Thyracont SP363, Operating Instructions Manual

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24
4 Operation
4.1 General
Measurement Principle
VSR vacuum transducers are equipped with an internal combination sensor of ty-
pe Piezo / Pirani.
Under the influence of pressure the thin diaphragm of the piezo-resistive sensor
is bent, on the back of which a resistor-bridge is applied. The bending forces the
measuring bridge to come out of tune, which is a measure for the applied pressure.
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.
Output Signal
The output signal 4.0 - 20 mA of your VSR has a logarithmic dependence on pres-
sure over the whole measurement range 5.0x10
-5
- 2000 mbar. Conversion of out-
put signal and pressure is done according to the following formula:
I
out
(mA) = 50/26 x log(p(mbar)) + 354/26
p(mbar) = 10
( 26/50 x (I
out
(mA) - 354/26) )
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.
Dependence on Gas Type
The output signal of the Pirani sensor depends on composition and type of the gas
being measured. The unit is adjusted for N
2
and dry air.
VSR-4-20 mA-210805