INFICON HPG400 ATM to High-Vacuum Gauge - Manual (Page 24)

24
tina31e1 (2004-05) HPG400 v1.om
Within certain limits, the thermal conductance of gases is pressure dependent. This
physical phenomenon is used for pressure measurement in the thermal conduc-
tance vacuum meter according to Pirani. A self-adjusting bridge is used as meas-
uring circuit. A thin tungsten wire is used as sensor element. Wire resistance and
thus temperature are kept constant through a suitable control circuit. The electric
power supplied to the wire is a measure for the thermal conductance and thus the
gas pressure. The basic principle of the self adjusting bridge circuit is shown below.
U
m
Pirani sensor
+ –
The bridge voltage U
m
is a measure for the gas pressure and is further processed
electronically (linearization, conversion).
The HPG400 covers the measuring range 2×10
-6
... 1000 mbar.
1000 mbar 2 × 10 -6 mbar
Hot cathode HP Pirani
5×10 -2 mbar ... 1 mbar user-
definable changeover threshold
• The Pirani permanently monitors the pressure.
• The hot cathode (controlled by the Pirani) is activated only at pressures
<1 ... 5×10
-2
mbar (threshold can be set with a switch).
If the measured pressure is higher than the set changeover threshold (which can
be selected with a switch within the range of 1 ... 5×10
-2
mbar), the hot cathode
remains off and the Pirani value is output as signal (
→ Appendix A).
When the pressure measured by the Pirani drops below the threshold, the hot
cathode is activated. This is indicated by a green lamp. After warming up of the
measuring system, the hot cathode value is output. When the pressure rises above
the set threshold, the hot cathode is deactivated, and the Pirani value is output
again.
Pirani (control range)
Measuring range,
changeover threshold