Measuring Microns - different methods can mean different results
What Methods are Available to Measure Micron?
There are three methods of measuring the micron of raw wool:
1. The Airflow Method
This method measures the flow of air through a sample of wool. The coarser the wool, the more air that flows through the sample, and vice-versa. The Airflow therefore provides a very indirect measurement of fibre fineness. It must be calibrated using standard wool samples where the Mean Fibre Diameter has been measured using a more direct technique.
The Airflow method does not give measurements of SD and CVD, it only gives a measure of the average fibre diameter.
Airflow is one of the oldest methods and has proven itself over many years of commercial use. However, there are some special wool types for which we know that the Airflow produces the wrong result e.g. lambs wool and coarse crossbreeds. This is because these wools are less dense than normal and the Airflow assumes that the wool density is always constant.We also know that the Airflow can also be affected by large variations in Coefficient of Variation of Diameter.
If the CVD is very large then the Airflow will provide a coarser result and if the CVD is very small the Airflow will provide a finer result.Until 1st July 2000 the Airflow was the normal measurement for micron results are shown in auction catalogues.
2. OFDA
OFDA is an abbreviation for Optical Fibre Distribution Analyser.
This instrument uses a video camera to take electronic images of fibres, distributed over a horizontal glass slide, through a low powered microscope. The instrument uses computer software to analyse these images and so derive measurements of diameter of a large number of fibre cross sections. Because it measures fibres this method measures SD and CVD as well as Mean Fibre Diameter.
Although the OFDA provides a more direct estimate of fibre fineness than the Airflow, the instrument must still be calibrated using the same standard samples of wool.
OFDA is a relatively new means of measuring fibre diameter (IWTO standard approved in 1995).
3. LASERSCAN
The LASERSCAN is a instrument developed by CSIRO, supported by funds provided by wool growers through the Research & Development Levy. Individual fibre snippets are dispersed in a fluid (iso-propanol and water), and the fibres are transported by the fluid through a patented glass cell where each fibre intersects a laser beam. The instrument measures the change in the signal generated when the shadow cast by the fibre snippets falls on a very sensitive light detector. These signals, which are directly proportional to the fibre diameter, are recorded electronically and analysed on a computer. This method measures SD and CVD as well as Mean Fibre Diameter.
Similarly, although the LASERSCAN provides a more direct estimate of fibre fineness than the Airflow, the instrument must still be calibrated using the same standard samples of wool.
LASERSCAN is also a relatively new means of measuring fibre diameter (IWTO standard approved in 1995). The speed of operation and potential for automation lends itself to high volume testing applications.
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There is only one instrument that is universally accepted for the direct measurement of Mean Fibre Diameter of wool. This is the Projection Microscope. Fibres are distributed on to a glass slide and placed under a microscope. The magnified image of the fibres is projected onto a screen. An operator measures, using a calibrated ruler, the width of these projected fibre images, taking great care to ensure that each image is in focus when it is measured. Because the magnification is accurately known, the actual widths of the fibres can be readily calculated. This procedure has existed for many years and is still used, but it is time consuming and expensive and consequently is not suitable for routine measurements of sale lots, or fleece samples.
However, the Projection Microscope is used as the reference method. All other instruments are calibrated using standard wool samples where the diameter and distribution values have been determined using the Projection Microscope. |
Which Method is Best?
It depends on what you mean by ‘ best’ . The three technologies measure fibre diameter in different ways and all are capable of producing precise results. Only OFDA and LASERSCAN can produce SD and CVD results.
A significant problem with using three different technologies to measure the same thing is that the three methods will not always produce the same results. The reason for this is that the three technologies all define fibre diameter differently, and consequently, whenever specific cases occur where the definition used does not incorporate those specific characteristics of the fibre which make it different from the norm then differences will occur.
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The reasons for the differences are numerous and somewhat technical, and in all cases are not well understood. What is quite clear from all the available research is that LASERSCAN is the only instrument that is relatively unaffected by all the factors that are known to influence the Airflow and the OFDA.
A key difference between LASERSCAN and OFDA is that OFDA is sensitive the calibration employed. OFDA can be calibrated for Mean Fibre Diameter, but this will result in biases in the Standard Deviation. Alternatively, OFDA can be calibrated for Standard Deviation, but this will result in biases in the Mean Fibre Diameter. This does not occur for LASERSCAN. Such behaviour indicates that the OFDA instrument is in some way sensitive to other characteristics of the fibre such as fibre curvature. This problem has yet to be resolved. |
In general, for ultrafine wools, OFDA produces a finer average result than LASERSCAN, which is again finer than Airflow. For other wools, there is generally good agreement for results using LASERSCAN, OFDA and Airflow, although on average, for Australian wools, LASERSCAN is 0.1 microns coarser than Airflow. Larger differences can occur with medullated wools, or wools with unusually high or low CVD’s.
The major concern of wool producers and wool processors must be to get repeatable results, year in and year out, and results, which can be reliably correlated with processing performance
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FURTHER INFORMATION
Ian Ashman
General Manager Customer Relations
AWTA Ltd
Ph: 03 9371 4103
Fax: 03 9371 4191
Email: ian.ashman@awta.com.au