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Vacuum Gauges - problems with the Pirani/ Penning combination


QUESTION: I’m using a wide range gauge to measure vacuum pressure. It includes a combination of the Pirani and Penning principles. The gauge gets dirty quite quickly. What is the reason for this and what can I do to avoid pollution?

ANSWER: These gauges employ two measurement principles in one device. The problems arise when the two measurement ranges are incorrectly reconciled. If you can program the cross-over pressures, set the switching point for the operation of the low pressure gauge as low as possible.

BACKGROUND: Wide range gauges employ two measurement principles in one device. The so-called Pirani gauge is used to measure from atmospheric pressure down to around 10-2 or 10-3 mbar. This is a measurement principle based on the heat conductivity of gases. This principle is ineffective when the gas density no longer dominates heat transport, i.e. at very low pressures. In this range, heat radiation and heat dissipation at the filament mounting dominate. At these low pressures, another measurement principle is employed utilizing high-energy electrons to produce charged gas molecules for pressure measurement. The electrons originate either from a filament (hot cathode) or glow discharge (cold cathode). They are accelerated in an electric field and subsequently collide with electrons of the gas molecules thus creating positive ions. These positively charged particles are collected by a negatively charged electrode. The resulting ion flow is a measure for the vacuum pressure.

The higher the gas density, the more positively charged ions will be found in the measurement cell. In cold cathode gauges, high voltages are required in order to ignite and maintain the glow discharge as an electron source. These high voltages create high energy ions, which collide with the walls of the measurement cell where they vaporize the wall material. This material forms unwanted deposit layers inside the gauge together with gas molecules, especially oil vapour from vacuum pumps. The organic molecules are cracked by the glow discharge, thereby forming polymer-like deposits on the cold metal surfaces. The ion bombardment supports chemical reactions which lead to insulating thermosetting material, which carries the space charges that affect the measurement.

The degree of contamination depends on the gas density, so operation of the ion gauge at high pressures should be avoided. The lower the cross-over pressure between the Pirani and cold cathode part of the wide range gauge, the lower the degree of contamination.


Pirani vacuum meter: Vacuum meter frequently named after its inventor, Berlin physicist Marcello Pirani (1880-1964). It works within the pressure range from around 10-4mbar to 1000 mbar and uses the dependency of pressure on heat conductivity as an indirect measured variable for pressure. The change in heat conductivity is measured on the basis of the change in resistance of an electrically heated wire.

Penning: The Penning vacuum meters operate at pressure ranges between around 10-2 and 10-9 mbar. The filament-less measurement cell generates glow discharge by applying a high voltage. The electrons obtain high energy from the glow discharge. They collide with and ionise neutral gas particles. Gas particles generated in this way are accelerated through an electrical field to a collector. The resulting ion flow is a measurement of the pressure in the measurement cell.

Sputtering (atomisation): Sputtering is the bombardment of a surface with high-energy ions. The bombardment generates a pulse transfer similar to snooker balls in which individual atoms are released from a solid and enter their gaseous phase.

Electron: Electrons are negatively charged elementary particles, which encircle the positively charged core of an atom. Freely moving electrons can transport an electrical current.

Ion: An ion is an electrically charged atom or molecule. It may comprise one or more atoms and be positively (cation) or negatively (anion) charged. The number of positively charged core modules (protons) in atoms is identical to the number of negatively charged electrons which encircle the core. An atom is outwardly electrically neutral. An ion is charged by the imbalance of positive charges in the core and negative charges in the electron shell.

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