Vacuum Electronics: Bellows, Thermal, and Ionization Transducers

Vacuum electronics utilize various types of transducers for pressure measurement. These include bellows mechanical transducers, diaphragm transducers, thermal transducers, and ionization transducers.

Bellows and Diaphragm Transducers

Bellows and diaphragm transducers operate based on the pressure differential between atmospheric pressure and the process being measured. They can be offset relative to atmospheric pressure and calibrated in absolute units. These transducers are suitable for measuring high vacuum. They may be attached to strain gauges or capacitive elements for electrical output.

Thermal Transducers

Thermal transducers are based on the principle of proportionality between the energy dissipated from the hot surface of a filament heated by a constant current and the pressure of the surrounding gas at low absolute pressures.

Pirani Transducer

The Pirani transducer uses a Wheatstone bridge circuit to compare the resistance of two filaments. One filament is sealed in a high vacuum, while the other is in contact with the gas being measured and loses heat through conduction. In this transducer, the resistance of the filament reflects the pressure rather than its temperature. The pyramidal transducer is compact and easy to operate and can withstand atmospheric pressure without the risk of combustion. However, its calibration depends on the composition of the measured gas.

Bimetallic Transducers

This transducer uses a bimetallic strip heated by a stabilized voltage source. Any change in pressure produces a deflection in the spiral, which is coupled to a scale for measurement.

Ionization Transducers

Vacuum ionization transducers are based on the formation of ions produced by collisions between molecules and electrons (or alpha particles in radiation-based types). The rate of ion formation, i.e., the ion current, varies directly with pressure.

Hot Filament Transducer

The hot filament transducer consists of an electronic tube with a tungsten filament surrounded by a grid (forming a coil), which is, in turn, surrounded by a collector plate. Electrons emitted by the hot filament are accelerated toward the positively charged grid, passing through it. Some electrons collide with gas molecules on their way to the negatively charged collector plate. The resulting positive ion current is a function of the number of ions and, therefore, a measure of gas pressure. These transducers are very sensitive and capable of measuring extremely high vacuum. Their electrical output signal is linear with pressure. However, they are sensitive to gas composition, and the hot filament can sometimes cause significant changes in the composition between the measured volume and the volume within the electronic tube.