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Appendix A. Turbidity Standards
AMCO Clear, supplied by GFS Chemicals (www.gfschemicals.com), is an
approved calibration standard and is the one we use to certify our instruments.
It is made from styrene divinylbenzene (SDVB) microspheres. SDVB spheres
have a median size and standard deviation of 0.28μm (~1/5 that of formazin
particles) and 0.10 μm respectively and a refractive index of 1.56. As shown
on the SEM image, they are dimensionally uniform. SDVB standards are
formulated especially for OBS meters and cannot be used with different
meters. Superior physical consistency of AMCO Clear results in a more
precise calibration standard, giving standard errors less than 1% compared to
2.1% for formazin and better linearity, 0.15 NTU compared to 0.32 for
formazin.
(Photos courtesy of GFS Chemicals)
The key benefits of SDVB standards are: 1) < 1% lot-to-lot variation in
turbidity; 2) consistent optical properties from 10 to 30°C; 3) guaranteed one-
year stability; 4) mixing and dilution are not required; and 5) they are not toxic.
Two drawbacks are that SDVB standards can only be used with the instruments
for which they are made and they are more expensive than formazin. For
example, one liter of 4000-NTU standard costs about twice as much as an
equivalent amount of 4000-NTU formazin. Our instruction manuals explain
how to use turbidity standards and the instructions provided by the suppliers
tell how they should be handled.
A-1
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