The hot-filament

ionization gauge Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure ...

, sometimes called a hot-filament gauge or hot-cathode gauge, is the most widely used low-pressure (vacuum) measuring device for the region from 10⁻³ to 10⁻¹⁰

Torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one " millimeter of merc ...

. It is a

triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...

, with the filament being the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

. ''Note: Principles are mostly the same for hot-cathode ion sources in

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particle ...

s to create electrons.''

Function

A regulated electron current (typically 10 mA) is emitted from a heated

filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar filament ...

. The electrons are attracted to the

helical Helical may refer to: * Helix, the mathematical concept for the shape * Helical engine, a proposed spacecraft propulsion drive * Helical spring, a coilspring * Helical plc, a British property company, once a maker of steel bar stock * Helicoil A t ...

grid by a DC potential of about +150 V. Most of the electrons pass through the

grid Grid, The Grid, or GRID may refer to: Common usage * Cattle grid or stock grid, a type of obstacle is used to prevent livestock from crossing the road * Grid reference, used to define a location on a map Arts, entertainment, and media * News ...

and collide with gas molecules in the enclosed volume, causing a fraction of them to be ionized. The gas ions formed by the electron collisions are attracted to the central ion collector wire by the negative voltage on the collector (typically −30 V). Ion currents are on the order of 1 mA/ Pa. This current is amplified and displayed by a high-gain differential amplifier/

electrometer An electrometer is an electrical instrument for measuring electric charge or electrical potential difference. There are many different types, ranging from historical handmade mechanical instruments to high-precision electronic devices. Modern ...

. This ion current differs for different gases at the same pressure; that is, a hot-filament ionization gauge is composition-dependent. Over a wide range of

molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...

density, however, the ion current from a gas of constant composition is directly proportional to the molecular density of the gas in the gauge.

Construction

A hot-cathode ionization gauge is composed mainly of three electrodes, all acting as a

, wherein the

is the filament. The three electrodes are a collector or plate, a

, and a

. The collector current is measured in picoamperes by an

. The filament voltage to ground is usually at a potential of 30 volts, while the grid voltage at 180–210 volts DC, unless there is an optional

electron bombardment Electron ionization (EI, formerly known as electron impact ionization and electron bombardment ionization) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of t ...

feature, by heating the grid, which may have a high potential of approximately 565 volts. The most common ion gauge is the hot-cathode Bayard–Alpert gauge, with a small collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the nude gauge is inserted in the vacuum chamber directly, the pins being fed through a ceramic plate in the wall of the chamber. Hot-cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or even low vacuum while hot. Electrons emitted from the filament move several times in back-and-forth movements around the grid before finally entering the grid. During these movements, some electrons collide with a gas molecule to form a pair of an ion and an electron (

electron ionization Electron ionization (EI, formerly known as electron impact ionization and electron bombardment ionization) is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of t ...

). The number of these

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

is proportional to the gas molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. Since the gas molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current. The low-pressure sensitivity of hot-cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce X-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10⁻⁸

and the Bayard–Alpert gauges to about 10⁻¹⁰ Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a * Faraday cup, *

quadrupole mass analyzer The quadrupole mass analyzer, originally conceived by Nobel Laureate Wolfgang Paul and his student Helmut Steinwedel, also known as quadrupole mass filter, is one type of mass analyzer used in mass spectrometry. As the name implies, it consists o ...

with Faraday cup, * microchannel plate detector with Faraday cup, *

with microchannel plate detector Faraday cup, * ion lens and acceleration voltage and directed at a target to form a sputter gun; in this case a valve lets gas into the grid cage.

Types

* Bayard–Alpert (uses sealed tube).D. Alpert, "New Developments in the Production and Measurement of Ultra High Vacuum" J. App. Phys. 24, 860 (1953); . * Nude gauge (uses the vacuum chamber to make a complete seal).

References

{{Reflist

External links

How does an ion gauge work?Hot filament ionization gauge tubes
- Hot cathode ionization pressure gauge

- Hot-cathode ionization pressure gauge including a sequence of electrodes arranged at a distance from one another in sequence along an axis Vacuum gauges Pressure gauges

Function

Construction

Types

See also

References

External links