Operation; General - thyracont VSM77E Betriebsanleitung

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4

Operation

4.1

General

Measurement Principle
The VSM vacuum transducer is equipped with an internal combination sensor of
type Pirani / Cold Cathode.
The Pirani principle uses the heat conduction of gases for measuring vacuum. A
sensor filament in a Wheatstone circuit is heated to a constant temperature, so
the bridge voltage is a measure for total gas pressure.
The cold cathode sensor of inverted magnetron type creates ionized gas mole-
cules by an electric discharge. The resulting ion current is a measure for the
number of gas molecules present in the sensor and proportional to the absolute
pressure.
Output Signal
The measured absolute pressure can be read out digitally via the transducers
serial RS485 or via EtherCAT interface. Additionally you can set various parame-
ters like gas correction factors. For further information see chapter 5.
Warm-Up Time
The signal output of VSM is available approx.. 2 s after the unit is switched on.
To take advantage of the maximum accuracy of the unit it is appropriate to allow
for stabilization time of 5 minutes, especially when extreme pressure changes
have occurred.
Accuracy
The unit is adjusted ex works in upright position and at 24V voltage supply.
Through contamination, ageing, extreme climatic conditions or different mounting
orientation the need for readjustment may arise. Accuracy is reduced in the
range above 10mbar.
Dependence On Gas Type
The output signal depends on composition and type of the gas being measured.
The unit is adjusted for N and dry air. For other gases correction factors for both
2
sensor types can be set via RS485 (see chapter 5.7). This results in a correct
pressure display below 0.1 mbar.
VSME-de-171111