Uster testing is the set of laboratory measurements made on evenness testers and related instruments from the Swiss test-equipment maker Uster, which have become the standard way to describe yarn quality worldwide. The main report gives evenness, imperfections, hairiness and strength, and each value can be placed against Uster statistics, which rank yarns against world production in percentiles. A yarn at the 5 percent level is among the best produced, while a yarn at the 95 percent level is among the worst.
The instruments do not test for everything a buyer cares about, and they say nothing about colour, contamination or shade variation unless separate tests are run. They do give a reliable, comparable picture of how regular and strong the yarn is, which predicts how it will run on knitting and weaving machines and how the fabric surface will look.
The main values on an evenness report
- U percent or CVm: the mass variation of the yarn along its length, where lower is more even. CVm is the coefficient of variation and is the figure used in modern reports.
- Thin places: the number of places per kilometre where the yarn is thinner than a set threshold, usually minus 50 percent of average mass.
- Thick places: the number per kilometre thicker than a set threshold, usually plus 50 percent.
- Neps: small entangled fibre knots counted per kilometre, usually at plus 200 percent for ring-spun and plus 280 percent for rotor yarn.
- Hairiness: the H value, representing the total length of protruding fibres per centimetre of yarn.
- Strength and elongation: measured separately on a tensile tester, reported as tenacity in centinewtons per tex and elongation percent.
Using the percentiles
Uster statistics are updated periodically and are published by fibre type, spinning system and count, so a combed ring-spun cotton yarn at Ne 30 is compared only with other combed ring-spun Ne 30 yarns. A spinner who says its yarn is at the 25 percent level on evenness and imperfections is making a verifiable claim, and you can ask for the report to confirm it. Be wary of a claim that all parameters sit at 5 percent, since evenness, hairiness and strength tend to pull in different directions and few mills achieve top percentiles on all of them at once.
Specifying percentile levels in a contract is a practical approach. Many buyers ask for 25 percent or 50 percent on the main values, depending on end use, and leave hairiness looser for knitting yarns where it matters less.
Limits of the report
A mill test report reflects the bobbins or cones the laboratory chose to test. Sampling matters more than the instrument, and a report from the mill's own lab is a starting point, not proof. Ask for the number of cones tested, the date, and whether the results are from the actual lot being shipped. Where the order is large or the fabric is demanding, an independent test on sealed samples is worth the small extra cost.
How Creston & Co helps
We draw sealed samples from the cones destined for your order and send them to an NABL-accredited lab for evenness, imperfection, hairiness and strength testing, and we compare the results against the mill's report before shipment. We audit the mill laboratory during our visit, supervise loading and send weekly updates. If you would like help turning Uster values into a contract specification, book a free 20-minute sourcing review.