Logo of the Physikalisch-Technische Bundesanstalt

Optical humidity metrology for cutting-edge international meteorological research


Water vapor measurements rank among the most important – but also the most difficult – measurements aboard research aircraft. During the past five years, PTB has been developing (partly in cooperation with Forschungszentrum Jülich) two completely new hygrometer instrument families (“SEALDH” and “HAI”) which – due to consistent metrological traceability – promise more reliable measurement data.

Hygrometer for Atmospheric Investigation.

Both instrument families are based on Direct Tunable Diode Laser Absorption Spectroscopy (dTDLAS), which was combined with a so-called “first principles” evaluation method. Above all, this means that the two hygrometers do not require any calibration by means of reference gases. Instead, the values are determined by means of a complete physical model. The key factors for implementation included: many years of experience with dTDLAS systems; the extensive validation of the measurement principle carried out over a period of almost two years at PTB’s primary humidity standards; and the use of PTB’s traceable spectral data. Meanwhile, four dTDLAS systems have been realized which have already successfully completed four national flight campaigns on a Learjet 35A (approximately 50 flight hours) and four international measurement campaigns on a Gulfstream G550 (approximately 300 hours). The success is based on manifold technical detailed solutions which permit – also in flight – metrologically validated measurement uncertainties of 4.3 % in relative terms, or at least 3 μmol/mol which has so far been unattained in atmospheric research. For the first time, traceable absolute results can be achieved – also in flight – by means of spectrometric hygrometers by combining all metrological findings and capabilities available.