Photoacoustic hygrometer for icing wind tunnel water content measurement: design, analysis, and intercomparison

This work describes the latest design, calibration and application of a near-infrared laser diode-based photoacoustic (PA) hygrometer developed for total water content measurement in simulated atmospheric freezing precipitation and high ice water content conditions with relevance in fundamental icing research, aviation testing, and certification. The single-wavelength and single-pass PA absorption cell is calibrated for molar water vapor fractions with a two-pressure humidity generator integrated into the instrument. Laboratory calibration showed an estimated measurement accuracy better than inline-formula3.3 % in the water vapor mole fraction range of 510–12 360 inline-formulappm (inline-formula5 % from 250–21 200 inline-formulappm) with a theoretical limit of detection (3inline-formulaσ) of inline-formula3.2 inline-formulappm. The hygrometer is examined in combination with a basic isokinetic evaporator probe (IKP) and sampling system designed for icing wind tunnel applications, for which a general description of total condensed water content (CWC) measurements and uncertainties are presented. Despite the current limitation of the IKP to a hydrometeor mass flux below inline-formula90 inline-formula $M9inlinescrollmathmlunit\mathrm{normal\; g}{\mathrm{normal\; m}}^{-\mathrm{normal\; 2}}{\mathrm{normal\; s}}^{-\mathrm{normal\; 1}}$ 46pt15ptsvg-formulamathimg7c357c7c8d9a58020f6dc78efa08ad26 amt-14-2477-2021-ie00001.svg46pt15ptamt-14-2477-2021-ie00001.png , a CWC measurement accuracy better than inline-formula20 % is achieved by the instrument above a CWC of inline-formula0.14 inline-formulag m⁻³ in cold air (inline-formula−30 inline-formula^∘C) with suitable background humidity measurement. Results of a comparison to the Cranfield University IKP instrument in freezing drizzle and rain show a CWC agreement of the two instruments within inline-formula20 %, which demonstrates the potential of PA hygrometers for water content measurement in atmospheric icing conditions.