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’s checklot 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.
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.
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.
In addition to calibrating the instruments, each classing office monitors its testing level through an in-house system designed to ensure that the instruments within a classing office provide uniform results. In this system, known-value cottons are tested on each instrument at regular intervals throughout each work shift. If any test value deviates from the known value by more than a specified tolerance, corrective action is taken, such as recalibration or diagnostic testing of the instrument.
USDA’s checklot program ensures that all USDA classification facilities across the Cotton Belt provide uniform test results. Following instrument testing and classer grading, samples are randomly selected by computer during each work shift, at a rate of approximately one percent of the classing volume. These randomly selected samples are referred to as “checklots.” The samples are forwarded by overnight delivery to USDA’s Quality Assurance Division, in Memphis, Tennessee, where they are retested. The results are compared with the original classification, and this information is communicated to the originating office, where adjustments are made as necessary. USDA maintains a record of comparisons for each instrument and classer on a daily, weekly, and seasonal basis.