SPECTROSCOPY
IN THE FIELD G
SPECTROMETERS
Janel Kane reports on a novel deployment of miniature spectrometers for grain protein concentration sensing
rain protein concentration (GPC) is a critical measure in grain production, with commodity pricing based on protein content.
Protein content is also directly proportional to nitrogen uptake. T e measurement of protein content and nitrogen uptake allows the producer to manage application of nitrogen fertilisers to reduce over-use and operate more economically while maximising production yield and return. Near-infrared absorbance spectroscopy is a standard and nondestructive analysis technique that has been used to measure the GPC in wheat and other grains. Since 2000, several systems have been developed to deploy this technology in the fi eld, on-combine, but these systems are too expensive for many small producers and have not been widely adopted for that reason. Researchers at the United States Department of Agriculture (USDA) Oregon Field Offi ces endeavoured to develop a low cost near infrared analyser for in fi eld, real-time grain analysis.
SYSTEM DESCRIPTION T e instrument adapted for the
USDA study was the Avantes AvaSpec
ULS2048x16-USB2 spectrometer (next generation instrument is the AvaSpec- ULS2048X64-Evo) with a 2,048 pixel charged coupled device (CCD) linear array detector and confi gured with a 600 lines mm-1
grating to measure the range
650-1,150 nanometres. T e spectrometer is fi bre coupled to a fi bre optic cable that connects to an integrating sphere (AvaSphere 50-LS-HAL) with built-in halogen lamp. Light is collected through an aluminosilicate glass covered aperture for a sealed, light transmitting collection window. T ermal stabilisation of the system was achieved with the help of a thermoelectric cold plate and a styrofoam insulated enclosure. An inexpensive notebook computer, off -the-shelf GPS and programmable controller completed the equipment list for a total cost of less than US$9,000, putting this system in reach of a wider segment of small producers. Designing the system began with lab testing, but it was also important to mimic fi eld conditions. A test stand was used to simulate grain fl ow conditions on an auger conveyor system like that on a combine harvester. T e probe head was secured to an opening in the auger housing at 30° above
the centre of the grain stream at a fl ow rate of 1.63 kg/s-1
. T is allowed further testing
of the calibration model under simulated harvest conditions. Field evaluation of the adapted system was conducted with the integrating sphere head mounted to the housing of the exit auger of a Case International Harvester 1470 combine. Steel mesh screens were used to protect the fan ports on the sphere from debris and fi bre optics from the sphere led to instrumentation mounted in the cab. Dark and light referencing was performed each time the combine was stopped for unloading the bulk collection tank, multiple times per day. Data was collected on the notebook computer during operation with processing at the end of each harvest day. T e ruggedised Avantes system withstood the rigors of four growing seasons and saw about 200 hours of run time.
DEVELOPMENT OF THE CHEMOMETRIC MODEL T e USDA pursued a research product aimed at adapting relatively inexpensive NIR refl ectance spectrometers to map
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