Thyracont VSH87D, Руководство по эксплуатации, Страница: 40 / 64

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4 Operation
4.1 General
Measurement Principle
The VSH vacuum transducer is equipped with an internal combination sensor of
type Pirani / Hot 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 Bayard Alpert type hot cathode sensor ionizes gas molecules by electron bom-
bardment. 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 1.219 V - 8.6 V of your VSH has a logarithmic dependence on
pressure over the whole measurement range 5.0x10
-10
- 1000 mbar. Conversion of
voltage signal and pressure is done according to the following formula:
V
out
/V = 0.6log(p/mbar) + 6.8
p/mbar = 10
((V
out
/V - 6.8) / 0.6)
The output voltage characteristic
can be scaled by software com-
mand, see section 5.8.
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
2
and dry air. For other gases correction factors for both
sensor types can be set (see section 5.8). This will result in a correct pressure
display below 0.1 mbar.
VSH-RS485-200116