This paper describes a photoacoustic spectroscopy-based detector of hydrogen sulfide (H2S) in biogas, natural gas and oil process technology. The instrument is capable of measuring H2S concentrations over four orders of magnitude (from a few ppm level up to several per cent) in changing gas mixtures. Problems caused by harsh industrial circumstances, contamination and widely varying composition of gases can be overcome by optimizing wavelength modulation, resonance frequency tracking and an easy-to-use method enabling in situ monitoring calibration. A diode laser emitting around 1.57 µm served as the excitation source; at this wavelength spectral overlap between H2S and CO2 is substantial. Spectral interference was eliminated by optimizing the amplitude of wavelength modulation; furthermore, a simplified calibration method was implemented taking advantage of a nearby absorption line of CO2 providing fast and economical measurements. Frequency dependence of the photoacoustic signal was determined by two methods to ensure accuracy. For 10 s integration time and 6800 Hz modulation frequency, the minimum detectable concentration was 6 ppm (3σ).

Abstract. This paper describes a tunable diode laser-based dual-channel photoacoustic (PA) humidity measuring system primarily designed for aircraft-based environment research. It is calibrated for total pressure 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 that 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 intercomparisons, which proved that the repeatability, the estimated accuracy and the response time of the system are 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 theoretically about 85 000 ppmV, limited only by condensation of water vapor on the walls of the 318 K heated PA cells and inlet lines, and was experimentally verified up to 20 000 ppmV. The unique advantage of the presented system is its applicability for simultaneous water vapor and total water volume mixing ratio measurements.

This paper describes a membrane permeability measuring setup based on photoacoustic spectroscopy using continuous carrier gas flow to transport the permeated analyte molecules into a photoacoustic detection cell. The permeability parameters of the sample were determined from the measured permeation curves by using a numerical curve fitting algorithm. The method was applied to different membrane samples for determining methane and carbon-dioxide permeability at various carrier gas flow rates (CFRs). For each sample, a characteristic threshold flow rate (TFR) value can be identified below which a strong dependency of the determined permeation parameters on the CFR was found. For those cases when the CFR value cannot be set to be sufficiently high (i.e. above the TFR value), an extrapolation method was presented giving an accurate estimation of the permeation parameters.

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.

Laser absorption spectroscopy by photoacoustic (PA) detection technique is increasingly used in trace gas analysis. Our research group focuses on related instrument and application development. In this paper the design and test results of a newly developed field programmable gate array based data acquisition and control system (DACS), are introduced. It became necessary to develop this new system since the limits of the old one had been reached; furthermore, it was challenging to implement new advanced measuring protocols. The system was designed to be scalable in order to be able to implement further ideas or measurement protocols without significant hardware or software modifications. In the current configuration, approximately 60% of the resources of the system are used. A complete PA measuring system was built around the new DACS to determine the analytical properties: the normalized noise equivalent absorption is 7.5 Â 10 À10 cm À1 W Hz À1/2 , while the estimated dynamic range for measuring optical absorption is 7.5 orders of magnitude.