The Pirani gauge is a widely used instrument for measuring vacuum pressure in the range of 10^-4 to 1000 Pa. It operates based on the principle of thermal conductivity, where the heat transfer rate from a heated filament to the surrounding gas depends on the gas pressure.
- Heated Filament: A thin metal filament (typically tungsten or platinum) is heated electrically by passing a current through it.
- Heat Transfer: The heated filament transfers heat to the surrounding gas particles through collisions.
- Pressure Dependence: The rate of heat transfer depends on the pressure of the gas. Higher pressure means more gas particles, leading to faster heat transfer and a lower filament temperature.
- Resistance Change: The filament’s resistance changes with temperature, and this change is measured using a Wheatstone bridge circuit.
- Pressure Measurement: The measured resistance change is calibrated to provide a corresponding pressure reading.
Advantages of the Pirani Gauge:
- Simple and robust design
- Wide measurement range
- Fast response time
- Relatively inexpensive
Disadvantages of the Pirani Gauge:
- Accuracy decreases at higher pressures
- Susceptible to contamination and outgassing
- Cannot distinguish between different gas types
Applications of the Pirani Gauge:
- Vacuum coating processes
- Leak detection systems
- Refrigeration systems
- Chemical analysis equipment
- Spacecraft and satellites
Other Important Information:
- The Pirani gauge was invented by Marcello Pirani in 1906.
- Several variations of the Pirani gauge exist, including the pulsed Pirani gauge and the cold cathode Pirani gauge.
- The Pirani gauge is often used in conjunction with other vacuum gauges, such as the ion gauge, to cover a wider pressure range and improve accuracy.