Updated: Jan 8, 2021
Sample is a comparatively small portion selected from a population; a sample is the true representative of the population.
It is impossible to test all the raw materials and finished products due to its time and cost constraints.
As many of the tests are destructive there would not be any material left after it has been tested due to this only a representative sample of the material has been tested.
The main aim of sampling is to produce an unbiased sample for testing.
Types of sample:
Random sample: The sample which is randomly picked from the population is known as random sample. In this type of sample every individual in the population has an equal chance of being included in it. It is free from bias, therefore truly representative of the population.
Numerical sample: A sample in which the proportion by number of, say, long, medium, and short fibers would be the same in sample as in the population.
Biased sample: A sample whose selection is influenced by factors other than chance is known as biased sample. A biased sample will; never represents the whole population.
Causes of Biased sample:
Bias due to physical characteristics: In this type of bias physical characteristics of fibres will influence the sampling process. Ex. Long fibers have a greater chance of being selected.
Bias due to position relative to the person: This type of bias is generally faced when the sampling person has taken the sample from only one particular location like surface, edges etc. This will lead to position bias, Ex. Fibre collected from top layer of bale.
Subconscious bias: If the person collects the samples which is free from ridges without thinking about it will lead to subconscious bias.
Aim of Sampling:
The main aim of sampling is to produce an unbiased sample.
** Only 20-25mg of sample is tested from 250kg of bale.
Terms used in sampling:
Population: It is defined as the entire pool from which a statistical sample is drawn.
Sample: It is a small proportion selected from the population.
Consignment: It is the quantity of material delivered at the same time. Each consignment may consist of one or several lots.
Test lot/batch: It consists of all the containers of a textile material of one defined type and quality, delivered to one customer according to one dispatch note. The material is presumed to be uniform so that this is the whole of the material whose properties are to be characterised by one set of tests. It can be considered to be equivalent to the statistical population.
Laboratory sample: it is the sample used for carryout the measurement in the laboratory, this is derived by appropriate random sapling methods from the test lot.
Test specimen: this is the actual sample used for testing, it is derived from the laboratory sample. Generally, measurements are made from several test samples.
Package: It is defined as a unit (which can be unwound) within each container in the batch. They might be bump top, hanks, skeins, bobbins, cones or other support on to which have been wound tow, top, sliver, roving or yarn.
Container or case: A shipping unit identified on the dispatch note, usually a carton, box, bale or other container which may or may not contain packages.
Sampling methods are controlled by:
Form of material (fibre, yarn, fabric).
Availability of material.
Type of test.
Type of testing instrument.
Type of information required.
Accuracy of the test results required.
** In Textile Testing sampling can be done at different stages like fibre stage, yarn stage, fabric stage and garment stage.
Fibre Sampling Techniques:
Zoning technique: This is the most popular and important technique for sampling of raw cotton or wool or any other fibre.
Collect at least 40 handful of samples (tuft) from different zones of the bulk raw material.
Take ¼ from each tuft to make a final sample which is free from defectives.
From the bulk, a sample of 2oz is prepared by selecting about 8 large tufts chosen.
Divide this sample into 4 parts.
Take 16 small tufts at random from each part at a weight of 20mg.
Now each tuft is divided into 4 parts, discarded alternately with right and left hands and turning the tuft through a right angle between successive halving’s. 16 'wisps' are thus produced from each part of the sample.
Combine each set of wisps into a tuft.
Mix each tuft in turn by doubling and drawing between the fingers.
Divide each tuft into four parts.
Obtain four new tufts by combining a part of each of former tufts.
Mix each new tuft again by doubling and drawing.
Take apart from each tuft to make the final sample.
This method is used to evaluate the amount of grease, vegetable matter present in the sample taken from unopened bales of raw wool.
It means half way into the bale i.e. samples from centre. The tube enters in the direction of compression, so perpendicular to the layers of fleece.
Cutting tip dia is lesser than coring tube.
helps sliding the core upside the tube penetrates.
helps retaining the core as it is withdrawn.
No. of cores are extracted and combined.
Different sizes of tube 14, 15, and 18mm.
After removal cores are kept in air tight container immediately.
Hydraulic coring machine for large number of samples.