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United States Department of Agriculture

Agricultural Research Service

Title: Calibrating a Free Space Measurement of the Dielectric Properties of Lint Cotton

item Pelletier, Mathew

Submitted to: ASAE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: August 25, 2003
Publication Date: August 25, 2003
Citation: Pelletier, M.G. Calibrating a free space measurement of the dielectric properties of lint cotton. 2003. American Society of Agricultural Engineers. Paper No. 031149. 8 p.

Interpretive Summary: In recent years, it has been shown that cotton moisture restoration systems that put moisture back into the cotton immediately before the bale press can dramatically reduce the force required to press the bale. This force reduction, in turn, translates into less stress on the bale press system leading to less breakage and down time. The industry is now embracing this new technology (Nelson and Turner, 2003). As such, there is a need to control these systems to regulate the amount of water that is added to the bale. Currently, there are few commercially available sensors that can measure the moisture content of the cotton bales. The few that are available are prohibitively expensive and as such have not been readily accepted by the cotton ginning industry. Recent research by the author has produced a low-cost cotton bale microwave sensor that shows promise in solving this problem. In order to test this new sensor, it became apparent that for commercial acceptance, there will have to be a rapid method by which to evaluate and calibrate the sensor upon installation. This lead to the immediate question as to whether samples could be presented to the industry standard resistance sensor to provide the spot check on the moisture content. The only problem with this approach is that all of the resistance sensor research has been based upon utilizing cotton that was placed in environment chambers for several days before taking any measurements. None of this work utilized resistance sensors with cotton that has been flash conditioned with moisture restoration systems. As flash conditioning produces a product with varying levels of moisture even in the same sample, it became apparent that the resistance sensors should be tested under this more extreme utilization. To test the resistance sensor's ability to perform on rapidly conditioned cotton, a commercial resistance sensor was tested on a series of lint samples that were flash conditioned using a custom steam conditioner. In conjunction with the resistance sensor, another new experimental hand-held microwave moisture sensor was tested as well. The research indicates that for cotton below 8-9% moisture content (dry basis) that has been flash conditioned with a moisture restoration system, a resistance sensor when utilizing a simple protocol of averaging three readings that are composed of readings from different portions of the lint sample, can be used to provide an accuracy of +/- 1.5% moisture content with a 95% confidence in the reading. While this level of accuracy is less than expected, it was shown to have the same slope as the manufacturers calibration (to within 3%). Over the full range of the test, and still within the limits of the instrument as detailed in the manufacturing specifications, the instrument produced a coefficient of determination of 0.634 with an accuracy of +/- 3.02% moisture content (95% confidence). The research also indicates that the hand-held microwave sensor provides a sensing prediction capability with a standard error of 1.17% moisture across the full range of the experiment. The coefficient of determination for the test was 0.725, with an accuracy of +/- 2.34% moisture content with a 95% confidence in the reading. Further work is required to determine if this new microwave sensor performance will improve if used with conditioned cotton.

Technical Abstract: Currently, there is a new influx of cotton moisture restoration systems into the cotton ginning industry. These systems are designed to add moisture back to the cotton immediately before the bale press. As such, there is a need to control these systems to regulate the amount of water that is added to the bale. Resent research by the USDA indicates that putting too much water back into the bale can result in a degradation of cotton quality while in long term storage. Today's commercially available sensors are either prohibitively expensive or do not provide a method by which to determine the final cotton bale moisture content. Recent research by the author has demonstrated a new free space microwave sensor that looks promising as a low cost method of measuring bale moisture. What is now required is a rapid method to calibrate the system for a commercial deployment of this sensor. To date, little research is available that details the accuracy of commercially available hand-held moisture sensors when used with flash moisture restoration systems. This paper examines both a commercially available resistance based hand-held moisture sensor as well as a new microwave hand-held prototype of the author's design. Both systems are evaluated for accuracy and suitablilty and use with flash moisture restored cotton lint.

Last Modified: 4/18/2014
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