Dual channel photoacoustic hygrometer for airborne measurements: background, calibration, laboratory and in-flight inter-comparison tests

Abstract

Abstract. This paper describes a tunable diode laser based dual channel photoacoustic (PA) humidity measuring system called WaSul-Hygro primarily designed for aircraft based environment research. It is calibrated for total pressures and water vapor (WV) volume mixing ratios (VMRs) possible during airborne applications. WV VMR is calculated by using pressure dependent calibration curves and a cubic spline interpolation method. Coverage of the entire atmospheric humidity concentration range which might be encountered during airborne measurements is facilitated by applying an automated sensitivity mode switching algorithm. The calibrated PA system was validated through laboratory and airborne inter-comparisons, which proved that the repeatability, the estimated accuracy and the response time of the system is 0.5 ppmV or 0.5% of the actual reading (whichever value is the greater), 5% of the actual reading within the VMR range of 1–12 000 ppmV and 2 s, respectively. The upper detection limit of the system is about 85 000 ppmV, limited only by condensation of water vapor on the walls of the 318 K heated PA cells and inlet lines. The unique advantage of the presented system is its applicability for simultaneous water vapor and total water volume mixing ratio measurements.

Embedded system based data acquisition and control system for photoacoustic spectroscopic applications

“…In recent years the number of reported photoacoustic (PA) instruments to prove their applicability in various areas has been steadily increasing [1][2][3][4]. Amongst these instruments are those which were developed by our research group for various applications, including in the natural gas industry [5][6][7], airborne hygrometry [8][9], source apportionment of urban light-absorbing aerosols [10][11], measurements of gas permeability [12], and clinical research [13]. The main components of our instruments are basically always the same: a data acquisition and control system (DACS), one or more longitudinal differential detection cells [14], (a) modulated light source(s) which is/are either telecommunication-type diode lasers [15] or frequency converted Nd:YAG lasers [10] and gas handling units in various configurations.…”

“…In most cases, the DACS has various additional functions such as controlling the gas handling system and communicating with other instruments. Our recently developed systems [5][6][7][8][9][10][11][12][13] were based on a highly integrated target DACS. This system turned out to be one of the bottlenecks during the application developments due to its limited capabilities; therefore, a fundamental redesign of our DACS became essential.…”

“…The different parts of the gas handling system were connected by 6 mm stainless steel tubes using Swagelok tube fittings. This PA system is identical to that used in our previous study [8]; only the DACS has been changed, thus making it possible to compare the performance of the two DACSs.…”

“…To determine sensitivity the system was calibrated for water vapor from a homemade humidity generator [8] which is capable of generating humidified gas in the 10-25,000 ppm range. First, calibration occurred on the strongest water vapor absorption line (1392.5 nm, 3.7 Â 10 À5 ppm m À1 ) available in the tuning range of the laser with optimized modulation parameters.…”

Fotoakusztikus spektroszkópia alkalmazási lehetőségei a kilélegzett levegő gázösszetételének vizsgálatában

“…Ez az érték közelítőleg 2,5 nagyságrenddel nagyobb, mint az longitudinális rezonátort tartalmazó, zárt kamrával rendelkező fotoakusztikus mérőrendszerek által tipikusan mérhető legkisebb kimutatható koncentráció [25]. Egy nagyságrendnyi különbség abból az összefüggésből következik, hogy a fotoakusztikus mérések zaja (σ) fordítottan arányos a mérés átlagolási idejének (τ) négyzetgyökével, azaz:…”