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 climate model computations - to be improved.
Humidity measurements in the atmosphere are of essential importance, since water vapor, as the most important natural greenhouse gas, has a strong influence on the Earth's atmospheric radiation balance and, thus, decisively influences our climate. In addition, water is responsible for meteorological phenomena such as the formation of clouds and precipitation.
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 (Selective Extractive Airborne Laser Diode Hygrometer) that 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 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 that 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. 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.