Vaisala DRYCAP DMT143, Руководство пользователя

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3. Functional Description
3.1 DRYCAP Technology
Vaisala DRYCAP
â
dew point 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 dew point
measurement. The self-diagnostic procedures that are conducted are called autocalibration,
sensor purge, and sensor warming.
DMT143 and DMT143L use different versions of the Vaisala DRYCAP
â
sensor:
• DMT143 uses the DRYCAP
â
180D sensor designed for applications such as plastic dryers
and small industrial compressed air dryer (sensor filter length 22.5 mm, probe length 91.6
mm).
• DMT143L can be ordered with two versions of the DRYCAP â 180 sensor: either the 180M
sensor designed for standard dry gas and desiccant dryer applications, or the 180S sensor
designed for more humid applications such as refrigeration dryers. The sensor filter length
of both DMT143L variants is 51 mm, probe length 120 mm.
3.1.1 Autocalibration
The autocalibration feature of the DMT143 transmitter is an automatic procedure which greatly
reduces the possible drift in the dry end of the dew point measurement. It is performed at 1-
hour intervals, and when the power is switched on. When measuring very dry conditions, the
transmitter performs the autocalibration at shorter intervals. A significant change in dew point
or temperature may also trigger the autocalibration.
During autocalibration 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 autocalibration the
transmitter outputs the T
d
value prior to the procedure.
Autocalibration 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 that the patented technology offers. These criteria include for example a stable
enough moisture level in the measured atmosphere. If the conditions are not fulfilled, the
autocalibration function is postponed until satisfactory conditions are reached.
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 day 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 autocalibration, results in a very small
drift of the sensor due to the very linear behaviour of the polymer technology. During the
sensor purge the transmitter outputs the T
d
value prior to the procedure.
DMT143 User Guide M211435EN-J
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