Physics
Ionization Gauge
An ionization gauge is a device used to measure the pressure of a gas by measuring the current produced when gas molecules are ionized within the gauge. It operates by applying a high voltage to a metal electrode, causing gas molecules to ionize and produce a measurable current. Ionization gauges are commonly used in vacuum systems to monitor and control gas pressure.
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7 Key excerpts on "Ionization Gauge"
eBook - PDF- K. Diels, R. Jaeckel(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
In spite of the relative inaccuracy of the gauge, so that to all intents and purposes only the order of magnitude of pressure is indicated, the gauge is frequently used in industrial applications because of its insensitivity to inrush of gas. 1.8.3.2 Ionization Gauge For accurate measurement of pressure in the high vacuum range, a simple triode can be used. The operating principle is as follows: Electrons emitted from a hat filament collide with gas molecules while travelling to a positively charged anode, and in consequence produce positive ions. These ions are collected on a negatively charged grid. The ratio of ion-cur-rent i + to electron current i~ is a measure of the number of collisions which the electrons undergo during their passage to the anode, and this number is proportional to the gas pressure. Measuring range: 10 -2 to 10 -7 Torr. Measuring principle: Gas ionization by electron impact. Measured quantity: Electric current. Beading: Total pressure. Advantages: Remote and continuous pressure indication. Disadvantages: If tungsten filaments are used as an electron source, the filament is likely to burn out during a gas inrush. This can be avoided by using Pt-cathodes coated with Th0 2 . The reading depends on the nature of the gas. Measurement of very high vacuum by means of the BAYARD-ALPERT gauge In the arrangement described above the lower limit of the measuring range is 5 X 10~ 8 Torr. This limitation is caused by the following mechanism : The electrons which arrive with a certain energy at the anode produce soft X-rays there. A fraction of these X-rays hit the negatively charged ion-collector producing photo-electrons which cannot be distinguished as a current in the measuring circuit from the ion current. This error is avoided in the Ionization Gauge designed by BAYABD-ALPERT. In their design, the ion collector is only a very thin wire which is not hit by the great majority of all X-rays produced at the anode.
eBook - PDF- A. Roth(Author)
- 2012(Publication Date)
- North Holland(Publisher)
(1986) describe an Ionization Gauge for use in a magnetic field. 70 Ρ ( T o r r ) Fig. 6.30 Calibration curves of an Ionization Gauge for various gases (Grid current 5 mA). ( § 7 ) Ionization GaugeS 315 Initial operation of an Ionization Gauge results in the heating of the electrodes and the emission of large quantities of adsorbed gases from the surfaces. Unless the gauge elements are heated vigorously to outgas them, the reading will remain high as compared with the system pressure. The grids can usually be heated elec-trically, the anode can be heated by electron bombardment by connecting the anode and grid together at the same positive potential. Finally, it is necessary to heat the glass or metal envelope of the gauge tube. After the gauge head has been outgassed, gas entering the tube is readily adsorbed especially on the tube walls. Chemical reactions induced by the hot filament pro-duce further sorption. These processes are responsible for the pumping action of gauges, which causes the pressure at the gauge to be lower than that in the system (Langmuir, 1915; Riddiford, 1951; Close and Vaughan-Watkins, 1976; Gear, 1976; Poulter and Sutton, 1981; Berman, 1982; Chapman and Hobson, 1985). If diffusion pump vapour is present in the system, these vapours react with the hot tungsten and change the electron emission, which produces an apparent change in pressure. It is therefore essential to employ a liquid nitrogen trap between the diffusion pump and the gauge. If diffusion pump vapour is present in a system operating at very low pressures, a normal tubulated Ionization Gauge indicates lower pressures than the nude gauge. This phenomenon is known as the Blears effect. Blears (1947) found that this phenomenon lies in the vastly different con-ductance of the small connecting tube for oil vapour and permanent gases in conjunction with cracking of the oil molecules by the gauge. Haefer and Hen-gevoss (1961) confirmed the original results of Blears.
