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 molecu-
les by an electric discharge. The resulting ion current is a measure for the number
of gas molecules present in the sensor and thus a measure for absolute pressure.
Output Signal
The output signal 1.82 V - 8.6 V of your VSM has a logarithmic dependence on
pressure over the whole measurement range 5.0x10
voltage signal and pressure is done according to the following formula:
V /V = 0.6log(p/mbar) + 6.8
out
((V
p/mbar = 10
Serial Interface RS485
The measured absolute pressure can be read out digitally via the transducer's seri-
al RS485 interface. Additionally you can set various parameters like gas correction
factors. For further information see chapter 5.
Warm-Up Time
The signal output 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 a
stabilization time of 5 minutes, especially when extreme pressure changes have
occurred.
Accuracy
The unit is adjusted ex works in upright position and at 24 VDC 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 20 mbar.
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 (see section 5.8). This will result in a correct pressure
display below 0.1 mbar.
VSM-RS485-200116
/V - 6.8) / 0.6)
out
and dry air. For other gases correction factors for both
2
-9
- 1000 mbar. Conversion of
The output voltage characteristic
can be scaled by software com-
mand, see section 5.8.