INFICON BCG450 ATM to Ultra-High Vacuum Triple Gauge - Manual (Page 26)

26
tina40e1-b (2011-04)
V b
Pirani sensor
The bridge voltage V
m
is a measure for the gas pressure and is further processed
electronically (linearization, conversion).
A capacitance diaphragm sensor consists of a reference vacuum chamber, sepa-
rated from the measured gas pressure by a diaphragm. The capacitance of a pair
of electrodes attached to the diaphragm and the chamber is measured electroni-
cally. A pressure difference acting upon the diaphragm will deflect it and cause a
change of capacitance between the electrodes. A pressure related measuring sig-
nal is produced, ready for further processing.
Capacitance
measurement
Measured gas
pressure
Reference vacuum chamber Diaphragm
Electrodes
Measuring
signal
The BCG450 gauges continuously cover the measuring range
5×10
-10 mbar … 1500 mbar.
• The Pirani constantly monitors the pressure.
• The hot cathode (controlled by the Pirani) is activated only at pressures
<2.4×10 -2 mbar.
If the measured pressure is higher than the switching threshold, the hot cathode is
switched off and the Pirani measurement value is output.
If the Pirani measurement drops below the switching threshold (p = 2.4×10
-2 mbar),
the hot cathode is switched on. After heating up, the measured value of the hot
cathode is fed to the output. In the crossover range of 5.5Ч10
-3 … 2.0Ч10 -2 mbar,
the output signal is generated from both measurements.
Pressure rising over the switching threshold (p = 3.2×10
-2 mbar) causes the hot
cathode to be switched off. The Pirani measurement value is output.
• Above 10 mbar (up to 1500 mbar) the measurement signal of the capacitance
diaphragm sensor is used.
In the crossover range 1 … 10 mbar a mixture of the Pirani sensor signal and the
capacitance diaphragm sensor signal is processed.
Schematic
Capacitance diaphragm sensorr
Schematic
Measuring range