Development and Metrological Characterization of a Tunable Diode Laser Absorption Spectroscopy (TDLAS) Spectrometer for Simultaneous Absolute Measurement of Carbon Dioxide and Water Vapor

Abstract

Simultaneous detection of two analytes, carbon dioxide (CO2) and water vapor (H2O), has been realized using tunable diode laser absorption spectroscopy (TDLAS) with a single distributed feedback diode laser at 2.7 μm. The dynamic range of the spectrometer is extended from the low parts per million to the percentage range using two gas cells, a single-pass cell with 0.77 m, and a Herriott-type multipass cell with 76 m path length. Absolute measurements were carried out, i.e., amount fractions of the analytes were calculated based on previously determined spectral line parameters, without the need for an instrument calibration using gas standards. A thorough metrological characterization of the spectrometer is presented. We discuss traceability of all parameters used for amount fraction determination and provide a comprehensive uncertainty assessment. Relative expanded uncertainties (k = 2, 95% confidence level) of the measured amount fractions are shown to be in the 2-3% range for both analytes. Minimum detectable amount fractions are 0.16 μmol/mol for CO2 and 1.1 μmol/mol for H2O for 76 m path length and 5 s averaging time. This corresponds to normalized detection limits of 27 μmol/mol m Hz(-1/2) for CO2 and 221 μmol/mol m Hz(-1/2) for H2O. Precision of the spectrometer, determined using Allan variance analysis, is 3.3 nmol/mol for CO2 and 21 nmol/mol for H2O. The spectrometer has been validated using reference gas mixtures with known CO2 and H2O amount fractions. An application example of the absolute TDLAS spectrometer as a reference instrument to validate other sensors is also presented.

Traceable amount of substance fraction measurements in gases through infrared spectroscopy at PTB

“…Traceability in absolute, laser-based amount fraction measurements is feasible and described by the TILSAM protocol [4]. Absolute TDLAS measurements based on TILSAM have been shown to yield relative expanded uncertainties in the low percent range [5,17] and TILSAM instrumentation may be used to certify gas mixtures or as optical transfer standard [10]. TILSAM can also be applied for absolute measurements based on CRDS [30,31], extending its scope to the measurement of trace concentrations.…”

“…If line data from spectral databases like HITRAN [27] or GEISA [28] are employed, uncertainties can be considerably higher (typically 5 % to 10 %) and traceability is missing for the vast majority of lines. Our uncertainty assessment [10] for the line area includes laser tuning (i.e. the wavenumber axis), parasitic absorption [29], laser side mode emission, detector non-linearity and the uncertainty in line fitting.…”

“…the wavenumber axis), parasitic absorption [29], laser side mode emission, detector non-linearity and the uncertainty in line fitting. The latter depends on the achieved signal-to-noise level and how well the probed line shape is represented by a Voigt profile, while the other uncertainties can be evaluated empirically in separate measurements [10]. For example, the uncertainty in the wavenumber axis may additionally be estimated by comparing line positions to reference values or experimentally determined Doppler widths of low pressure signals to calculated values based on measured gas temperatures [22,23].…”

“…A study presented in ref. [10] showed that uncertainty figures do not suffer when CO2 and H2O amount fraction measurements are combined by scanning the laser wavelength over two neighboring CO2 and H2O absorption lines. Here, the relative expanded uncertainties were estimated as 2 % to 3 % for nominal amount fractions of 100 μmol/mol, documenting a slight improvement for CO2 upon the earlier work [17].…”

“…We developed absolute laser hygrometers which are used in atmospheric research [5], combustion research [6,7], or to measure humidity in natural gas or biogas for quality assurance purposes [8]. In the framework of the completed EMRP project MacPoll [9] we developed an absolute spectrometer targeting CO2 and H2O to DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015 2015 m 0 00 etrolo / gy validate the CO2 concentration and humidity in zero gas which is used to check the zero-reading of calibrated sensors and as dilution gas for reference gas mixtures [10]. In the currently running EMRP projects MetAMC [11] and MetNH3 [12] we work on reliable online detection of NH3 in clean rooms based on cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS), and on a traceable optical transfer standard for NH3 detection based on a commercial CRDS spectrometer, respectively.…”