What Are Pirani Gauges Used to Measure?

The Pirani gauge measures the thermal conductivity of gasses at various pressures within a vacuum. In the simplest terms it tells how quickly a given gas will absorb heat at different levels of pressurization. These measurements must be taken within a vacuum to ensure accuracy, and eliminate the possibility of contamination of the sample being measured.

History

• Marcello Stefano Pirani, a renowned German physicist, was born in Berlin in 1880. He was educated and attained his Ph.D. in the fields of mathematics and physics. In 1904, he joined a subsidiary of Siemens and Halske where he focused on the refinement of light bulb technology. In 1906, Pirani developed the Pirani gauge to help measure the properties of heat transfer in different gasses as a means of refining and furthering his work on gas discharge lamps. Still used today, the Pirani gauge has been pivotal in the development of the modern high efficiency gas discharge bulbs used today.

Function

• A metal filament is suspended in a gas-filled vacuum tube. An electric current is passed through the filament to cause it to heat to a given temperature by means of direct resistance. This is the same process used in modern electric heaters with metal heating coils. The filament loses heat when molecules of the gas come in contact with it. The measurement derived is a function of the thermal transfer properties of the gas being used and the level of pressure within the vacuum system. As a result both factors can be measured using the Pirani gauge.

Design

• The filament of the Pirani gauge is usually platinum, but other metals can be used. The vacuum system is normally of glass construction, and the system is sealed with a rubber O-ring around a metal connector or glass joint. The filament is connected to an electrical circuit that must be calibrated to make the accurate measurements of the thermal transfer that will take place within the vacuum.

Alternatives

• The Pirani gauge is one of the most popular tools of its kind. However, it is not the only gauge that can be used to measure pressure levels within a vacuum and the thermal transfer of gasses. Another, similar gauge is the thermocouple. The thermocouple functions in much the same way as the Pirani, though it can only produce accurate measurements at a much lower range of pressures than the Pirani gauge. However, if augmented with digital gauge controller, an industry standard thermocouple can measure within the same pressure ranges as the Pirani gauge and at a much lower price.

Cost

• A Pirani gauge is not something you can get at the local hardware store. In fact, most distributors of these gauges are headquartered in Europe. Cost of a Pirani gauge before shipping or tax charges can range from $600 to $1,000 as of November 2009. However, if you have some technical and engineering expertise, it is possible to construct your own Pirani gauge using a do-it-yourself kit purchased through a number of reputable online distributors. Unfortunately, after the cost of professional calibration is added to the cost of the kit, you may find you are not saving enough money to justify the effort.