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

• Main Authors: B. Lang, W. Breitfuss, S. Schweighart, P. Breitegger, H. Pervier, A. Tramposch, A. Klug, W. Hassler, A. Bergmann
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-03-01
• Series: Atmospheric Measurement Techniques
• Online Access: https://amt.copernicus.org/articles/14/2477/2021/amt-14-2477-2021.pdf

<p>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 <span class="inline-formula">3.3</span> % in the water vapor mole fraction range of 510–12 360 <span class="inline-formula">ppm</span> (<span class="inline-formula">5</span> % from 250–21 200 <span class="inline-formula">ppm</span>) with a theoretical limit of detection (3<span class="inline-formula"><i>σ</i></span>) of <span class="inline-formula">3.2</span> <span class="inline-formula">ppm</span>. 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 <span class="inline-formula">90</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">s</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="7c357c7c8d9a58020f6dc78efa08ad26"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-14-2477-2021-ie00001.svg" width="46pt" height="15pt" src="amt-14-2477-2021-ie00001.png"/></svg:svg></span></span>, a CWC measurement accuracy better than <span class="inline-formula">20</span> % is achieved by the instrument above a CWC of <span class="inline-formula">0.14</span> <span class="inline-formula">g m⁻³</span> in cold air (<span class="inline-formula">−30</span> <span class="inline-formula">^∘</span>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 <span class="inline-formula">20</span> %, which demonstrates the potential of PA hygrometers for water content measurement in atmospheric icing conditions.</p>