INFICON BPG400 ATM to Ultra-High Vacuum Gauge - Manual (Page 28)

28
tina03e1-b (2004-02) BPG400 v1.om
U
B
Pirani sensor
The bridge voltage U
B
is a measure for the gas pressure and is further processed
electronically (linearization, conversion).
The BPG400 gauges continuously cover the measuring range
5×10
-10 mbar … 1000 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 overlapping 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.
The output signal is gas type dependent. The characteristic curves are accurate for
dry air, N
2
and O
2
. They can be mathematically converted for other gases
( → Appendix B).
The measuring currents of the Bayard-Alpert and Pirani sensor are converted into a
frequency. A micro-controller converts this frequency into a digital value repre-
senting the measured total pressure. After further processing this value is available
as analog measurement signal (0 … +10 V) at the output (sensor cable connector
Pin 2 and Pin 12). The maximum output signal is internally limited to +10 V
(atmosphere). The measured value can be read as digital value through the
RS232C interface (Pins 13, 14, 15) (
→ 31). Gauges with a display show the
value as pressure. The default setting of the displayed pressure unit is mbar. It can
be modified via the RS232C interface (
→ 31).
In addition to converting the output signal, the micro controller's functions include
monitoring of the emission, calculation of the total pressure based on the meas-
urements of the two sensors, and communication via RS232C interface.
Schematic
Measuring range
Gas type dependence
4.2 Operational Principle of
the Gauge