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01/07/2005 REVOLUTIONARY TECHNOLOGY FOR MOISTURE MEASUREMENT  Aplein Ingenieros, S.A. and Xentaur, the American manufacturer of transmitters for measuring water in fluids and gases, has reached an agreement for the distribution of its analysers in Spain. Based on an aluminium oxide sensor with registered hyper thin-film technology (HTFTM), they provide a reliable solution for the measurement of moisture and dew point in gases and liquids. It provides readings with a level of sensitivity several orders of magnitude better than other sensors using different technologies. The HTFTM sensors are insensitive to temperature changes in most ranges, as well as to drift. The measuring principle is shared with that of other aluminium oxide sensors: the capacitance measured between the aluminium core of the sensor and a gold film deposited on the upper part of the oxide layer varies with the water vapour content in the pores of the oxide layer. 
Xentaur includes 3 structural improvements in the
oxide layer that grant superior sensitivity and stability in comparison
with other manufacturers’ products: a very much finer oxide layer
which provides greater sensitivity to capacitance changes, a perfectly defined
layer separating the aluminium and the aluminium oxide that provides a high
dry to wet ratio of the sensor capacitance reducing the drift with time,
and a unique pore geometry that improves water molecule capture and avoids
the need for continually recalibrating.Xentaur offers the following configurations within its product range: - Model XDT for on-line measurement of the dew point in its different versions: safe area and classified area. - Model LPDT for measuring the dew point in a closed loop. - Model XPDM, a handheld instrument for measuring dew point. The design of these sensors avoids the need to periodically ship the sensors back to the factory for calibration, and the necessity to have spare units for these occasions. The transmitter provides a 4-20 mA output signal that can be taken directly to the DCS. It is ATEX-certified so it need not be installed in a safe area cabinet/enclosure. The transmitter also provides a HART digital signal. These instruments are based on Henry's Law: Concentration of H2O (ppmw) = Vapour pressure of the H2O x K(t) The validation or recalibration would be carried out in the plant laboratory using a Karl Fischer validator with which the value of the constant K would be calculated. This would be introduced in the DCS or in the controller so that it can perform the multiplication with the value of water vapour pressure, which is linear with the signal delivered by the sensor. In this way the value of the water concentration is obtained in ppmw. Applications: Refinery and petrochemical industry, breathing gases, compressed air, food and drink handling, industrial gases, insulation gases, natural gas, ozone production, plastics, semiconductors, laboratories, defence, etc. More news |
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