5. Theory of Operation
The functional parts of a typical ionization gauge tube are the filament (cathode), the grid (anode), and the ion collector (refer to Figure 4 below). The
A current passed through the filament causes it to heat up sufficiently enough to cause electron emission. The IGE 3000 control circuit controls the filament current to maintain a constant electron emission. The difference of potential between the filament and the grid causes these emitted electrons to accelerate toward the grid. While most of the electrons collide directly with the grid, many of these electrons traverse the region inside the grid. When an electron collides with a gas molecule, electrons may become disassociated from the gas molecules leaving a positively charged ion. These ions accelerate to the collector. The rate of the electron/molecule collisions is proportional to the density of gas molecules. Therefore ion current is proportional to the density (i.e. pressure) of the gas. The collector’s ion current is fed into a precision electrometer where it is converted to a DC voltage. This DC voltage is then processed through a precision analog to digital converter and the data is passed to the
The B/A tube sensitivity is expressed by the term “K” factor and is dependant on the physical design of the tube. The definition of K=ion current/ (emission current x pressure). The K factor for the
Figure 4- Bayard Alpert (B/A) Tube
Figure 5- Output Graph
IGE 300 | Page 14 of 21 |
