IG9972BEN (2017-09) Plug-In Boards.om 5
2 Description
The CP300C9 board is a combined board containing one Pirani and one cold
cathode measurement circuit.
The Pirani measurement circuit has one gauge cable connector, two trimmer po-
tentiometers and one analog signal output. When the control unit is on, the Pirani
measurement circuit is in continuous operation. The analog signal is constantly
available, independent of what is shown on the pressure display.
The cold cathode measurement circuit for the measurement of high and ultra high
vacuum has one gauge cable connector and one analog signal output. When the
gauge is turned on, the analog signals are constantly available, independent of
what is shown on the pressure display.
This measurement board contains special electronics to limit the measurement
current to 100 µA, a feature that considerably extends the lifetime of the gauge.
The IF300P board contains a RS232C interface and a Profibus-DP interface. The
board has five relays with one floating changeover contact each.
Within certain limits the thermal conductivity of gases is a function of the pressure.
Pirani thermal conductivity vacuum gauges exploit this phenomenon for pressure
measurements.
The measurement element consists of a thin filament with a high temperature co-
efficient. The resistance of the wire and consequently its temperature are main-
tained at a constant value by means of a suitable control circuit. The electrical
power supplied to the filament is, therefore, a measure of the thermal conductivity
and consequently the gas pressure.
The current flowing in a self-sustained gas discharge with a cold cathode (inverted
magnetron) depends on the applied voltage, the gas composition, and the pres-
sure. A magnetic field that penetrates the measurement chamber has the effect
that the electrons move along a spiral trajectory from the cathode to the anode and
thereby cause even at low gas densities a sufficient number of ionizing impacts for
maintaining the discharge. If (with a known gas type) the anode voltage and mag-
netic field are kept constant, the discharge current is a measure of the pressure.
2.1 Pirani / Cold Cathode
Measurement Board
CP300C9
Pirani measurement circuit
Cold cathode measurement
circuit
2.2 Interface and Relay
Board IF300P
2.3 Pirani Measurement
2.4 Cold Cathode
Measurement
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