Thyracont VSI17D, Operating Instructions Manual

Page: 34 / 52

34
VSI-de-170326
4 Operation
4.1 General
Measurement Principle
The VSI vacuum transducer is equipped with an internal cold cathode sensor.
The cold cathode sensor of type inverted magnetron 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 output signal 2.199 - 7.801 V of your VSI has a logarithmic dependence on
pressure over the whole measurement range 5x10
-9 - 2x10 -3 mbar.
Conversion of voltage signal and pressure is done according to the following for-
mula:
The output voltage characteristic can be scaled by software command, see
chapter 5.7.
Serial Interface RS485
The measured absolute pressure can be read out digitally via the transducers
serial RS485 interface. Additionally you can set various parameters like gas cor-
rection factors or setpoints. For further information see chapter 5 communication.
Warm-Up Time
The signal output of VSI is available approx. 2s after the unit is switched on.
Switching-on of the sensor can be done manually or automatically (see chapter
5.7). 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.
Dependence On Gas Type
The output signal depends on composition and type of the gas being measured.
The unit is adjusted for N
2
and dry air. For other gases correction factors for both
sensor types can be set via RS485 (see chapter 5.7).
V
out
/V = log (p/mbar) + 10.5
p /mbar = 10
( Vout /V – 10.5)