Vaisala PTB220 User Manual download pdf (Page 12)

USER'S GUIDE_______________________________________________________________________

6 ____________________________________________________________________ M210194EN-A

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 (FIGURE 2-2 )

out

FIGURE 2-2 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.

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Vaisala PTB220 User Manual Page 12