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Refractometry
(continued)
What is refractometry?
(continued)
Examples
(continued)
Therefore one can neither say straight lines nor parabola are adequate for a device that should
reach a precision of 0.01 Bx!
With a third-order function the maximum error at the seven support points would be only 0.01 Bx.
But be careful ! The result of a measuring of a 65 - Bx-sample would be 65.04 Bx with a third or-
der function. It is a general problem of all polynomal fits that they are unsteady beyond the support
points. If you only have values from 0 to 60 Bx, it means, that the scale is only secured up to 60 Bx.
Let us take a look to the temperature correction. If you are using a manual Abbe-refractometer
with a defined brix scale, you also got a table with temperature corrections that came with it. Ac-
cording to this table the measured value is changed per degree Centigrade about 0.06 Bx at water
and about 0.08 Bx at 60 Bx. Since the parabola does provide good results, a linear correction of
0.07 Bx / °C is adequate. But please consider that this linear correction of 0.07 Bx / °C is not good
enough for a refractometer that should guarantee a precision of 0.04 Bx!
Therewith c
would be 0.07 and all the other coefficients zero in this simplified case.
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This example shows that the calculation of a special scale is not that simple. Many users probably
have a calculator that is able to determine a best-fit curve. But in the example shown above you
saw that this was not sufficient, because you can achieve an accuracy range of 2 Bx.
Starting point of all calculations is respectively a table with values that the refractometer should
show (set values) and the corresponding refractive indexes (or also brix values). Such a table can
also be computed with a formula you find in technical literature. For a number of particular cases we
provide you already calculated coefficients.
-continued-
1400700-101 Rev. A
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