Vaisala Drycap DMT152, User Manual

Page: 14 / 72

3. Functional Description
3.1 Advanced DRYCAP Technology
Dew point transmitter DMT152 utilizes an advanced, patented measurement technology to
ensure 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 dew point measurement. The self-diagnostic procedures that are
conducted are called auto-calibration, sensor purge, and sensor warming.
3.1.1 Auto-Calibration
The auto-calibration feature of the DMT152 transmitter is an automatic procedure that greatly
reduces the possible drift in the dry end of the dew point measurement.
Auto-calibration is performed:
• at 45-minute intervals
• at shorter intervals if T
d
is near the dry end of the measurement range
• when the transmitter is powered up
• when the measured environment changes rapidly.
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. Measurement and output remain active
during auto-calibration.
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.
3.1.2 Sensor Purge
Sensor purge is also an automatic procedure that minimizes the drift at the wet end readings
of the dew point measurement. Sensor purge is performed once a week or when 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. The measurement output of the transmitter is frozen to the last measured value for
the duration of the sensor purge.
DMT152 User Guide M210865EN-J
12