Vacuum Transducers and Pressure Control Mechanisms

Vacuum and Pressure Measurement Devices

Cold Cathode Ionization Transducer

The cold cathode transducer is based on the principle of measuring the ion current produced by a high-voltage discharge. Electrons detached from the cathode take a spiraling motion as they move through a magnetic field on their way to the anode. This spiraling motion ensures that the mean free path of the electrons is greater than the distance between the electrodes.

This extended path increases the possibility of collisions with residual gas molecules, leading to increased ion current. Consequently, the cathode discharge is maintained at a lower pressure, indicating a higher vacuum level. While this instrument cannot degas as quickly as a hot filament gauge, it is significantly more robust and eliminates the risk of filament burnout.

Thermocouple Vacuum Transducer Technology

The thermocouple temperature transducer contains a filament (often a strand of resistive material) which incorporates a small thermocouple. If a constant current is passed through the filament, its temperature is inversely proportional to the absolute pressure of the surrounding gas. The electromotive force (EMF) generated by the thermocouple indicates the filament's temperature and, therefore, points to the vacuum environment.

To compensate for ambient temperature, a second reference unit is used, contained within a vacuum-sealed tube. The differential output signal of the two thermocouples is directly proportional to the pressure.

Alpha Source Radiation Transducer (Ion Gauge)

A radiation transducer utilizes a sealed radioisotope source that produces alpha particles. These particles ionize the gas molecules within the vacuum chamber. The resulting ions are collected at an electrode, generating a current that varies directly with the number of particles in the vacuum chamber and is therefore proportional to the system's pressure.

Key characteristics of this instrument include:

• It incorporates no hot filament.
• It is highly robust and can withstand exposure to atmospheric pressure without damage.
• It has stable emissivity and is reliable.

At very low pressures, a preamplifier is required because the generated currents are extremely small, typically on the order of $10^{-11}$ to $10^{-13}$ A. Its typical measuring range is $760$ to $10^{-4}$ mm Hg.

Pressure Switch (Presostato) Functionality

A pressure switch, also known as a presostato, is a device that closes or opens an electrical circuit depending on the pressure reading of a fluid. The pressure of a fluid exerts force on an internal track (such as a diaphragm or piston), causing it to move until it joins two electrical contacts.

When the pressure drops, a spring pushes the track in the opposite direction, separating the contacts. A screw is used to adjust the trigger sensitivity of the pressure switch by applying more or less force on the internal mechanism via the spring.

Pressure switches usually have two separate settings:

1. The ON pressure setting.
2. The OFF pressure setting (or differential setting).