Ametek 1400700, Руководство пользователя

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13
1400700-101 Rev. A
Refractometry
(continued)
What is refractometry?
(continued)
Calculating of a special scale
(continued)
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 valid for computation type “
Type 1
“
r = Input value valid for computation type “ Type 2 “
“ Type 3“ : special computation formula, details subject to Reichert
Scale (t) := c
1
+ c
2
· r + c
3
· r
2
+ c
4
· r
3
+ c
5
· r
4
+ c
6
· r
5
+ c
7
· r
7
+ c
8
· c
7
Some of the coefficients c
1
to c
8
could be zero, so don’t be irritated by the complexity of the formula.
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:
dT := r - reference temperature
Scale := Scale (t) + c
12
· dT + c
13
· dT
2
+ c
14
· dT
3
+ ( c
12
· dT + c
23
· dT
2
+ c
24
· dT
3
) · Scale (t)
+ (
c
32
·
dT + c
33
·
dT
2
+
c
34
·
dT
3
) ·
Scale (t)
2
+ ( c
42
· dT + c
43
· dT
2
+ c
44
· dT
3
) · Scale (t)
3
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:
Bx_2 = 2.66 + 2.0 · r
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:
c
1
= 2,66
c
2
= 2,0
c
3
= 0,0
...
c
8
= 0,00