Why can different instruments produce different results when measuring the same wool?

There are three major factors.

Firstly, differences will arise because of measurement variation. However, this is also the case when the same samples are re-measured using the same instruments. IWTO Specifications for these technologies are specifically designed to minimise such variation while simultaneously minimising the cost of the measurements. The simplest and most effective way to reduce such measurement variation is to increase the number of individual measurements conducted, since the major proportion of the variation can be attributed to sampling and sample preparation.

Secondly, LASERSCAN, OFDA and Airflow are indirect, calibrated systems, which do not measure exactly the same geometric characteristics of wool fibres. Consequently, they "define" fibre diameter differently. IWTO Specifications attempt to minimise differences arising from this source by requiring each instrument to be calibrated using standard wool tops, which have been standardised by repeated measurements in a large number of laboratories, using a direct measurement system, the Projection Microscope. However, when the sample being measured has different characteristics than the tops used to calibrate the instruments, and the instruments are affected differently by these specific characteristics, then differences will result.

Thirdly, in cases where the fibre characteristics are distinctly different from those of the calibration tops, systematic biases can occur. The best-known example of this is the effect of wool density on the Airflow instrument. This is particularly severe for medullated (or 'hollow") wool fibres. In such cases the Airflow will invariably measure considerably finer than either LASERSCAN or OFDA.

View a fact sheet on this topic.