The story behind the Pirani Vacuum Gauge

Marcello Pirani – A Hero of Vacuum

Born of Italian descent in Berlin in 1880, Marcello Pirani was destined to make a major input to vacuum technology at a very early age. He completed his studies in Mathematics and Physics and then postgraduate research in 1904, thereafter joining the Siemens & Halske (Gluhampenwerk) incandescent lamp factory. He was mainly concerned with sources of light but also the manufacture of tantalum lamps, the manufacture of which required a higher vacuum than carbon filament lamps.

A particular problem was in the use of glass McLeod gauges for vacuum measurement. They presented problems in being both manually operated and particularly sensitive to breakage; spilling poisonous mercury when doing so. Pirani considered this problem and as a result in 1906 he published his paper entitled the ‘Directly Indicating Vacuum Gauge’ which became known as the ‘Pirani gauge’: the first automatically reading gauge.

The Pirani vacuum gauge was designed to measure low pressures by utilising the variation of heat loss from a wire with the pressure of the surrounding. A heated metal filament (typically platinum in modern gauges) loses heat to the gas from collisions of gas molecules with the wire. The heat loss is dependent on the number of collisions made with the wire and hence the pressure/density of the gas. As the vacuum level increases the number of molecules present will fall proportionately. This has a reduced cooling effect for the wire.

The electrical resistance of a wire varies with its temperature. The Pirani vacuum gauge operates in one of three modes: constant voltage, constant current or constant resistance (i.e. temperature). The Wheatstone bridge circuit is usually used where the Pirani vacuum gauge filament is one arm of a four-armed bridge. The readings of the gauge have to be corrected or calibrated for different gases (which have different thermal conductivities). Compared to the McLeod gauge the Pirani Gauge has the advantage of being automatic. Modern day gauges can measure from 100/10 to 10-4 mbar with an extension to higher pressure by exploiting the pressure dependence of convection losses.

1. Compensating cell
2. Power supply
3. To recorder
4. Measuring cell (Pirani gauge chamber)
5. Filament (platinum)
6. Applied pressure (unknown) vacuum

Pirani worked further on optical measurements of high temperatures and then joined Osram in 1919 as head of the scientific-technical bureau. There he researched widely on topics ranging form the sorption of gases by tantalum to the transition from incandescent to gas-discharge lamps. During his time in industry he held several positions at the Technical University and Technishe Hochschule, both in Berlin.

From 1936 Pirani woked in the UK on activities as varied as high temperature resistant materials to the utilization of fine coal dust. He returned to Germany in 1953 consulting for Osram before dying at the age of 88 years in the city of his birth.