March 4, 2013
Weather And Climate Models Improved Using SEALDH
A humidity sensor developed by the Physikalisch-Technische Bundesanstalt (PTB), the SEALDH laser hygrometer, has proven its worth when used aboard an aircraft; it fulfills all pre-conditions to be used as a transfer standard for conventional humidity-measuring instruments. This would allow the quality of air humidity measurements in the Earth's atmosphere - and, thus, also climate model computations - to be improved.
In order to improve the quality of atmospheric water vapor measurements and to provide better comparability, PTB scientists have developed the traceable laser hygrometer SEALDH. SEALDH stands for "Selective Extractive Airborne Laser Diode Hygrometer" and works according to the principle of tunable diode laser absorption spectroscopy (TDLAS). Its use aboard an aircraft requires the system to be small, light and insensitive to vibrations; in addition, it must be able to perform rapid measurements and to work autonomously, e.g. to constantly monitor itself or to be able to resume operation itself after a fault after an unplanned shutdown. But SEALDH is even more than this: it is self-calibrating. This is a clear advantage compared to conventional hygrometers which have to be calibrated frequently - and often under adverse conditions (e.g. in an airplane hangar).
SEALDH has now been tested under field conditions within the scope of a scientific mission on a Learjet 35A, a former passenger plane that was adapted for research purposes; it was able to give proof of its performance during seven flights after having passed a blind comparison with several established aircraft hygrometers at the environmental simulation chamber of Forschungszentrum JÃ¼lich. During its in-flight operation, the laser hygrometer proved a detection limit in the ppm range, a very large measuring range between 25 ppm and 25,000 ppm of water volume fraction, as well as an excellent temporal resolution of clearly below 1 s.
Furthermore, PTB scientists have compared SEALDH with PTB's national humidity standard and attained a mean deviation of less than 2 % - without prior calibration. Based on these features, it will, in the medium term, be possible to use SEALDH as a transfer standard for the quality assurance of air humidity measurements in atmospheric research.
What does "traceable measurement" mean?
Numerous research groups all around the world are investigating climate-relevant processes in the atmosphere; they use different measurement procedures under different conditions. To make their measurement results comparable, traceable measurements would make sense, i.e. the measurement uncertainty of a measuring device compared to the best possible standard must be known. In the medium term, SEALDH could be such a standard, since its capacity as a transfer standard has been demonstrated also under real in-flight conditions and compared to PTB's primary humidity standard.
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