Patthar

Calibration Principle HVI requires calibration from time to time, in order to ensure that the instrument provides the correct levels of measurement results. In fact, the instrument’s results are adjusted to a specific level of measurement, based on internationally accepted cotton standards. These standards are established and recommended by the International Cotton Standards Committee, a joint committee representing the international cotton trade exchanges. There are different means of calibration used for different HVI™ measurements: • Length and Strength Standard Calibration Cottons • Micronaire Standard Calibration Cottons • Color and Trash Standard Ceramic Tiles

3.6.1

Length and Strength

Today, there are two types of standard calibration cottons available through the USDA. These are: • Universal HVI Calibration Cotton Standards • ELS (Extra Long Staple) HVI Cotton Standards For both cotton types, there are two pairs of calibration cottons available, a short/weak and a long/strong type. These pairs are required because a two-point calibration is applied as shown in the Fig. 3-4 below. For each parameter, a low and a high value is required to establish what is called a “regression curve”. This will “translate” the actual measured value to the level of the standard cottons, which will make the results comparable to industry standards. The “agreement line” shown in Fig. 3-1 is an ideal regression curve, with the slope equaling 1, and the offset being zero. This would assume that each measured value exactly matches the standard value. In reality, however, this seldom happens, which is due to the standards that have been developed over the past decades. Hint: • Slopes and offsets will be different for each parameter. It is recommended to follow the trends of the slopes and offset, which are displayed after running a calibration or a calibration check procedure on the HVI. Normally, slopes and offsets should not change significantly over time. However, large shifts in the slopes and offsets could indicate a mechanical problem.

The approximate ranges in length and strength values for both standard calibration cotton types are listed in the Table 3-10 below: Standard Type Length Range Uniformity Range Strength Range Micronaire (“) (%) (g/tex) Range Universal < 1.00 – 1.20 77 – 90 22 – 34 3.6 – 4.4 ELS 1.10 – 1.30+ 82 – 90 30 – 37+ 3.6 – 4.4 Table 3-10 Fiber Property Ranges in HVI Calibration Cottons It is advisable that a spinning mill chooses calibration cottons that cover the entire range of cotton parameters that is usually tested. For example, Universal cotton standards are used in spinning mills that mainly consume short/medium staple type cottons. The ELS cotton standards, however, are mainly used in spinning mills that use long staple type cottons, which are longer, and stronger by nature. Frequency of Calibration Each time a calibration is carried out, the slopes and offsets of the parameters that are adjusted to the calibration standard levels are slightly changed. Thus, the regression or “adjustment” curve for this parameter changes each time as the slopes and offsets change. This affects the overall test level, and although it is necessary to be on the same measurement level as the cotton standards, it is not recommended to calibrate the machine too often. Hints: There are only a few scenarios when a regular cotton calibration is required: • Something has changed with the instrument’s mechanical setup. (Hopefully, only an authorized Uster Technologies service technician will carry out mechanical setup) • The results obtained by the “calibration check” procedure requires a new calibration (see comments below) • There is a consistent deviation from the calibration cotton standards level. 2.2 Micronaire (Mic) A fiber sample of constant weight is measured by passing air through the fibers and measuring the drop in pressure. The micronaire scale has been established empirically with a standard set of cottons and is not linear. Other factors such as fineness and maturity have an influence on micronaire results. Micronaire Description Less than 3.0 Very fine 3.0 to 3.6 Fine 3.7 to 4.7 Medium 4.8 to 5.4 Coarse 5.5 and higher Very coarse Table 2-2 Micronaire Ranges for Cottons

(short/medium staple)

2.13

Trash Count (Tr Cnt)

Trash is measured on the same glass window as the color measurement. The sample is illuminated from underneath the glass window, and a black and white CCD camera analyzes the changes in pixels. A trash particle is counted if it exceeds the grayness threshold set in the camera. All particles that cover the glass window are counted one by one, and the results are expressed in trash particle counts per surface area.

2.14

Trash Area (Tr Area)

The area that is covered by a trash particle is measured at the same time the particle is counted. The area that is covered by a single particle is summed up with all areas of the other particles counted on the surface of the glass window. The result is expressed in relation to the entire measuring area of the

glass window and is expressed as “% Area”. For example, a small number of trash counts can result in a high “% Area” values if the particles are large. On the contrary, a high number of particle counts with a small “% Area value” indicates the trash is smaller in size (pepper trash, for example).

2.15

Trash Grade (Tr Grade)

This is the trash or leaf grade that is determined by calibrating the HVI with known samples (i.e., trash standards). These samples are usually numbered from 1 to 7 with increasing amounts of trash as the number gets larger. The standards used for calibration can be supplied by the USDA or established by the official cotton standards agency in any cotton producing country.

2.12

Color Grade (C Grade)

The color grade of a cotton sample is determined in a two-filter colorimeter. This objective method was developed by Nickerson and Hunter in the early 1940’s to check the USDA cotton grade standards. Today, it is intended to completely replace the subjective visual grade determined by the cotton classer.