The original Pirani vacuum gauge measures the vacuum gas pressure indirectly by measuring the pressure dependent heat-loss of a suspended hot metal wire to the surrounding gas.
The Pirani gauge was invented and first published in 1906 by Marcello Stefano Pirani. The invention of the Pirani gauge was a classic example of experiencing a problem and then finding an innovative solution to that problem.
Marcello S. Pirani was born in Berlin on July 1st, 1880 and studied physics at the Technical University in Berlin-Charlottenburg, where he completed his Ph.D. in 1904. In the same year, Marcello Pirani joined the lamp factory at Siemens & Halske, where he was engaged in the lamp manufacturing and gearing up the manufacturing of tantalum lamps for high volume production.
In the lamp manufacturing industry, McLeod vacuum gauges were used for measurement of vacuum level during the air evacuation process and gas filling of the light bulbs. The McLeod gauge is based on the measurement of liquid mercury column volume and height, which depends on the vacuum pressure. The internal volume of the McLeod gauge consists of approximately 2 kg of liquid mercury. The gauge is made of glass and consequently broken McLeod gauges were common on the lamp manufacturing floor with toxic mercury liquid and vapor exposed to the surroundings and workers environment as a result.
Marcello Pirani was determined to find an alternative solution to overcome the toxicity and hazardous nature of the McLeod gauge and noted that using thermal conductivity for low pressure measurement was a promising approach.
The result of Pirani’s idea was a metal wire-based thermal conductivity vacuum gauge arranged in a four-element Wheatstone bridge circuit. Instead of determining the vacuum pressure by reading the level of a mercury column, the vacuum pressure could now indirectly be determined by measuring the thermal conductivity of the gas. The thermal conductivity of the gas was converted to an electric signal and read on a voltmeter for conversion to vacuum pressure level.
The Pirani gauge solved the challenges and problems with mercury hazards. However, the gas dependency of the measurement of thermal conductivity required that the gauge was calibrated to the gas or gas mixture being used. The Pirani gauge became an industry standard gauge and is to this day still widely used in many applications.
Marcello Pirani would later become the head of the Osram Research Laboratories, where his main activities were outside of the vacuum technology area. During his career, he made many significant inventions in the field of light sources. Pirani passed away in 1968 and became a legend in the vacuum science for his invention of the Pirani vacuum gauge.
The Pirani gauge remains the most commonly used vacuum measuring principle in the vacuum industry today. It has earned its popularity because of its robustness, low cost and wide dynamic measurement range.
Today’s wire Pirani gauges are not based on suspended wire in glass ball, but a thin metal wire made of nickel suspended in a tube.
In 1993 Pirani gauges based on MEMS (Micro Electrical Mechanical system) sensor technology were introduced. The improved design resulted in a wider measurement range and better overall performance. In 2019, a next-generation MEMS Pirani transducer pushed the limits further by extending the measurement range down to 1.0E-6 mbar (7.5E-7 Torr).
The Pirani vacuum measurement technology invented more than 100 years ago is still improving and the technology is being developed to enhance performance and extend the useable applications for the heat-loss Pirani gauge.
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