What Is a Pirani Vacuum Gauge?
A Pirani vacuum gauge is a sensor that measures vacuum levels using the piezoelectric effect. It is often used in various situations due to its small size, portability, and ease of operation. In this device, the current flowing to the measuring element changes according to the vacuum level, enabling the measurement of vacuum levels. Generally, a Pirani gauge consists of a controller and a measuring element, covering a range from about 0.5kPa to 2kPa.
Though Pirani gauges have limitations in accuracy at lower ranges, they can operate from atmospheric pressure to vacuum without concerns of breakage. Some models feature a separable measuring element and controller.
Uses for Pirani Vacuum Gauges
Pirani gauges are commonly used in fluid circuits where vacuum reach control is crucial, such as in chambers and quartz tubes, and in applications requiring surface cleanliness. They are particularly effective in the medium vacuum range, achievable with oil-rotary vacuum pumps. These gauges are also used in high vacuum to ultra-high vacuum pumping systems, medical instruments, analyzers, semiconductor manufacturing, and electron microscopes.
However, calibration is necessary for accurate absolute pressure measurements, as the indicated value varies with the type and composition of the gas.
Principle of the Pirani Vacuum Gauges
Pirani gauges function by measuring the thermal energy dissipation when gas impinges on a heated platinum wire. The pressure is inferred from the current value required to maintain the wire’s temperature. The platinum wire inside the gauge is heated to about 200 degrees Celsius. When air contacts the wire, it cools it down, requiring more power to maintain the temperature. Conversely, in a vacuum, less power is needed to keep the wire heated. The power change indicates the vacuum level.
Regular maintenance of the platinum wire is vital for accuracy. Over time, the wire wears out and requires more power to reach 200°C, necessitating periodic replacement and cleaning to avoid dust adhesion.
Other Information on Vacuum Gauges
Definition of Vacuum
Vacuum levels are classified into low, medium, high, and ultra-high vacuum, with Pirani gauges effectively measuring from low to medium vacuum. According to JIS standards, vacuum levels are categorized as follows:
- Low vacuum: 105Pa to 102Pa
- Medium vacuum: 102Pa to 10-1Pa
- High vacuum: 10-1Pa to 10-5Pa
- Ultra-high vacuum: 10-5Pa to 10-8Pa
- Extremely high vacuum: 10-8Pa or less