Vaisala DPT146 - Manual (Page 20)

User's Guide _______________________________________________________________________
18 ___________________________________________________________________ M211372EN-D
DRYCAP® Technology
Vaisala DRYCAP® dewpoint measurement technology ensures accurate
measurement with excellent long term stability. This results in very low
maintenance requirements for the transmitter. The lasting performance is
achieved with microprocessor technology and software that automatically
performs self-diagnostic functions in addition to the normal dewpoint
measurement. The self-diagnostic procedures that are conducted are
called auto-calibration, sensor purge, and sensor warming.
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 once 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 behavior of the polymer technology.
Sensor Warming in High Humidities
DPT146 transmitter has a warming feature which prevents the sensor and
filter from becoming wet in high humidities. High humidity may be
present when the dewpoint temperature rises close to the gas temperature.
Sensor warming is switched on automatically when the humidity in the
measured gas increases to a level where dew can start to form. The