The triode or thermionic emission ionization transducer is a hot-cathode vacuum gage. The top cross-sectional view is shown diagrammatically in Fig. 5.35.
|Fig5.35 Hot-cathode vacuum gage.|
It consists of a tungsten filament and cathode in the center. Concentric with the cathode is the grid, which is made from a fine nickel wire helix, held in position by upright supports. Surrounding the grid is the external nickel plate electrode, which is concentric with the grid. The gage is housed in an enclosure with an opening that is connected to the chamber whose pressure is being measured.
In this configuration, the electrons are accelerated from the cathode to the grid (+180 V), but most electrons not collected by the fine helix move toward the plate (-20 V). As they move in the vicinity of the plate, the electrons get repelled by the negative potential of the plate.
These electrons undergo several oscillations between the grid and the plate, and during this process they collide with the gas molecules, creating positive ions. The ions are attracted toward the negatively biased plate collector, and an ion current flows in the plate circuit.
This ion current, within a certain pressure range, is proportional to the total pressure (vacuum) in the chamber. The effective range for the gage is 10-8 to 10–3 torr.
The Bayard Alpert vacuum transducer is an example of a hot-cathode ultra-high-vacuum gage. The collector is in the center and consists of a fine nickel wire. The filament and cathode located outside the grid. Its operation is similar to that of the triode gage.
The ion current is proportional to the gas pressure down to 10-10 torr, because the undesired x-rays generated by the grid are minimally intercepted by the wire collector. Its operating range is from 4×10-10 to 5×10-2 torr. To measure pressures lower than torr, the glass envelope is not used. The operating range of the nude gage is 2× 10–11 to 1×10–3 torr.