Pirani gauge
Encyclopedia
The Pirani gauge is a robust thermal conductivity
gauge used for the measurement of the pressures
in vacuum systems. It was invented in 1906 by Marcello Pirani
.
) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or a flanged
metal connector, sealed with an o-ring
. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken.
The electrical resistance of a wire varies with its temperature. This resistance, and hence the pressure of the gas, can therefore be used to measure the vacuum surrounding the wire. In many systems the wire is maintained at a constant temperature, the current required to achieve this is therefore a measure of the vacuum being studied.
The gauge may be used for pressures between 0.5 Torr
to 10−4 Torr. The thermal conductivity and heat capacity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement other instruments such as a Penning gauge are used.
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
gauge used for the measurement of the pressures
Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges....
in vacuum systems. It was invented in 1906 by Marcello Pirani
Marcello Pirani
Marcello Stefano Pirani was a German physicist known for his invention of the Pirani vacuum gauge, a vacuum gauge based on the principle of heat loss measurement...
.
Structure
The Pirani gauge consists of a metal filament (usually platinumPlatinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
) suspended in a tube which is connected to the system whose vacuum is to be measured. Connection is usually made either by a ground glass joint or a flanged
Flange
A flange is an external or internal ridge, or rim , for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel...
metal connector, sealed with an o-ring
O-ring
An O-ring, also known as a packing, or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a disc-shaped cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, creating a seal at the interface.The O-ring...
. The filament is connected to an electrical circuit from which, after calibration, a pressure reading may be taken.
Mode of operation
A heated metal filament suspended in a gas will lose heat to the gas as its molecules collide with the wire removing heat, and accelerating in the process. If the gas pressure is reduced the number of molecules present will fall proportionately and the wire will rise in temperature due to the reduced cooling effect.The electrical resistance of a wire varies with its temperature. This resistance, and hence the pressure of the gas, can therefore be used to measure the vacuum surrounding the wire. In many systems the wire is maintained at a constant temperature, the current required to achieve this is therefore a measure of the vacuum being studied.
The gauge may be used for pressures between 0.5 Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
to 10−4 Torr. The thermal conductivity and heat capacity of the gas may affect the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement other instruments such as a Penning gauge are used.