FAQ: "We recently supplied a LabScan XE to a customer for the measurement of various samples including pellets and plaques.
I believe the instrument that this is replacing a competitor.
The customer is worried about the repeatability of the LabScan XE as well as the difference in results between LabScan XE and the competitor.
He is also worried about repeatability on samples.
I have asked him to record 500 readings and he has supplied us with spreadsheets with data."
If there is no enlightenment after 500 readings, I would suggest a simpler approach.
There are two separate issues here - measurement repeatability on translucent pellets and plaques and inter-instrument agreement with the competitor's old instrument.
One more variable that can occur with plastic plaques and pellets is UV optical brightening in the sample that can be affected by the UV content of the LSXE pulsed xenon lamp. If this is the case, create a measurement mode in the LSXE software to put the UV Filter IN the path to exclude the UV.
The first issue to explore is repeatability. The LSXE is a spectrophotometer designed to measure opaque, solid materials in reflectance.
Take the extra white tile that came with the unit. Measure it ten times with replacement (on and off the port). The range in readings will show that the HunterLab LabScan XE is highly repeatable.
Uniform plastic plaques and non-uniform plastic pellets are translucent and trap light. This causes more variation in the measurement. Measure the plastic plaque 10 times with a 90 degree rotation between each reading (this is the same as replacement at the port) and backed with the extra white tile. Look at the spread and you will see that it is larger than the white tile
To ensure a repeatable reading of a plastic plaque, our recommended measurement protocol would be to average 2 readings of the plaque backed by the white tile, with a 90 degree rotation between each reading (with replacement).
Pellets are non-uniform and more variable. Measure the plastic pellets 10 times with a dump and re-fill between each reading (this is the same as replacement at the port) . Look at the spread and you will see that it is larger than the white tile or the plastic plaque.
To ensure a repeatable reading of a plastic plaque, our recommended measurement protocol would be to average 3 -4 readings of the plastic pellets in a sample cup with a dump-and-fill of the pellets between each reading (with replacement).
Once we have a sense of how repeatable the measurement is, we can move on to the issue of agreement with the competitor's old unit.
I have enclosed a spreadsheet if you wish to use it.