YOKOGAWA SC200 - Page 21

IM 12D7B2-E-H
17
1. Why is temperature compensation
necessary?
The conductivity of a solution is very strong-
ly influenced by temperature. Typically for
every 1°C change in temperature the solu-
tion conductivity will change by approxima-
tely 2%.
The effect of temperature varies from one-
solution to another and is determined by
several factors: solution composition,con-
centration and temperature range. In very
pure water the temperature influence increa-
ses dramatically with increasing purety to
approximately 6%/°C.
A coefficient (a) is used to express the
amount of temperature influence in % chan-
ge in conductivity/°C.
In almost all applications this temperature
influence must be compensated before the
conductivity reading can be interpreted as a
measure of concentration or purity.
2. Automatic temperature compensa-
tion with EXA SC200
From the factory EXA SC200 instruments
are calibrated with a general temperature-
compensation function based on a sodium-
chloride salt solution. This is suitable for
many applications and is compatible with
the compensation functions of typical labo-
ratory or portable instruments.
As conductivity decreases into the range of
pure water the instrument will follow the
non-linear compensation function of neutral
NaCl solutions. In the high conductivity
region the effect of temperature decreases
and the compensation factor is decreased
too. In these extreme regions the tempera-
ture compensation factor is calculated with
a square equation.
A temperature compensation factor is de-
rived from the following equation:
α
= Temperature compensation factor (in
%/°C)
T = Measured temperature (°C)
K
t
= Conductivity at T
T
ref
= Reference temperature (25°C)
K
ref
= Conductivity at T
ref
3. Manual setting of temperature com-
pensation with EXA SC200
If the general compensation function is
found to be inaccurate for the sample in
question, the instrument can be set manual-
ly for a linear factor on site to match the
application.
The procedure is as follows:
1. Take a representative sample of the pro-
cess liquid during operation.
2. Heat or cool this sample to the reference
temperature of the instrument (usually
25°C). Note: see §7-4.
3. Measure the conductivity of the solution
with the SC200 and note the reading of
the display.
4. Bring the sample to the typical process
temperature. (Check the temperature with
the display routine.)
5. Adjust the reading of the display to the
previously noted value at the reference
temperature.
6. Check that the temperature compensa-
tion factor has been changed (from the
display routine).
7. Insert the conductivity cell into the pro-
cess again
4. Other possibilities
Manual temperature coefficient §7-12.
4-4. Temperature compensation
K
t
- K
ref
x 100
α =
X
T - T
ref
x K
ref