Technical Note: Vacuum Pressure Measurement Gauges

A pirani gauge is used for a high vacuum because it measures from atmospheric down to approximately 1 millitorr. The word approximately is used because pirani gauges tend to be accurate to 2% of the actual reading. When requiring better accuracy a capacitance manometer would be the choice. Capacitance manometers are typically accurate to 0.25% to 0.5% of the actual reading.

In the Pirani Gauge design, a sensor wire is heated electrically and the pressure of the gas is determined by measuring the current needed to keep the wire at a constant temp. The thermal conductivity of each gas is different so the gauge will measure the relative vacuum. Due to the fact that it consists of an exposed wire, a pirani gauge can't be used when dealing with solvents.

The Capacitance manometer operates by measuring the absolute pressure. The design consists of a moveable tensioned metal diaphragm with one side exposed to the gas and the other side containing an electrode assembly in a sealed high vacuum reference cavity. The gauge has the ability to detect the smallest changes in the pressure because of the smallest movements of the diaphragm. This construction also makes it immune to contamination so it can be used in working with corrosive products, products with solvents, and steam sterilizable applications.