Fourier Transform Infrared gas analyzers have been widely used for speedy quantitative analyses of gases, and it is found that in many cases field maintainability determines the instruments’ online applicability instead of the instrument’s accuracy as is desired. To be maintenance-free is both the target of online instruments and the key to their field applications. Analyses show that if a background can be collected simultaneously with the sample spectrum, the transmittance will be only a function of concentration. Gas spectra collected on Nicolet670 of attenuated inputs and adjusted gains are examined via OMNIC software. Collected data exhibit that the absorbance spectra keep constant when input energy increases 8 times and the instrument gains becomes to 2.0 and 4.0 times. On viewing the absorption peaks vary with wavenumber in high “frequency” and that of the spectrometer itself in low frequency, by subtracting its instrumental response function of a transmittance spectrum extracted by wavelet transform, an absorbance spectrum of air is obtained and comparisons demonstrate that it is a summation of absorbance of the sample and that of gases in the optical path of the spectrometer. Calibration-free and background-free principles are thus exhibited and they construct maintenance-free principles of FTIR gas analyzers.
Recently, chromatography column and gas sensor have been used in online monitoring device of dissolved gases in transformer oil. But some disadvantages still exist in these devices: consumption of carrier gas, requirement of calibration, etc. Since FTIR has high accuracy, consume no carrier gas and require no calibration, the researcher studied the application of FTIR in such monitoring device. Experiments of “Flow gas method” were designed, and spectrum of mixture composed of different gases was collected with A BOMEM MB104 FTIR Spectrometer. A key question in the application of FTIR is that: the absorbance spectrum of 3 fault key gases, including C<sub>2</sub>H<sub>4</sub>, CH<sub>4</sub> and C<sub>2</sub>H<sub>6</sub>, are overlapped seriously at 2700~3400cm<sup>-1</sup>. Because Absorbance Law is no longer appropriate, a nonlinear calibration model based on BP ANN was setup to in the quantitative analysis. The height absorbance of C<sub>2</sub>H<sub>4</sub>, CH<sub>4</sub> and C<sub>2</sub>H<sub>6</sub> were adopted as quantitative feature, and all the data were normalized before training the ANN. Computing results show that the calibration model can effectively eliminate the cross disturbance to measurement.
FTIR spectra have been widely used in quantitative analyses of mixed gases. Although Beer's Law regulates the relationships between absorbency and the product of concentration and path length, its deviations have been found rather complicated. Here we present the complexity of quantitative relationships between methane's infrared spectra and concentrations and resolutions. Measurements of the same methane sample's spectra under different resolutions demonstrate that both area absorbency and height absorbency vary with resolutions; spectra at lower resolution have bigger area absorbency for most of the peaks and are more likely to saturate for peaks of strong absorption. Standard methane sample of certain concentration is diluted with super pure nitrogen via mass control flowmeters and continuously passes through a 2-meter gas cell, such that the spectra of methane of different concentrations are collected. The area absorbencies of different peaks are carefully calculated via OMNIC software and results show that peaks with lower absorption are more likely to fit to linearity but more reluctant to changing concentrations. Area absorbencies are integrated through characteristic absorption regions, height absorbencies and area absorbencies are calculated at the two main absorption peaks and measurements show that approximative linearity fits all the areas and the best linearity appears at 1035.6cm<sup>-1</sup>.
Recently, Semiconductor sensor and thermal conductivity sensor are widely used for gas detection in transformer online monitors. Since the long-time stability or precision of these sensors is not satisfactory, the present researcher studied the application of FTIR in such monitors. In the wide measuring range of online monitoring, Absorbance Law is not always applicable, thus a non-linear calibration model was necessary. Experiments were done to set up the calibration model. A gas dilution system was designed. With the system, standard samples of fault gas including CH<sub>4</sub>, C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub> and C<sub>2</sub>H<sub>6</sub> were diluted to different concentration. BOMEM MB104 FTIR Spectrometer was used to collect spectra of gases. Curve fitting of the output of FTIR was done, and the effect of quantitative feature and concentration range on quantitative analysis was investigated. In addition, the lowest detection limit was tested. Experiment and calculation results show: accuracy can be improved by taking strong peak height at low concentration range, taking peak area or weak peak height at high concentration range as quantitative feature, and using third order polynomial to fit the output curve of FTIR. The lowest detecting limit of C<sub>2</sub>H<sub>2</sub> with 2.4m gas cell is below 0.3ml/l and that of 10cm cell is below 3ml/l.