back To Main VACUUM  TECHNOLOGIES

Basic Terms of Vacuum Metrology

 

Today, the total range of vacuum pressure accessible to measurement extends from atmospheric pressure (about 1000 mbar (750 Torr)) down to 10^-12 mbar/Torr, i.e. it extends over 15 powers of ten. The instruments used for measuring the pressure within this wide range are called vacuum gauges. For physical reasons it is not possible to create a single vacuum sensor through which it might be possible to perform quantitative measurements within the entire pressure range. Therefore, a variety of different vacuum gauges are available, each with their own characteristic measurement range which commonly extends over several powers of ten. A difference is made between direct and indirect pressure measurements. In the case of direct (or absolute) pressure measurements, the readings obtained through the vacuum gauge are independent of the type of gas and the pressure which is to be measured. Common are so-called mechanical vacuum gauges where the pressure is determined directly by recording the force acting on the surface of a diaphragm. In the case of so-called indirect pressure measurements the pressure is determined as a function of a pressure dependant property of the gas (thermal conductivity, ionization probability, for example). These properties do not only depend on the pressure, but also on the molar mass of the gases.

For this reason, the pressure readings obtained through vacuum gauges which rely on indirect pressure measurements, depend on the type of gas. The readings usually relate to air or nitrogen as the measurement gas. For the measurement of other vapors or gases the corresponding correction factors must be applied.

 

Vacuum Gauges where the Pressure Readings are Independent of the Type of Gas (Mechanical Vacuum Gauges)

 

BOURDON Vacuum Gauge

The inside of a tube which is bent into a circular arc (the so-called Bourdon tube) is connected to the vacuum system. Due to the effect of the external atmospheric pressure, the end of the tube bends more or less during the evacuation process. This actuates the pointer arrangement which is attached to this point. The corresponding pressure can be read off on a linear scale. With Bourdon gauges it is possible to roughly

determine pressures between 10 mbar (7.5 Torr) and atmospheric pressure.

 

Capsule Vacuum Gauge

This vacuum gauge contains a hermetically sealed, evacuated, thin-walled diaphragm capsule which is located within the instrument. As the vacuum pressure reduces, the capsule bulges. This movement is transferred via a system of levers to a pointer and can then be read off as the pressure on a linear scale.

 

Diaphragm Vacuum Gauge

In the case of the diaphragm vacuum gauge which is capable of absolute pressure measurements, a sealed and evacuated vacuum chamber is separated by a diaphragm from the vacuum pressure to be measured. This serves as the reference quantity. With increasing evacuation, the difference between the pressure hich is to be measured and the pressure within the reference chamber becomes less, causing the diaphragm flex. This flexure may be transferred by mechanical means like a lever, for example, to a pointer and scale, or electrically by means of a strain  gauge or a bending bar for conversion into an  electrical measurement signal. The measurement range of such diaphragm vacuum gauges extends from 1 mbar (0.75 Torr) to over 2000 mbar  (1500 Torr).

 

Capacitance Vacuum Gauge

The pressure sensitive diaphragm of these capacitive absolute pressure sensors is made of Al2O3  ceramics. The term “capacitive measurement” means that a plate capacitor is created by the diaphragm  with a fixed electrode behind the diaphragm. When the distance between the two plates  of this capacitor changes, a  change in capacitance  will result. This change, which is proportional to the pressure, is then converted into a corresponding electrical measurement signal. Here too, an evacuated reference chamber serves as the reference for the pressure measurements. With capacitance gauges it is possible to accurately measure pressures from 10^-5 mbar/Torr to  well above atmospheric pressure, whereby different capacitance gauges having diaphragms of different thickness (and therefore sensitivity) will   have to be used.

 

Vacuum Gauges where the  Pressure Readings Depend of the Type of Gas

 

Thermal Conductivity Gauge (Pirani)

This measurement principle utilizes the thermal conductivity of gases for the purpose of pressure measurements in the range from 10-4 mbar/Torr to atmospheric pressure. Today, only the principle

of the controlled Pirani gauge is used by LEYBOLD in order to attain a quick response.

The filament within the gauge head forms one arm of a Wheatstone bridge. The heating voltage which is applied to the bridge is controlled in such a way, that the filament resistance and thus the temperature of the filament remains constant regardless of the quantity of heat given off by the  filament. Since the heat transfer from the filament to the gas increases with increasing pressures,  the voltage across the bridge is a measure of the pressure. Improvements with regard to temperature  compensation have resulted in stable pressure readings also in the face of large temperature changes, in particular when measuring low pressures.

 

Cold Cathode Ionization Vacuum Gauge (Penning)

Here the pressure is measured through a gas discharge within a gauge head whereby the gas  discharge is ignited by applying a high tension. The resulting ion current is output as a signal which is proportional to the prevailing pressure. The gas discharge is maintained also at low pressures with the aid of a magnet. New concepts for the design of such sensors permit safe and reliable operation of these socalled Penning ensors in the pressure range from 10^-2 to 1 x 10^-9 mbar/Torr.

 

Hot Cathode Ionization Vacuum Gauge

These sensors commonly use three electrodes. A hot cathode emits electrons which impinge on an anode. The gas, the pressure of which is to be measured, is thus ionized. The resulting positive ion current is detected through the third electrode - the so-called ion detector - and this current is used as the signal which is proportional to the pressure. The hot cathode sensors which are mostly used today, are based on the Bayard-Alpert principle. With this electrode arrangement it is possible to make measurements in the pressure range from 10^-10 to 10^-2 mbar/Torr. Other electrode arrangements permit access to a higher range of pressures from 10-1 mbar/Torr down to 10-10 mbar/Torr.  For the measurement of pressures below 10^-10 mbar/Torr so-called extractor ionization sensors after Redhead are employed. In extractor  ionization gauges the created ions are focused onto a very thin and short ion detector. Due to  the geometrical arrangement of this system, interfering influences such as X-ray effects and ion desorption can be almost completely eliminated. The extractor ionization gauge permits pressure measurements in the range from 10^-4 to 10^-12 mbar/Torr.

 

Selection of the Right Vacuum Gauge

 

When selecting a suitable instrument for pressure measurements, the pressure range is not  the only critera. The operating conditions for the instrument play an important part. If, for example,  there is the risk of excessive contamination, vibrations, or if air inrushes are to be expected  etc., the instrument must be rugged enough. Thus for industrial applications diaphragm gauges,  controlled thermal conductivity gauges as well as cold cathode ionization gauges after  Penning are strongly recommended. Precision instruments are very often quite sensitive to rough operating conditions. These should therefore  only be used while observing the corresponding applications information.