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VACUUM GAUGES

FILSERV produces a range of hot cathode ionisation gauges of the triode and Bayard Alpert type. These gauges are used for the measurement of low pressures in the 10e-1 to 10e-10 Torr range and operate on the same basic principle whereby electrons are emitted from a hot filament and are attracted towards a positively charged collector.

The electrons collide with and ionise gas molecules and the positive ions formed are collected and produce an ion current which is proportional to the number of gas molecules and hence, the gas pressure. Ionisation gauges have different sensitivities for different gases and although they are usually calibrated for nitrogen they can be used for different gases.


The sensitivity of a gauge is defined as:

S =          Collector current (A)
                  Grid current (A) x Pressure (Torr)

Most gauges contain two filaments which may be straight, hairpin or coiled wire or ribbon. Iridium wire and ribbon are most commonly used with the addition of a thoria or yttria coating to reduce the work function. This has the effect of producing more electrons at a given current or a similar electron flux at a lower current thereby increasing the lifetime of the filament.

Work function is defined as the energy (eV) needed to remove an electron from the Fermi level in a metal to a point an infinite distance away from the surface. When applied to an electron escaping from a heated negatively charged filament in a vacuum tube, the work function is known as the thermionic work function. Tungsten filaments are sometimes used in aggressive environments such as those containing halogens (Cl, Fl, Br) or hydrogen.

 

Gauges may be of the glass encapsulated type or nude where they are mounted, typically, on a stainless steel flange. The incorporation of a heat loss sensor based on the Pirani principle with the Bayard Alpert gauge provides a wide range pressure measurement from 10e3 to 10e-10 Torr.

The low pressure limit of these gauges is determined by the fact that some electrons collide with the electron collector and produce soft X-rays which in turn strike the ion collector producing photoelectrons. This effectively sets the lowest pressure, also known as the X-ray limit, that can be measured since the system is unable to distinguish between the electron current so produced and an ion current arriving at the ion collector.


Hot cathode ionisation gauges are used extensively for general pressure measurement and control in high vacuum systems, semiconductor processing and coating and evaporating systems.

FILSERV also makes a range of filaments for surface science applications including LEED, organic and x-ray spectroscopy and electron microscopy.


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