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Refractometry
What is refractometry?
Calculating of a special scale
Because of the fact, that the refractive index changes with the temperature, the result of the first step also
changes with a varying temperature. This is symbolized by '(t)'. The two formulas in relation:
r = Input value - 1.33
r = Input value
"Type 3" : special computation formula, details subject to Reichert
Scale (t) := c
Some of the coefficients c
In the second step the temperature dependency is considered. This is necessary for most of the special
scales as long as a stable temperature isn't guaranteed during a measurement. The formula uses the re-
sult of the first step as well as the temperature, which is called <input b> here. The reference temperature,
the temperature that does need any corrections, is used also.
The formulas:
This formula with its twelve coefficients seems very complicated first, but as in step one they are not all
quite necessary.
Examples
Let us start with a very simple example. If you want to dilute samples 1:1 before you determine the brix
value with the refractometer, you have to multiply the value with two – or you create the special scale
BX_2, which is doing that automatically. Take a look at the formula for the first step. There is a value called
"Input[1]", which is normally the refractive index. But in this case it is more useful to take the brix value as
the input (value) ! Since must be subtracted from the input value (which is very annoying in this case) the
conversion is:
Remember: "r" is the auxiliary quantity, so input value minus 1.33. In order that the coefficients are:
This was pretty easy. Take care, that the brix value considers the temperature and you do not have to apply
a temperature correction.
Let us observe an example that is a little bit more exciting: the brix scale itself. For instance, you have this
table with values at 20°C:
1400700-101 Rev. A
(continued)
(continued)
(continued)
valid for computation type "Type 1"
valid for computation type "Type 2"
+ c
· r + c
· r
1
2
3
2
to c
could be zero, so don't be irritated by the complexity of the formula.
1
8
dT := r - reference temperature
Scale := Scale (t) + c
+ (c
· dT + c
· dT
12
23
+ (c
· dT + c
· dT
32
33
+ (c
· dT + c
· dT
42
43
Bx_2 = 2.66 + 2.0 · r
+ c
· r
+ c
·r
+ c
· r
4
3
5
4
6
5
· dT + c
· dT
+ c
2
12
13
+ c
· dT
) · Scale (t)
2
3
24
+ c
· dT
) · Scale (t)
2
3
34
+ c
· dT
) · Scale (t)
2
3
44
c
= 2,66
1
c
= 2,0
2
c
= 0,0
. .
3
.
c
= 0,00
8
+ c
· r
+ c
· c
7
7
8
7
· dT
3
14
2
3
13
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