The Classification of Cotton
Quality and Reliability of Classification Data
Both manual and instrument classification are closely monitored to assure high-quality results. The monitoring of quality is performed mainly through the operations of the Cotton and Tobacco Program’s Quality Assurance Division. Several tools and programs are in place to manage quality. These include laboratory conditioning, sample conditioning, equipment performance specifications, instrument calibration, in-house monitoring, and USDA Quality Management Program.
Atmospheric conditions influence the measurement of cotton fiber properties. Therefore, the temperature and humidity of the classing laboratory must be tightly controlled. Temperature is maintained at 70°F plus or minus 1°F (approximately 21°C plus or minus 1/2°C), and relative humidity is maintained at 65 percent plus or minus 2 percent.
Samples are conditioned to bring the moisture content into equilibrium with the approved atmospheric conditions. Conditioned samples will have a moisture content between 6.75 and 8.25 percent (on a dry-weight basis). The conditioned samples are randomly checked to verify that the appropriate moisture content has been reached. Samples may be conditioned passively or actively. In passive conditioning, the samples are placed in single layers in trays with perforated bottoms to allow free circulation of air.
The samples must be exposed to the approved atmosphere until the specified moisture level is reached, which usually takes at least forty-eight hours.
In active conditioning, a Rapid Conditioning unit is used to draw air set at the approved atmospheric conditions through the sample until the required moisture content for high volume instrument testing is attained. Through active conditioning, the time required to condition samples can be reduced to ten minutes.
Equipment Performance Specifications
It is essential to verify that classing equipment meets minimum performance specifications. “Precision” refers to the ability of an instrument to produce the same measurement result time after time. “Accuracy” refers to how well an instrument measures a certain property in relation to its true value.
Newly purchased equipment must pass a series of thorough tests before being accepted and put into operation. Specifications for the delivery of new equipment include the maximum allowable tolerances for precision shown in the table at right.
Furthermore, all instruments are evaluated annually, typically before each cotton season begins. Testing is done to verify both the precision and the accuracy of instrument measurements.
Calibration of Instruments
Instruments are calibrated for fiber length, length uniformity, micronaire, and fiber strength through the use of calibration cottons. Tiles are used to calibrate color and trash measurements. Calibration is performed at regular intervals for each quality factor. USDA calibration tolerances are shown in the table below.
Quality Management Program
USDA’s Quality Management Program (QMP) ensures that all USDA classification facilities across the Cotton Belt provide uniform test results. QMP utilizes a series of known value cottons and tile materials that are tested every two hours to verify cotton measurement levels. Results of the QMP verifications are analyzed and graphed utilizing specialized data analytics programs. Large screen monitors in the classification laboratories display the graphical results in real-time so that technicians can take immediate action if any instrument measurement begins to drift out of calibration. In addition to the two-hour QMP checks, special USDA round test samples are tested weekly to further verify that instrument test levels are consistent between all USDA classification facilities.