electrical arrangement again consisted of a bridge circuit, and a constant current was fed in at two opposite corner points while a linear voltage signal proportional to the pressure was picked up at the two other corner points. Fig. 3.4 illustrates the principle of this arrangement. Such instruments were designated as PIEZOVAC and currently as the DI/DU2000 units and are still in use in many cases.
Measuring the strain as part of a bridge circuit gets you high levels of accuracy, but has a limited range. A more advanced method is to where the deflection of the diaphragm is measured as the change in capacitance of a plate capacitor: one electrode is fixed, the other is formed by the diaphragm. When the diaphragm is deflected, the distance between the electrodes and thus capacitance of the capacitor is altered. Fig. 3.5 illustrates the principle of this arrangement. This is utilized in the CEREVAC CTR range. A distinction is made between sensors with metallic and those with ceramic diaphragms. Capacitance diaphragm gauges are used from atmospheric pressure to 1·10-4 mbar (below 10-4 mbar the measurement uncertainty rises rapidly). To ensure sufficient deflection of the diaphragms at such low pressures, diaphragms of varying thicknesses are used for the various pressure levels. In each case, the pressure can be measured with the sensors to an accuracy of 3 powers of ten:
1000 to 1 Torr
100 to 10–1 Torr
10 to 10–2 Torr
1 to 10–3 Torr
10–1 to 10–4 Torr
In order to further increase the accuracy levels, you can also heat the capacitive cell. In this instance the capacitive cell is heavily insulated against the ambient temperature and a small heating filament heats the cell to a fixed temperature. This helps to eliminate any variation due to fluctuating ambient temperatures. It also is used when operating with gases where you would not want them to condense inside the gauge, which would otherwise be a cold spot in the system.
If the pressures to be measured exceed these range limits, it is recommended that a multichannel unit with two or three sensors be used.
The capacitance diaphragm gauge thus represents, for all practical purposes, the only absolute pressure measuring instrument that is independent of the type of gas and designed for pressures under 1 mbar. Today two types of capacitive sensors are available: