Pirani gauges are a group of indirect measuring pressure gauges which measure the thermal conductivity of the gas sample. Pirani gauges are typically used in the fine vacuum range (1 torr – 10-3torr), as this is the range where they achieve the best accuracy. A typical accuracy spec for these indirect-reading gauges is ±15% of reading in the fine vacuum range. Specially-designed Pirani gauges have the capability to provide accurate pressure readings to as low as 10-4 torr.
Pirani gauges are built upon a common type of electric circuit called a Wheatstone bridge. These circuits use a series of three known resistors in order to measure the resistance of a fourth unknown resistor with a high degree of precision. In the present case, the fourth resistor represents our Pirani gauge. As the thermal conductivity of the gas sample varies, the resistance itself will vary and produce a change in the output of the circuit. For those interested in a more detailed description of Wheatstone bridges, Engineersedge.com and Wikipedia both provide good reference information.
Since Pirani gauges rely upon the thermal conductivity of the gas in order to ascertain the absolute pressure level, the accuracy of Pirani gauges is inherently gas-dependent. Gases with similar thermal characteristics will result in similar readings; gases with different thermal characteristics will result in starkly different measurements. Consequently, it is important to calibrate your Pirani gauge with the gas mixture that you intend to measure. When using a Pirani gauge, it is also important that you allow the gauge to reach equilibrium with respect to both temperature and flow, since heat transfer is strongly affected by temperature variations (conduction) and mass flow (convection). Following these and other recommendations from the manufacturer of your Pirani gauge will help you to achieve the most accurate possible readings.
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