Vaisala DMT152 - Manual (Page 15)

Chapter 3 _______________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 13
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the advanced functionality of Dewpoint
Transmitter DMT152, including the auto-calibration, sensor purge, and
sensor warming functions.
Advanced DRYCAP
®
Technology
Dewpoint 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 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 DMT152 transmitter is an automatic
procedure which greatly reduces the possible drift in the dry end of the
dewpoint measurement. The 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.