Operation; General - thyracont VSM77D 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 type inverted magnetron creates ionized gas
molecules 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 1.82 - 8.6 V of your VSM has a logarithmic dependence on
pressure over the whole measurement range 5x10
Conversion of voltage signal and pressure is done according to the following
formula:
V /V = 0.6 log (p/mbar) + 6.8
out
p /mbar = 10
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
correction factors or setpoints. For further information see chapter 5 communica-
tion.
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
sensor types can be set via RS485 (see chapter 5.8). This results in a correct
pressure display below 0.1 mbar.
VSM-de-171005
Downloaded from www.Manualslib.com
( (Vout /V – 6.8) / 0.6 )
and dry air. For other gases correction factors for both
2
manuals search engine
-9
- 1000 mbar.
The output voltage characteristic can
be scaled by software command, see
chapter 5.8.