Operation
26
GA09420_002_C0 – (2016-10) –
Leybold
Pirani
Within certain limits, the thermal conductibility 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 (→ schematic). A thin tungsten wire forms the sensor element. Wire
resistance and thus temperature are kept constant through a suitable control cir-
cuit. The electric power supplied to the wire is a measure for the thermal conduc-
tance and thus the gas pressure. The basic principle of the self-adjusting bridge
circuit is shown in the following schematic.
Schematic
The bridge voltage U
B
is a measure for the gas pressure and is further processed
electronically (linearization, conversion).
Measuring range
The ITR 90 transmitters 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 5.5×10
-3
… 2.0×10
-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.
Gas type dependence
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).
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