26
4 Operation
4.1 General
Measurement Principle
VSP and VCP vacuum transducers are equipped with an internal Pirani sensor.
The sensor filament is heated to a certain temperature threshold in pulsed mode,
the time required for heating is a measure for total gas pressure.
Output Signal
The output signal 1.5 V - 8.5 V of your VSP (VCP: 2.2 V - 8.5 V) has a logarithmic
dependence on pressure over the whole measurement range 1.0x10
-4
- 1000 mbar
(VCP: 5.0x10
-4
- 1000 mbar). Conversion of voltage signal and pressure is done
according to the following formula:
V
out
(V) = log(p(mbar)) + 5.5
p(mbar) = 10
(V
out
(V) - 5.5)
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 the pressure reading can
be corrected below 0.1 mbar by means of gas type factors, e.g in combination with
Thyracont display units.
Correction Factor Pirani:
Ar CO CO
2
H
2
He N
2
Ne Kr
1.6 1.0 0.89 0.57 1.0 1.0 1.4 2.4
VSP/VCP-0-10 V-201126
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