eBook - PDF- H. Adam(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
Die durch diese Methoden erhaltenen Resultate werden mit Ergebnissen der Massenspektrometrie verglichen. Zum Schluss wird ein Bericht iiber eine Anzahlheute verwendeter Eichmethoden fur lonisationsvakuummeter bei niedrigen Drucken gegeben. I. Introduction The purpose of this review is threefold: (a) To compare the more important properties of some recently reported Ionization Gauges; (b) To compare the information obtainable at low pressures by adsorption-desorption techniques and total pressure gauging with that given by a mass spectrometer; (c) To summarize the present status of low pressure gauge calibration methods. It is hoped that it will be of some assistance to those wishing to choose and/or calibrate an Ionization Gauge for a particular application. The discussion will be concerned mainly with developments which have occurred since the Second Inter-national Vacuum Congress in Washington (1961). The list of gauges and important properties chosen is meant to be illus-trative rather than comprehensive. II. General We would like our low-pressure measurements to tell us either (a) the number density of molecules in some specified portion of the vacuum system, or (b) the rate of impingement of mole-cules on some specified surface. The determination of these quantities from the output signal of an Ionization Gauge, in general located at some other position in the system, is the basic problem of low-pressure measurement. Some difficulties in pressure measurement depend little on the type of gauge being used, but rather are characteristic of low-pressure systems generally. When the pressure in a system is reduced to the ultrahigh vacuum (uhv) range, the number of molecules in the gas phase becomes small compared to the number adsorbed or adsorbable on its surfaces. The high pumping speed caused by adsorption (3.638 /T/M l./sec/ cm 2 for unit sticking probability) can result in quite large, long-term variations of the pressure of adsorbable gases with position.
eBook - PDF- Glenn F. Knoll(Author)
- 2015(Publication Date)
- Wiley(Publisher)
Chapter 5 Ionization Chambers Several of the oldest and most widely used types of radiation detectors are based on the effects produced when a charged particle passes through a gas. The primary modes of interaction involve ionization and excitation of gas molecules along the particle track. Although the excited molecules can at times be used to derive an appropriate signal (as in the gas scintillators discussed in Chapter 8), the majority of gas-filled detectors are based on sensing the direct ionization created by the passage of the radiation. The detectors that are the topic of the following three chapters (ion chambers, proportional counters, Geiger tubes) all derive, in somewhat different ways, an electronic output signal that originates with the ion pairs formed within the gas filling the detector. Ion chambers in principle are the simplest of all gas-filled detectors. Their normal operation is based on collection of all the charges created by direct ionization within the gas through the application of an electric field. As with other detectors, ion chambers can be operated in current or pulse mode. In most common applications, ion chambers are used in current mode as dc devices, although some examples of pulse mode applications will be given at the end of this chapter. In contrast, proportional counters or Geiger tubes are almost always used in pulse mode. The term ionization chamber has conventionally come to be used exclusively for the type of detector in which ion pairs are collected from gases. The corresponding process in solids is the collection of electron–hole pairs in the semiconductor detectors described in Chapters 11–13. Direct ionization is only rarely exploited in liquids, although some developments of this type are described in Chapter 19. Many details that are omitted in the following discussions can be found in the classic books on ionization chambers by Rossi and Staub 1 and by Wilkinson.
