Robust, spatially scanning, open-path TDLAS hygrometer using retro-reflective foils for fast tomographic 2-D water vapor concentration field measurements

Abstract

Abstract.We have developed a fast, spatially scanning direct tunable diode laser absorption spectrometer (dTDLAS) that combines four polygon-mirror based scanning units with low-cost retro-reflective foils. With this instrument, tomographic measurements of absolute 2-D water vapor concentration profiles are possible without any calibration using a reference gas.A spatial area of 0.8 m × 0.8 m was covered, which allows for application in soil physics, where greenhouse gas emission from certain soil structures shall be monitored. The whole concentration field was measured with up to 2.5 Hz. In this paper, we present the setup and spectroscopic performance of the instrument regarding the influence of the polygon rotation speed and mode on the absorption signal. Homogeneous H 2 O distributions were measured and compared to a single channel, bi-static reference TDLAS spectrometer for validation of the instrument. Good accuracy and precision with errors of less than 6 % of the absolute concentration and length and bandwidth normalized detection limits of up to 1.1 ppmv · m (Hz) −0.5 were achieved.The spectrometer is a robust and easy to set up instrument for tomographic reconstructions of 2-D-concentration fields that can be considered as a good basis for future field measurements in environmental research.

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

“…Figure 2 summarizes some representative results from a number of sensor development and implementation studies in terms of detection limits and bandwidths. It compares species concentration measurements in a variety of combustion systems including laboratory environments [1,[16][17][18][19][20][21][22], shock tube studies [23][24][25][26][27][28], industry processes [29][30][31][32][33], and engines [34][35][36][37][38][39][40][41][42][43][44][45][46]. For the purpose of making a direct comparison between different studies, the reported absolute minimal detection limits for several commonly probed species in combustion, including CO, CO 2 , H 2 O, and C 2 H 2 , are plotted on the same diagram against detection bandwidths.…”

Simultaneous two cross-sectional measurements of NH₃ concentration in bent pipe flow using CT-tunable diode laser absorption spectroscopy

“…The absorption signal intensity becomes an integrated value of the optical path. In this study, several optical paths are intersected to each other to form the analysis grid, reconstructing 2D temperature distribution by a CT method (Ma et al, 2008;Wang et al, 2010;Kasyutich et al, 2011;An et al, 2011;Deguchi et al, 2012;Deguchi et al, 2015;An et al, 2015;Cai, et al, 2015;Jatana, et al, 2015;McCann, et al, 2015;Seidel, et al, 2015;Stritzke, et al, 2015;Kamimoto et al, 2016). Figure 4 shows concept of analysis grids and laser beam paths.…”

Two-Dimensional Temperature Measurement in a High-Temperature and High-Pressure Combustor Using Computed Tomography Tunable Diode Laser Absorption Spectroscopy (CT-TDLAS) with a Wide-Scanning Laser at 1335–1375 nm

“…Recently, combination of computed tomography (CT) and TDLAS has been studied and applied for two-dimensional (2D) measurements of temperature and concentration in combustion processes. 15–24 In past studies, H 2 O absorption spectra using narrow wavelength range scanning lasers were used for 2D temperature measurement. 20–24 However, these lasers are not applicable for temperature measurement in high-pressure condition, such as 1.0–5.0 MPa.…”