Measurement principle – Vaisala PTB220 User Manual
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Measurement principle
The measurement principle of the PTB220 series digital barometers is
based on an advanced RC oscillator with three reference capacitors
against which the capacitive pressure sensor and the capacitive
temperature compensation sensor are continuously measured. A
multiplexer connects each of the five capacitors to the RC oscillator
one at a time and five different frequencies are measured during one
measurement cycle (
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3
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F
IGURE
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RC-oscillator with five capacitors
The RC oscillator is designed to attenuate changes in stray impedance
and to achieve excellent measurement stability with time. Vaisala’s
electronic measurement principle emphasizes in the first place
stability over a wide environmental temperature and relative humidity
range and over a long period of time; yet it can achieve fast
measurement speed and high resolution at the same time.
In the fast measurement mode, a special measurement algorithm is
used. In this mode only the frequency from the BAROCAP
pressure
sensor is measured continuously while the frequencies from the three
reference capacitors and from the thermal compensation capacitor are
updated only every 30 seconds. This is quite justifiable as the changes
in the reference capacitors can be considered negligible over any
periods of time, and the internal temperature of the barometer remains
stable enough over a few tens of seconds. The fast measurement mode
achieves a speed of ten measurements per second at 1 pascal
resolution. Each measurement represents the pressure average during
the last 100 ms. When the reference frequencies are measured every
30 seconds the outputting stops for a short moment and typically one
measurement is lost during this time. The fast measurement mode can
be used only in barometers with one pressure transducer and in full
duplex communication.