Principles of Operation [ page 2]

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Electron-Stimulated Ion Desorption The Electron-Stimulated Ion Desorption (ESID) effect in Bayard-Alpert gauges is caused by ionization of adsorbed gas molecules on the electron collector by the arriving electrons. These ions then are driven by the internal electric fields in the gauge to the collector, along with the ions formed from the gas phase. The result is a falsely high apparent pressure measurement which gets proportionally worse as the pressure falls. ESID errors exhibit themselves as false high pressures or prolonged/inconsistent pump-down times and are noticeable at pressures as high as 10-7 Torr. General Vacuum Practice To insure that the measured pressure accurately mirrors the system pressure, several factors should be considered. The gauge should be well de-gassed, especially when measuring very low pressures. The gauge should be baked under high vacuum for one hour at a temperature of at least 250C or as high as is practical up to the maximum set by the particular gauge specification. The internal electrodes are de-gassed by heating them to a temperature of about 900C for approximately 15 minutes. The electrode heating is accomplished by either electron bombardment (EB) or by passing a high current at low voltage (IR) through the grid. Gauges equipped with squirrel-cage grids or those where the grid wire is welded, brazed or swaged to the support wire at multiple locations are usually only de-gassed by electron bombardment. The collector in present designs can only be de-gassed by electron bombardment. Grids consisting of a helix or double helix with both ends having external connections can be de-gassed by either the EB or high current IR method. Care must be taken to have a heavily grounded gauge circuit and to avoid touching exposed electrodes especially during EB de-gas when higher voltages are present. The ionization gauge has a certain pumping capacity due to both chemical and electrical effects. Chemical pumping is due, in general, to reactions on the hot filament and the adsorption of gases on very clean surfaces. As the surface becomes saturated the pumping decreases. Pumping due to electronic phenomena continue as long as voltages are applied. The vacuum connection between the ionization gauge and the rest of the system can have a great effect on the measuring ability and life of the gauge. If the tubulation has too small a diameter or is connected by long plumbing to the vacuum system, the gauge reading may differ from the system pressure by as much as a factor of ten. This difference is most pronounced when system pressures change rapidly and the system is at low pressures. It is wise to select the largest diameter tubulation possible or even use a nude gauge in the vacuum system. A glass gauge with 3/4-inch tubulation has adequate conductance for use down to the 10-8 Torr scale. A gauge with 1-inch tubulation can be used down to the 10-10 Torr. When considering gauge placement and tubulation size, a number of factors can come into play. If the application is one where material is evaporated or sputtered, care must be taken to keep the material out of the gauge. Contamination of the gauge can severely limit life and lead to grossly inaccurate measurements. Also, operations taking place in the 10-2 to 10-4 Torr range, with relatively high voltage and current, can lead to long path arc discharge. A simple right-angle connector has been shown to lessen both the long path arc and evaporation problems, but with some sacrifice of conductance. Manufacturer's Cross Reference Each manufacturer's gauge is made to custom requirements and, therefore, generic gauges purchased from unknown "second sources" should be approached with caution. In some cases, generic material or approximate tolerances are used. These generic gauges may not perform as intended by the original manufacturer or are of inferior quality. It is, therefore, recommended that you contact the original manufacturer of the gauge to assure that complete operational specifications are met. In the cases where the original manufacturer no longer provides this gauge, they can best guide you to a reliable source of gauges. Several companies, no longer in operation, have made arrangements with ETI to provide gauges that meet the exact original specifications. If you are uncertain about the source of your replacement gauges, please contact ETI.   About the Company | ETI Gauges: Principles of Operation | Ionization Gauges | Special ETI Gauges | OEM and Custom Designs Configurations of Standard Gauges | Earlier Model Gauges | Replacement Filament Assemblies | ETI Custom Filaments | ETI Stems | | Contact ETI | Return Home | Site best viewed using IE 4.0 or Navigator 4.0 web browsers. ©1999 ETI