This year marks the 101st anniversary of Marcello Pirani’s invention of the thermal conductivity gauge that bears his name. This talk reviews the historical developments from the early days that focused on the development of a simple and manometer to replace the hazardous, mercury containing McLeod gauge in an industrial environment, via the in-depth analyses and optimization studies of the fifties, to the current developments of this vacuum measurement workhorse in the rough and high vacuum range.
We discuss the different realizations of the gauge, the classical wire and coiled wire Pirani sensors as well as their counterparts produced with silicon micromachining techniques and review the advantages and weaknesses in terms of sensitivity but also robustness, inertness, stability, dynamics and finally cost of the different sensor types.
The talk addresses the different attempts to extend the measurement range of this type of gauge. Several techniques have been developed to improve upon the vanishingly small sensitivity at atmospheric pressure due to the pressure independent thermal conduction in the viscous regime, by utilizing the pressure dependent convection as used in a Convectron or by capitalizing on the pressure related heat capacity of the gas when the Pirani resistor is heated in pulsed mode. At low pressures, the residual heat loss by radiation and heat conduction through the Pirani element’s mounting structure starts to mask the pressure dependent heat loss by heat conduction in the gas and sets the low pressure sensing limit. The goal of a reliable extension to the high vacuum domain around 10-5 mbar has seen developments that aimed at reducing radiation and the heat flux through the contacts of the Pirani heating element by either optimizing the wire geometry or by utilizing a proper MEMS design. These measures are to be weighted with respect to their effects on stability, robustness and inertness of the sensor.
Finally, it seems that the centenarian does not show signs of aging and is, with little competition, still in full swing where reliable and cost-effective measurements of high and rough vacuum are needed.
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