page_title: Pirani gauge
section_title: Mode of operation
hierarchical_section_title: Pirani gauge / Mode of operation
caption_attribution_description: Deutsch: Gasartabhängigkeit des pirani-Vakuummeter
context_page_description: 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.
  Marcello Stefano Pirani was a German physicist working for Siemens & Halske which was involved in the vacuum lamp industry. In 1905 their product was tantalum lamps which required a high vacuum environment for the filaments. The gauges that Pirani was using in the production environment were some fifty McLeod gauges, each filled with 2 kg of mercury in glass tubes.
  Pirani was aware of the gas thermal conductivity investigations of Kundt and Warburg published thirty years earlier and the work of Marian Smoluchowski. In 1906 he described his "directly indicating vacuum gauge" that used a heated wire to measure vacuum by monitoring the heat transfer from the wire by the vacuum environment.
context_section_description: In order to understand the technology, consider that in a gas filled system there are four ways that a heated wire transfers heat to its surroundings.
  Gas conduction at high pressure  (r representing the distance from the heated wire)
  Gas transport at low pressure 
  Thermal radiation 
  End losses through the support structures
  A heated metal wire (sensor wire, or simply sensor) suspended in a gas will lose heat to the gas as its molecules collide with the wire and remove heat. If the gas pressure is reduced the number of molecules present will fall proportionately and the wire will lose heat more slowly. Measuring the heat loss is an indirect indication of pressure.
  There are three possible schemes that can be done. 
  Keep the bridge voltage constant and measure the change in resistance as a function of pressure
  Keep the current constant and measure the change in resistance as a function of pressure
  Keep the temperature of the sensor wire constant and measure the voltage as a function of pressure
  Note that keeping the temperature constant implies that the end losses(4.) and the thermal radiation loses (3.) are constant.
  The electrical resistance of a wire varies with its temperature, so the resistance indicates the temperature of wire. In many systems, the wire is maintained at a constant resistance R by controlling the voltage I through the wire. The resistance can be set using a bridge circuit.  The voltage required to achieve this balance is therefore a measure of the vacuum.
  The gauge may be used for pressures between 0.5 Torr to 1×10⁻⁴ Torr. Below 5×10⁻⁴ Torr, a Pirani gauge has only one significant digit of resolution. The thermal conductivity and heat capacity of the gas affects the readout from the meter, and therefore the apparatus may need calibrating before accurate readings are obtainable. For lower pressure measurement, the thermal conductivity of the gas becomes increasingly smaller and more difficult to measure accurately, and other instruments such as a Penning gauge  or Bayard-Alpert gauge are used instead.
caption: Curves to convert air readings to other gases
exif: {}