Malvern Mastersizer 2000, User Manual

Page: 109 / 154

Advanced features Chapter 9
Mastersizer 2000 Page 9-3
The lower limit is used mainly as a means of triggering measurements, particu-
larly dry powder measurements. The practical upper limit for wet dispersions
varies depending on the particle size of the sample.
3.
Click on the Raise alarm if the final obscuration is out of range check box
to display an alarm if the measurement’s final obscuration is out of range.
Guidelines
Use a maximum obscuration of about 5% for dry measurements (or even lower for
fine dry material).
For other samples, typically add enough sample to give the following obscurations:
„
10% obscuration – for very fine material (up to 3 or 4 microns). Because the
material is fine, there will be little problem in obtaining good sample represen-
tation but there is some risk of multiple scattering.
„
15% obscuration – if the material has a median size of 50 microns or greater.
„
20% obscuration – if the material has a broad distribution or is coarse. Here
achieving good sample representation is the most important consideration.
The required obscuration can be determined experimentally by measuring the par-
ticle size at different obscurations. At low obscurations the result reproducibility
may be poor due to poor sampling or signal-to-noise issues. At very high obscura-
tions the calculated result may shift to smaller particle sizes due to the effects of
multiple scattering. Obtain the ideal obscuration by finding the range where the
result reproducibility is good and the results are independent of the obscuration
chosen.
In each case, the amount of sample which was required to achieve the recom-
mended levels of obscuration should be noted and specified in the SOP.
Autostart
Sometimes it is necessary to wait for the sample flow to stabilise before making a
measurement, particularly in dry measurements, to avoid errors due to incomplete
dispersion. Obscuration limits can be set up manually or using default values for
the chosen sampler. Once the sample obscuration has stabilised within these limits
for the specified time, the measurement is started.
MAN0384-1.0 Mastersizer 2000.book Page 3 Tuesday, March 20, 2007 8:56 AM