INFICON BPG402-Sx ATM to Ultra-High Vacuum Gauge - Manual (Page 26)

26
tina46e1-a (2010-03) BPG402.om
Pirani sensor
B
V
The bridge voltage V
B
is a measure for the gas pressure and is further processed
electronically (linearization, conversion).
The BPG402 -Sx 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).
BPG402-Sx sensors are equipped with two identical filaments. They are perma-
nently monitored by the gauge electronics. In case of a filament breakage, the
gauge will immediately react and switch over to the second (undamaged) filament.
During the change over procedure, the last valid pressure value before filament
failure will be output. As soon as the second filament is operating and the emission
parameters have settled (t <4s), the measuring circuit resumes operation. A "Hot
Cathode Warning" is generated during this switch over cycle. The filament status
indicator LED on the gauge will display the incident (blinking green,
→ 28). The
filament status can also be read via the RS232C or field bus interfaces (
→ 30,
[1] and [2]). Additionally, the BPG402-S offers a relay contact "Filament status"
on the sensor cable connector ( → 19). In case of two broken filaments, a "Hot
Cathode Error" is generated. Again, this status can be read via the interfaces
(
→ 30, [1] and [2]) and is also displayed by a red filament status indicator
LED on the gauge ( → 28). In this case, the sensor has to be replaced (→ 44).
At the beginning of every "Emission ON" cycle, the gauge alternates between fila-
ments in order to age both filaments evenly. However, filament selection can be
commanded via the interfaces (
→ 28, 30, [1] and [2]).
INFICON recommends the replacement of the sensor as soon as the first filament
failure has been detected. The replacement can be carried out during an appropri-
ate off time such as the completion of a process or a planned maintenance brake.
Schematic
Measuring range
Gas type dependence
Dual filament feature