How does a Pirani gauge work?

A Pirani gauge is a thermal conductivity vacuum gauge used to measure low to medium vacuum levels, generally from about 0.5 Torr down to 10⁻⁴ Torr. Its operation is based on the principle that the thermal conductivity of gases changes with pressure.

Working Principle

1. Heating Element: The gauge consists of a thin filament, usually made of tungsten or platinum, which is heated by a constant electric current. When the filament is in a vacuum, the heat dissipates through conduction to the surrounding gas molecules.

2. Thermal Conductivity: As gas pressure decreases, fewer gas molecules collide with the filament, resulting in less heat being conducted away. At higher pressures, more gas molecules are present, leading to more efficient heat transfer.

3. Measuring Resistance Change: As the filament heats up, its electrical resistance changes. This resistance variation is measured and correlated to the gas pressure. When the vacuum level is high (fewer gas molecules), the filament stays hotter, leading to higher resistance. When the pressure is higher, the filament cools more due to increased molecular collisions, leading to lower resistance.

4. Calibration: The gauge is calibrated to translate this resistance into a pressure reading. Pirani gauges are generally calibrated for specific gases (usually air or nitrogen), so the accuracy can vary with different gases due to differences in thermal conductivity.

Applications and Limitations

• Applications: Pirani gauges are commonly used in vacuum systems that operate in the low to medium vacuum range, such as in vacuum furnaces, coating processes, and other industrial applications.
• Limitations: The accuracy of a Pirani gauge can be affected by changes in gas composition, as different gases have different thermal conductivities. Additionally, they are less effective at very high vacuums (e.g., below 10⁻⁴ Torr), where other types of gauges like ionization gauges are preferred.