Field Trip to Yocom-McColl: Interpreting that Histogram Report
Written by Melinda Cook, originally published on her blog, The Pacablogger.
Remember those field trips during your old school days? The excitement of going somewhere new, a day not sitting behind a desk, the chance perhaps to eat lunch out, getting your permission slip signed? I loved them way back when and still do. In pretty much all respects (except the permission slip thing), a visit to the venerable Yocom-McColl labs very much took me back to those days. When at the end of it all, you came away with eyes agog and brain stuffed to the last wrinkle and cell with new information .
Besides a hospitable welcome by Margaret and Angus McColl and their friendly staff, a tour of the facility is an eye opener to the fiber business beyond our growing alpaca industry. Much of the activity there focuses around sheep and other fibers , not just in raw form, but also in cloth, yarn, and finished product. It’s a foreshadowing of what’s to come for us as we move to a true commercial industry and the depth of analysis that will come. But for now, I’m concerned more about our relatively narrow world of alpaca fiber testing. What is the measurement process ? How do I use the numbers for my breeding program? What is the correct interpretation and use of the fiber stats? Beyond that, I am ready to soak up whatever other tidbits of precious information that Angus was willing to share from his vast wealth of experience.
So what does happen when your little bag of fiber lands at Elk Place in Denver? Well here’s the Cliffs Notes version -
Cutting the Test Sample: Like quite a few of the tools we saw, the machine to cut the sample off that fiber you sent in was customized by Angus himself. It has a guillotine-like blade that thumps down and whacks off 2mm off the cut end to ensure a testing sample that has experienced the same conditions e.g. environment, nutrition etc. This 2mm sample is then washed with a degreaser in a little cup and flash dried with a burst of air, leaving residual matter and grease behind.
Into the Conditioning Lab: After drying, the sample is conditioned in the Conditioning lab, where the humidity and temperature are held at 65% and 20 degrees Celsius, respectively. This creates an environment identical to those of measuring labs worldwide so that the stats generated in each lab are apples to apples (You know that little note on the bottom of your histogram reports that say “This test performed according to I.W.T.O. method 47 or 12″? Well, that one line sentence has a lot more meaning when you see the precision and complexity of the preparation and measurement process). Once conditioned, the fiber is prepared precisely on a glass slide for viewing. There are two machines in the lab that are used for measurement: the OFDA 100 (that’s when you get the spin fineness and curvature in addition to the usual AFD, SD, CV, and % fibers greater than 30m measurement) and the Sirolan Laserscan (which gives you just the AFD, SD, CV and % of Fibers greater than 30m). The OFDA 100 is what we observed in action (mostly because the Laserscan is a big enclosed box – not so fun for layman viewing ). The slide is placed on the black base and as the machine scans back and forth across the slide, it uses a camera to generate images of the magnified fibers. It then takes measurements across the width of each of fiber strands, averaging them together to get the Averge Fiber Diamenter (AFD) in microns. While we were watching, we observed it taking over 500 measurements in just seconds. Zowee. It also figures standard deviation (SD), Coefficient of Variation (CV), spin fineness and curvature – CV and spin fineness recalculating on screen dynamically. The histogram forms on the computer before your eyes. It’s amazing to see. And the margin of error in the AFD? According to Angus, not greater than 3/10th of a micron. Wow. That’s 3/10th of 1,000,000th of a meter, to put it in everyday terms that perhaps I can wrap my brain around. The Laserscan will give you the same AFD, SD, and CV measurements through a different technology.
OK, so now I have this great report with quantitative, objective measures that I can publish, quote, and use to make decisions regarding my breeding program and fiber use. What does it all mean?
Here are the conclusions I’ve come to:
AFD: Average Fiber Diameter. This is probably the most straightforward and easy to understand. This is the average diameter in microns of all the sample measurements taken. This is your measurement of fineness which translates into the grades of royal baby, baby, superfine,adult, coarse etc. As an average , however, this means that not each or even most of the fibers are equal to the AFD. It is best viewed with the SD or CV to consider variation around the AFD.
SD: Standard Deviation. This is the average deviation away from the AFD in microns. So if you have a small figure, you have better uniformity in micron. High SD means greater variation in micron. In general, you’ll hear a lot of people say they want less than 5m SD. Get close to 3m SD and you’re talking really uniform. You can look at the bell curve represented in your histogram report to see whether you have a skewed distribution. Most commonly, you might perhaps see a very tight distribution on the left side and then a more spread out distribution of points on the right side, signifying your variation is due perhaps to guard hair.
CV: Coefficient of Variation. This is simply a calculation where the SD is divided by the AFD and then multiplied by 100 or (SD/AFD)*100. Many people rely on the CV to track uniformity which is correct. However, it can be misleading as a standalone metric if fineness is of primary importance in your program. For instance, you can have an animal at 16m with an 3.8m SD and therefore a CV of 23.8m. But an animal at 23m with an SD of 3.8m has a CV of 16.5m. Which would you rather have? Well it depends – are you prioritizing fineness or uniformity? When you talk to mills they will sometimes say CV is the more important metric. But that is because from a processing standpoint,variation is the enemy. Variation in micron, staple length, tensile strength can really muck up the machines and consistency in end product. This is the difference between consistency of product and grade of product.
% Fibers Greater than 30m: 30m as a fiber diameter is the commonly used threshold for prickle factor. In other words, above 30 m and you have prickle, below and it’s fiber that is tolerated by the skin. So this measurement is useful for determining end use of the fiber.
Spin Fineness: What’s the difference between AFD and Spin Fineness? Spin fineness is a pure mathematical calculation that normalizes the AFD measure based on a CV of 24. In other words, if your fiber has a CV different from 24, the fiber diameter is recalculated to what it would be if the CV were equal to 24. That means if you have a very low CV, the spin fineness will reflect a number lower than your AFD. So what is this figure used for? This normalization is used to predict how the fiber will process when compared to other fiber. By assuming the same CV and adjusting the fiber diameter accordingly, you get a comparison that removes the variation consideration. This to me seems to be a measurement purely useful for processing.
Curvature: We’ve seen a lot about curvature lately. Curvature measures the angle in crimp in deg/mm. In general, higher curvature numbers correlate to higher frequency of crimp. Higher frequency of crimp correlates to greater fineness…in general. Greater fineness does not always show up as higher frequency of crimp or high curvature as we know. But since we’re talking quantitative measures to come to objective conclusions, what is the real value of this measure? It seems that the curvature is a good indicator of resistance to compression. The greater the curvature reading (the greater the crimp so to speak), the greater the resistance to compression. The greater the resistance to compression, the greater the memory. Lack of memory, as you remember, is why alpaca sweaters easily lose their shape and why it is often blended with wool which has great memory.
Needless to say, I came away from the visit with much more than an understanding of the testing process. I’ve summarized just a bit of it in the above. As a breeder in Colorado, I am always amazed at the depth of knowledge to which we have ready access right under our noses. A visit with Angus and Margaret will bring that point home quicker than most any activity. Besides an informative trip, it was a pure pleasure to spend time with them. Their generosity in sharing what they know will delight you and Margaret will have you rolling with laughter. So I can only say it was a day more than well spent and like those field trips of my childhood, filled me with more information than I could immediately absorb and eyes once again agog.