Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 19
Auto-Calibration
The auto-calibration feature of the MPS1 multiparameter sensor is an
automatic procedure which greatly reduces the possible drift in the
dry end of the dewpoint measurement. It is performed at one hour
intervals, and when the power is switched on. During auto-calibration
the sensor is warmed for a short period (< 1 min) and the sensor
capacitance values are evaluated at the elevated temperature. The
possible dry end drift is then corrected to correspond to the calibrated
values. During the auto-calibration the transmitter outputs the
dewpoint value prior to the procedure.
Auto-calibration is carried out only if several criteria for the
measurement environment are fulfilled. This ensures the reliability of
the adjustments, and maintains the excellent long term stability. These
criteria include, for example, a stable enough moisture level in the
measured atmosphere. If the conditions are not fulfilled, the auto-
calibration function is postponed until satisfactory conditions are
reached.
Sensor Purge
Sensor purge is also an automatic procedure that minimizes the drift
at the wet end readings of the dewpoint measurement. Sensor purge is
performed twice a day, and five minutes after the power is switched
on. The sensor is heated for several minutes which will then
evaporate all excess molecules out of the sensor polymer. This,
together with the auto-calibration results in a very small drift of the
sensor due to the very linear behaviour of the polymer technology.
BAROCAP® Technology
The BAROCAP® silicon capacitive absolute pressure sensor was
developed by Vaisala for barometric pressure measurement
applications. The BAROCAP® sensor has excellent hysteresis and
repeatability characteristics, low temperature dependence, and a very
good long-term stability. The ruggedness of the BAROCAP® sensor
is outstanding and the sensor is resistant to mechanical and thermal
shocks. The pressure measurement of the MPS1 multiparameter
sensor is based on an advanced RC oscillator and reference capacitors
against which the capacitive pressure sensor is continuously
measured. The microprocessor of the transmitter performs
compensation for pressure linearity and temperature dependence.