Principles of Operation

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Hot cathode ionization gauges such as the triode, Schulz-Phelps, Bayard-Alpert and the extractor gauge have the same basic operating principle. Electrons emitted from a hot filament at constant rate are accelerated towards a positively charged electron collector. In the space between the filamentary electron emitter and the ion collector these electrons collide with gas molecules, ionizing them. The positive ions thus formed are collected on the ion collector. The number of gas molecules ionized at a fixed electron flux is proportional to the gas density and, hence, to the gas pressure. We can define a proportionality constant S such that: S (Sensitivity) = Ion collector current / (Electron collector current) (Pressure) where the currents are in amperes and the pressure in Torr. The sensitivity defined in this manner is independent, over a wide range, of the electron current and dependent only to the gauge geometry and the gas species. Thus, if you know the sensitivity (from the manufacturer's data) and the ion current at a known electron current, the pressure can be computed. Ionization gauges have different relative sensitivities for different gases and, thus, will only give true pressure measurements if the gas composition is known. Table 1 gives the relative sensitivity of certain common gases. Thus, gauges, which are usually calibrated for nitrogen, can be used for other gases. Essentially, the different types of hot cathode gauges are determined by the shapes, sizes, and distribution of the three basic elements (electron emitter, electron collector and ion collector). Triode The earliest configuration was that of a simple triode vacuum tube. It was soon learned that if the functions of the grid and anode of the tube were interchanged so that the grid became the electron collector and the anode the ion collector, a higher sensitivity could be achieved. In this case electrons leaving the filament are attracted to the positively charged electron collector (grid). Due to the open nature of the grid, most of the electrons miss the grid and are repelled by the negatively charged ion collector and return to the grid. A significant fraction miss again, so that the electrons transit the grid-to-ion-collector space several times before finally striking the grid. This greatly increases the probability that an electron will ionize a gas molecule and that the resulting ion will strike the ion collector. Triode gauges of small dimensions or of unusual configurations are well suited to measurements of relatively high pressure approaching 10-1 Torr. However, they are limited at low pressures to 10-6 Torr, or in some configurations, 10-8 Torr. X-ray Limit It was learned that the low-pressure limit of the triode gauge was caused by energetic electrons striking the electron collector and producing soft X-rays. A fraction of these X-rays strike the ion collector releasing photoelectrons. An electron current leaving the ion collector is indistinguishable from an ion current arriving at the ion collector, thus setting a limit to the lowest pressure that can be measured. Bayard-Alpert (BA) Gauges The Bayard-Alpert Gauge is essentially a triode reconfigured so that only a small fraction of the X-rays discussed above are able to strike the collector. To achieve this reconfiguration, the filamentary electron emitter was moved outside the grid and the massive ion collector was replaced by a fine wire in the center of the grid. The improved performance at lower pressures requires some sacrifice of high-pressure performance. The range of measurement of glass Bayard-Alpert gauges is nominally from the 10-3 Torr scale to the 10-10 Torr scale. Nude Bayard-Alpert Gauges operate from 10-3 to 10-11 Torr. Special designs are under development to extend the low-pressure limit to less than 10-12 Torr. 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