Operation
8 | 44 Instruction Manual VACTEST PPP 700 D_EN_en
4 Operation
4.1 General
Measurement Principle
The VACTEST PPP 700 D is equipped with an internal combination of a piezo-resistive diaphragm
sensor and a Pirani sensor, which uses the heat conduction of the gas for measuring vacuum. An-
other piezo-resistive sensor measures ambient pressure.
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 tem-
perature, so the bridge voltage is a measure for total gas pressure. For relative pressure output the
ambient pressure is subtracted from the measured absolute pressure inside the sensor flange.
Warm-Up Time
The signal output is available approx. 2 seconds after the device 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 device is adjusted ex works in upright position with the flange axis horizontally. Through con-
tamination, ageing or extreme climatic conditions 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. For other gases correction factors can be set (see 4.6.3 Sensor
[➔20]). This will result in a correct pressure display below 0.1 mbar.
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