handbook of modern sensors

356 10 Pressure Sensors
Fig. 10.15. Pirani vacuum gauge with NTC thermistors operating in self-heating mode.
The same voltage E that feeds the reference plate is applied to the thermistor S v on
the sensing plate via R v = R r . The output voltage V is taken differentially from the
sensing thermistor and the bridge. The shape of the transfer function is shown in Fig.
10.14B. A vacuum sensor often operates with gases that may contaminate the sensing
plates so the appropriate filters must be employed.
10.9.2 Ionization Gauges
This sensor resembles a vacuum tube that was used as an amplifier in the old-fashioned
radio equipment. The ion current between the plate and the filament (Fig. 10.16A) is
a nearly linear function of molecular density (pressure) [16,17]. The vacuum gauge
tube has a reversed connection of voltages: The positive high voltage is applied to a
grid and negative lower voltage is connected to the plate. The output is the ion current
i p collected by the plate that is proportional to pressure and the electron current i g
of the grid. Presently, a further improvement of this gauge is the so-called Bayard–
Alpert vacuum sensor [18]. It is more sensitive and stable at a much lower pressure.
Its operating principle is the same as a vacuum tube gauge, except that the geometry is
different—the plate is substituted by the a wire surrounded by a grid and the cathode
filament is outside (Fig. 10.16B).
10.9.3 Gas Drag Gauge
The gas molecules interact with a moving body. This is the basic idea behind the
spinning-rotor gauge [19]. In the current implementation of the sensor, a small steel
ball having a diameter of 4.5 mm is magnetically levitated (Fig. 10.16C) inside a
vacuum chamber and spinning with a rate of 400 Hz. The ball magnetic moment