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Fiber Measurement
Terms
AFD
Average Fiber Diameter
(microns). This number reports how wide in cross section,
the individual fibers in a fiber sample are on an average.
To determine this number, a 2 inch square fiber sample is clipped from
the side of the alpaca's blanket as close to the skin as possible.
The sample is then sent in to a testing lab such as Yocom-McColl Testing
Laboratories where they take the fibers in the sample and cut a
2-millimeter sample from the base of the sample (the part which was
closest to the skin). A laser then scans through the fiber
sub-sample and measures the diameter of several thousand individual
fibers. The individual fiber diameters are then averaged to obtain
the AFD.
A typical alpaca at about 1
year of age has an AFD of about 25 microns. A micron is a unit of
length equal to 1/1,000th of a millimeter or 1/25,400th’s of an inch.
As an example of how small a micron is, consider the following ...
Take a piece of paper and a
ruler. Mark and cut off a 1 inch wide strip of any length from the
original piece of paper. Now cut the 1 inch wide strip in half to obtain
a strip 1/2 inches wide. Cut the half in halves for a 1/4 inch wide
strip. Now cut the half in halves again (1/8 inch). Now carefully cut
the strip into 3,175 equal pieces. You've now got a strip (actually a
whole bunch of strips) which are 1 micron wide.
The lower the AFD the
better. Sheep's wool is around 30 to 40 microns, meaning it is about
twice as coarse (read scratchy) as on a quality alpaca fleece which can
have a AFD of as low as 17 (or even lower). In fact, AFD's as low as 14 have
been observed on some very fine fleeced alpacas.
Yocom-McColl reports male
alpacas have, on an average, a higher AFD than female alpacas.
Other factors reported by Yocom-McColl as influencing fiber diameter are
age, level of nutrition, and genetics.
If the test to determine AFD
is run on a fleece before the alpaca is 1 year of age then the AFD can
be artificially low. Baby fiber is the very finest. An AFD
of less than 25 on an alpaca of greater than 2 to 3 years of age means
the alpaca has an unusually fine fleece.
(top)
SD
Standard Deviation (microns). The more variation within the
fleece the higher this number will be. If a fleece has lots of thick
fibers mixed in with the thin fibers then the standard deviation will be
high. You want this number to be such that the CV (see below) is less
than 24 to 25. Many breeders and fiber experts do not talk much
about the SD. They instead refer to the CV to express, in one way, how
consistent the fleece is.
(top)
CV
Coefficient of Variation (percent). This is the SD
(Standard deviation) divided by the AFD (Average Fiber Diameter).
Lets say we have an alpaca with a AFD of 20. If the SD for this alpaca
is 4 then 4/20 = CV = 20%, a very good number. CV is the one of
the keys to use to evaluate the variation in the fleece. The lower
the CV, the more consistent (the more alike) the fibers are within the
fleece sample. An excellent number for CV is 20 or below.
The average is around 24 to 25. A high number such as 30 would
mean a pretty poor quality fleece, even if the AFD was low!
Lets say we had a 25 micron
fleece (AFD), which is about normal for an alpaca, but the CV is
unusually high at 30. Although the average is acceptable, the
deviation is way too high. This would be like reporting a person's
body temperature as being normal, but, the person's head is in the
freezer and the feet are in a hot oven. With the head at zero
degrees and the feet at 200 degrees, the person's average temperature
would indeed be about normal, but the deviation (the variation) would be
ridiculous!
CV is also important in that
it allows the consistency of one alpaca to be compared to another.
If we have one alpaca with an AFD of 20 and a CV of 21, and another
alpaca with an AFD of 17 and a CV also of 21, then we can say although
one alpaca is finer than the the other, both have equal consistency.
(top)
%>30
Fibers Greater than 30 Microns (percent), or the
Prickle Factor.
Garments having contact against more sensitive skin (such as a scarf,
sweater, skirt, or suit) must be made of finer fleece or the clothing
will cause prickle or itch to the wearer. Comfort tests have
suggested that high quality garments of these types should be made of
fleece which has a Prickle Factor of less than 5%. When a fleece
analysis is run, the report will show what percentage of the fiber is
greater than 30 microns. A high quality fleece will have this
number at 5 percent or less.
As the alpacas ages, the
prickle factor will continue to become greater as the alpaca's fleece
become more coarse. It is truly a remarkable alpaca which
maintains this number below 5% when it is more than 3 to 5 years of age.
(top)
Histogram
A fiber analysis report (one
is pictured to the right) showing both a numerical and a graphical
representation of the AFD, SD, CV, and %>30 for a given fiber sample.
(top)
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