User manual VAISALA DMT142

[. . . ] USER'S GUIDE Vaisala DRYCAP® Dewpoint Transmitter DMT142 M210397EN-D PUBLISHED BY Vaisala Oyj P. O. Box 26 FIN-00421 Helsinki Finland Phone (int. ): +358 9 8949 1 Fax: +358 9 8949 2227 Visit our Internet pages at http://www. vaisala. com/ © Vaisala 2006 No part of this manual may be reproduced in any form or by any means, electronic or mechanical (including photocopying), nor may its contents be communicated to a third party without prior written permission of the copyright holder. The contents are subject to change without prior notice. Please observe that this manual does not create any legally binding obligations for Vaisala towards the customer or end user. All legally binding commitments and agreements are included exclusively in the applicable supply contract or Conditions of Sale. _________________________________________________________________________________ Table of Contents CHAPTER 1 GENERAL INFORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] 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 DMT142 transmitter 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 Td value prior to the procedure. VAISALA_________________________________________________________________________ 5 User's Guide _______________________________________________________________________ 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 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 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 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 autocalibration results in a very small drift of the sensor due to the very linear behaviour of the polymer technology. Sensor Warming in High Humidities Additionally the DMT142 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 advantage of sensor warming is the rapid response of dewpoint measurement. A wet sensor and filter would otherwise result in a dewpoint equal to ambient temperature (that is RH = 100 %). If in spite of sensor warming the sensor gets soaked, it will recover fully back to normal operation after it dries out. 6 ____________________________________________________________________ M210397EN-D Chapter 4 ________________________________________________________________ Installation CHAPTER 4 INSTALLATION This chapter provides you with information that is intended to help you install Dewpoint Transmitter DMT142. Selecting Location In the mounting of Dewpoint Transmitter DMT142 it is important that the point of installation represents well the gas to be measured. Temperature changes do not affect the dewpoint measurement, but pressure changes will have an effect on the measurement. All leaks in the system must be eliminated to avoid ambient humidity affecting the measurement. VAISALA_________________________________________________________________________ 7 User's Guide _______________________________________________________________________ 0507-094 Figure 2 Mounting Dewpoint Transmitter DMT142 The numbers in the following list refer to Figure 2 above: 1. Select a proper location to mount the DMT142 and remove the yellow sensor protective cap. When using the cable provided with DMT142 refer to the following wiring section. Self diagnostics performed at start-up freeze the output during the first minutes of operation. [. . . ] A simple field-checking operation can be performed by comparing the readings of the DMT142 and the DM70 hand-held dewpoint meter with a calibrated reference probe. For the comparison you need a connection cable, Vaisala order code: 211917ZZ. If there is need for calibration or adjustment contact Vaisala Service Center or your local Vaisala representative. Filter Change The DRYCAP® sensor in the DMT142 transmitter is protected with a stainless steel sintered filter. The filter is recommended to be changed whenever it looks dirty or as a part of the periodical maintenance procedure. [. . . ]