title: SEALDH-II-An Autonomous, Holistically Controlled, First Principles TDLAS Hygrometer for Field and Airborne Applications: Design-Setup-Accuracy/Stability Stress Test.
abstract: Instrument operation in harsh environments often significantly impacts the trust level of measurement data. While commercial instrument manufacturers clearly define the deployment conditions to achieve trustworthy data in typical standard applications, it is frequently unavoidable in scientific field applications to operate instruments outside these commercial standard application specifications. Scientific instrumentation, however, is employing cutting-edge technology and often highly optimized but also lacks long-term field tests to assess the field vs. laboratory performance. Recently, we developed the Selective Extractive Laser Diode Hygrometer (SEALDH-II), which addresses field and especially airborne applications as well as metrological laboratory validations. SEALDH-II targets reducing deviations between airborne hygrometers (currently up to 20% between the most advanced hygrometers) with a new holistic, internal control and validation concept, which guarantees the transfer of the laboratory performance into a field scenario by capturing more than 80 instrument internal "housekeeping" data to nearly perfectly control SEALDH-II's health status. SEALDH-II uses a calibration-free, first principles based, direct Tuneable Diode Laser Absorption Spectroscopy (dTDLAS) approach, to cover the entire atmospheric humidity measurement range from about 3 to 40,000 ppmv with a calculated maximum uncertainty of 4.3% ± 3 ppmv. This is achieved not only by innovations in internal instrument monitoring and design, but also by active control algorithms such as a high resolution spectral stabilization. This paper describes the setup, working principles, and instrument stabilization, as well as its precision validation and long-term stress tests in an environmental chamber over an environmental temperature and humidity range of ΔT = 50 K and ΔRH = 80% RH, respectively.
authors.affiliation: Physikalisch-Technische Bundesanstalt Braunschweig, Braunschweig 38114, Germany. buchholz@csi.tu-darmstadt.de.|Physikalisch Chemisches Institut, Universität Heidelberg, Heidelberg 69120, Germany. buchholz@csi.tu-darmstadt.de.|Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08540, USA. buchholz@csi.tu-darmstadt.de.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Buchholz
authors.affiliation: Physikalisch-Technische Bundesanstalt Braunschweig, Braunschweig 38114, Germany. s.kallweit92@web.de.
authors.forename: Sören
authors.initials: S
authors.lastname: Kallweit
authors.affiliation: Physikalisch-Technische Bundesanstalt Braunschweig, Braunschweig 38114, Germany. volker.ebert@ptb.de.|Physikalisch Chemisches Institut, Universität Heidelberg, Heidelberg 69120, Germany. volker.ebert@ptb.de.|Center of Smart Interfaces, Technische Universität Darmstadt, Darmstadt 64287, Germany. volker.ebert@ptb.de.
authors.forename: Volker
authors.initials: V
authors.lastname: Ebert
keywords: SEALDH; TDLAS; absorption spectroscopy; airborne; calibration-free; field; hygrometer; metrology; trace gas measurement; traceable