Gas Measurement Using Static Fourier Transform Infrared Spectrometers

Abstract

Online monitoring of gases in industrial processes is an ambitious task due to adverse conditions such as mechanical vibrations and temperature fluctuations. Whereas conventional Fourier transform infrared (FTIR) spectrometers use rather complex optical and mechanical designs to ensure stable operation, static FTIR spectrometers do not require moving parts and thus offer inherent stability at comparatively low costs. Therefore, we present a novel, compact gas measurement system using a static single-mirror Fourier transform spectrometer (sSMFTS). The system works in the mid-infrared range from 650 cm - 1 to 1250 cm - 1 and can be operated with a customized White cell, yielding optical path lengths of up to 120 cm for highly sensitive quantification of gas concentrations. To validate the system, we measure different concentrations of 1,1,1,2-Tetrafluoroethane (R134a) and perform a PLS regression analysis of the acquired infrared spectra. Thereby, the measured absorption spectra show good agreement with reference data. Since the system additionally permits measurement rates of up to 200 Hz and high signal-to-noise ratios, an application in process analysis appears promising.

Keywords: gas concentration; gas process monitoring; high-speed gas measurement; infrared spectroscopy; optical gas measurement; static Fourier transform spectroscopy; static single-mirror Fourier transform spectrometer.

Figure 1

Schematic overview of a static single-mirror Fourier transform spectrometer [11].

Figure 2

(a) Basic set-up of the White cell and focal rays for eight passes; (b) Schematic layout of the field mirror with indicated numbers of traversals.

Figure 3

(a) Measurement setup containing a customized White cell for sensitive quantitative gas analysis; (b) Measurement setup containing a single pass cell for high-speed qualitative gas measurement applications.

Figure 4

Recorded R134a absorption spectra with different concentrations by the sSMFTS system (solid lines) and the corresponding reference spectra measured with a conventional FTIR spectrometer (dashed lines) at a spectral resolution of 8 cm${}^{-1}$. All spectra were measured at an optical path length of 80 cm.

Figure 5

(a) Validation plot of the partial least squares (PLS) model applied to the measured absorption spectra; (b) Gas concentration increase in the system over time when measuring a 100 ppm R134a probe at a volume flow rate of 1 L min${}^{-1}$.

Figure 6

(a) Long-term gas measurement of a 100 ppm R134a probe with a measurement frequency of 50 Hz; (b) Corresponding Allan-plot in order to determine the ideal integration time.

Figure 7

(a) Background signal-to-noise ratio (SNR) of the measurement system for different integration times $t_{\mathrm{int}}$ at a measurement frequency of 50 Hz; (b) Corresponding resolution of the system for a of 100 ppm R134a probe.

Figure 8

(a) Decreasing transmittance around 1189 cm${}^{-1}$ for selected times when filling the single pass cell with a 2000 ppm R134a probe at a volume flow rate of 1 L min${}^{-1}$; (b) Corresponding gas concentration increase in the cell measured at 200 Hz.

Figure 9

(a) Minimum resolvable absorption coefficients ${\alpha }_{\mathrm{res}}$ of both measurement setups; (b) Minimum resolvable absorption coefficients ${\alpha }_{\mathrm{res},\mathrm{n}}$ normalized to the acquisition rate of both measurement setups.