Advanced portable four wavelength NIR analyzer for rapid chemical composition analysis

This paper discusses the requirements for a portable infrared analyzer and presents the design of a four wavelength NIR analyzer utilizing the integrated multichannel detector technique for wavelength separation and detection. This technique, together with an electrically modulated miniature tungsten filament source and surface mount electronics manufacturing, provides a compact, rugged handheld instrument construction without any moving parts. Improved accuracy and short stabilization time is achieved through a combination of thermoelectric temperature stabilization in the four channel detector and a calculated signal compensation for the residual temperature error. Parallel analog phase sensitive detection of the detector signal maximizes the S/N ratio and maintains the simultaneity of the measurement. All composition calculations are performed in a microcomputer built inside the analyzer. The weight of the prototype analyzer is about 1 kg and a NiCd battery pack provides capacity for hundreds of single measurements or about 3 hours of continuous operation. Two prototype instruments have been fabricated with optimized NIR wavelengths for the moisture measurement of milled fuel peat on production fields. The accuracy of digitized two-wavelength signal ratios were tested in the laboratory over time and against temperature. Full accuracy is achieved in 10 seconds after switch-on and the maximum short time peak-to-peak variation in the signal ratios is 0.2%. The errors due to temperature fluctuations in the range from +2 to +50 degree(s)C are between -0.4 to +0.6%. The instruments were calibrated using 102 samples of Finnish milled fuel peat. the cross-validation testing of calibration gave a standard deviation of 1.6% (moisture by weight) compared to the reference method. Other applications for the analyzer are being planned in wood processing and chemical industries as well as in agriculture.