Vaisala DPT146 Guide de l'utilisateur télécharger pdf (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

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Vaisala DPT146 Guide de l'utilisateur Page 20