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Evolution in cotton testing instruments
By :   V. Parthasarathi
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HVI 9000 Strength Measurement


HVI uses the "Constant rate of elongation" principle while testing the fibre sample. The available conventional methods of strength measurement are slow and are not compatible to be used with the HVI. The main hindering factor is the measurement of weight of the test specimen, which is necessary to estimate the tenacity of the sample. Expression of the breaking strength in terms of tenacity is important to make easy comparison between specimens of varying fineness.


The problem is overcome in the HVI 9000 by positioning the jaws and breaking the fibres at a constant "Amount" location across the beard. By breaking the fibres at a constant amount location, it is made sure that the samples are broken with a constant number of fibres between the jaws.


Therefore, raw data strength is directly proportional to the force to break the fibres. The raw data so obtained are then adjusted to desired Ievels by testing samples of designated values. In order to make the estimation of the specimen linear density accurate enough, a micronaire correction factor is normally introduced so that the strength values are not affected by variations in micronaire.


Fineness measurement


The micronaire module of HVI 9000 and the low volume fineness tester use the airflow method to estimate the fineness value of cotton. A sample known weight is compressed in a cylinder to known volume and subjected to an air current at a known pressure. The rate of airflow through this porous plug of fibre is taken to be a measure of the fineness of cotton.

The number of fibres in a given weight of cotton will be more in the case of finer fibres than in the case of finer fibres than in the case of coarser fibres. If air is blown through these samples, the plug containing finer fibres will be found to offer a greater resistance than the plug with coarser fibres. This is due to the fact that the totaI surface area in the case of the former will be greater than the latter and hence the drag on the air flowing past will be more. This differentiating factor is made use of to indirectly measure the fineness of cotton.


The instrument operates as follows. The chamber lid is closed; a piston at the chamber bottom compresses the fibre to a fixed and known volume. A regulated stream of air is then forced through the sample and the pressure drop across the sample is applied to a differential pressure transducer. The transducer outputs an analog signal voltage proportional to the pressure drop.


This analogue voltage is applied to an analogue to digital converter, which outputs a digital signal representing the voltage. Cotton with known fineness values is tested and the voltages obtained are used to obtain the calibration curve, which is used for all subsequent testing to display the cotton fineness.


The fineness is expressed in the form of a parameter called the micronaire value, which is defined as the weight of one inch of the fibre in micrograms. Maturity of cotton also influences the micronaire value.


Colour measurement


The HVI colour module utilises optical measuring principles to define colour. The colour module has a photodiode, which collects the reflected light from the sample. The photodiode output is converted into meaningful signals using signal conditioners. The illumination of the sample is done with the help of two lamps connected in parallel.


Light from the lamps is reflected from the surface of a cotton sample on the test window. The reflected light is diffused and transmitted to the Rd and +b photodiode. These two signals are conditioned to provide two output voltages, which are proportional to the intensity of light falling on the respective photodiodes. These voltages are converted to digital signals from which the computer derives Rd and +b readings to be displayed on the screen.


 

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