eBook - PDFVacuum Technology Transactions
Proceedings of the Sixth National Symposium
- C. Robert Meissner(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
F Advances in the Design of Vacuum Gauges using Radioactive Materials By J. R. ROEHRIG and G. F. VANDERSCHMIDT National Research Corporation, Cambridge, Massachusetts Tritium, the radioactive isotope of hydrogen, is advantageously used in radiological vacuum gauges. The tritium gas is absorbed in a thin layer of titanium, forming a stable compound at room temperature. Because of the complete absence of penetrating radiation, high intensity sources may be used to permit convenient measurement of pressures down to 10 -5 mm Hg, with essentially no hazard to personnel. A fundamental problem in the design of simple, portable radiological vacuum gauges is the difficulty of measuring the small current from such gauges. A method which makes use of the time of discharge of a capacitor permits a simple direct measurement of the current; the method provides an output which is ideally suited for telemetering, and makes the gauge especially useful for high-altitude meteorological measurement. I. Introduction Radiological vacuum gauges measure the density of air or other gas in a chamber by ionizing the gas with the radiation from a radioactive source. The ions are collected, and the ion current provides an analog indication of the gas density. A commercial version of this gauge, the Alphatron, (registered trade mark) permits the measurement of equivalent density* from a few times 10 -4 mm Hg to 760 mm Hg. The gauge uses radium as a source. 1 The difficulty in extending the operation of the gauge to lower equivalent densities is the inconvenience of measuring the very small ionization current produced by the gauge at low densities. A typical gauge uses a cylindrical ion chamber about 3 Jin. high and 2 in. in diameter with a radium source of 400 /xc at one end of the chamber. With dry air in the chamber such a source produces an ionization current less than 10 -12 A at an equivalent density of 10~ 3 mm Hg.
eBook - PDF- Michael F. L'Annunziata(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Gas ionization detectors consist of a gas volume in an enclosure that is ei-ther sealed or constructed in such a way as to permit a continuous flow of the filling gas. Within that gas volume an electric field is applied across the elec-trodes. The outer wall frequently serves as one of the electrodes, the cathode, while a wire rod, a grid, or a plate in the middle of the gas volume serves as the anode. Although there are many different variations in the design of gas ionization counters, a cylindrical system with a central wire or rod, called a counting tube, is very common. Many designs with different shapes and geometries have been realized. Some of them are suitable for a very wide range of useful appli-cations, some were designed for a very special investigation, and others have been realized only to learn more about the operating principles of ionization detectors in order to improve the performance of this type of radiation detection device. In this chapter a selection is given from numerous developments in the field of gas ionization detectors. It should be mentioned that radiation measurement methods today place emphasis mainly on radiation spectroscopy. Solid-state and scintillation detectors offer unique advantages in that field of applications. Nevertheless, a great deal of interesting and useful research work is still done using ionization detectors and new developments and applications are still re-ported in the literature. Although this type of detector is extremely useful, prob-lems and limitations have to be faced, and careful planning of experiments to recognize and deal with those limitations is extremely important (Bateman etal, 1 9 9 4 ) .
eBook - PDF- L.N. Rozanov(Author)
- 2002(Publication Date)
- CRC Press(Publisher)
Chapter 3 Measurement of Vacuum 3.1 Classification of Measurement Methods The range of pressures used in modern vacuum engineering is very wide (10 5 to 10 12 ) Pa. The measurement of pressure in this range cannot be provided by one device. In practice, the pressure of rarefied gases is usually measured by various manometric converters that are distinguished by the principle of action and the class of accuracy. Devices for the measurement of pressure are referred to in vacuum engineering as vacu-um gauges or vacuummeters. They usually consist of two parts: a manometric transducer and a measuring device. By the method of measurement vacuum gauges can be subdivided into absolute and relative. The indication of absolute devices does not depend on the type of gas and can be determined in advance. Devices for relative measurements use the de-pendence of the parameters of some physical processes that occur in vacuum on pressure. They require reference devices. Vacuum gauges measure total pressure of gases that are present in a vacuum system. Figure 3.1 shows the ranges of working pressure for various types of vacuum gauges. Measurers of partial pressure, as well as those of total pressure, are characterized by the lower and upper measured pressures, sensitivity, as well as their unique parameter, resolu-tion which determines working mass range. The resolution is the ratio of molecular weight of gas M to its least detected change A M , _ = M P m AM' Depending on the type of device, the values of p M , M/p M , Mp M can remain constant in the entire range of measurement. Experimentally, resolution is determined from a mass spectrum. The width of a peak is measured at 10 or 50 % of peak height. Measurement of partial pressure in vacuum systems is performed using two methods: ionisation and sorp-tion. Ionization is the method based on ionization and separation of positive ions according to the ratio of ion mass to its charge.
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