Optical-fiber measurements coupled with near-infrared spectra in the range of 5500 to 10,000 cm−1 are used to determine the optimum spectral region for remotely monitoring the energy content of natural gas at above ambient pressures. The system was configured with a Fourier transform near-infrared spectrometer and a fused-silica fiber bundle. The total energy values at a pressure of 100 psi were determined by the partial least-squares (PLS) calibration algorithm. The precision and accuracy for predicting the BTU content of an industry standard gas sample were less than 0.4% in the lower-frequency region of 5700 to 6400 cm−1.
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