Hach MP-4, User Manual

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Appendix C Temperature compensation
and TDS derivation
The MP series handheld meters contain internal algorithms for characteristics of the
three most commonly referenced compounds. The selected solution type is shown
on the left of the display. In addition to KCl, NaCl and 442, a User choice is
available. The User mode allows the user to enter the temperature compensation
and the TDS conversion ratio of a unique solution.
C.1 Conductivity characteristics
When taking conductivity measurements, the Solution Selection determines the
characteristic assumed as the instrument reports what a measured conductivity
would be if it were at 25 °C. The characteristic is represented by the temperature
compensation, expressed in %/°C.
If a solution of 100 µS at 25 °C increases to 122 µS at 35 °C, then a 22% increase
has occurred over this change of 10 °C. The solution is then said to have a
temperature compensation of 2.2%/°C.
Temperature compensation always varies among solutions because it is dependent
on their individual ionization activity, temperature and concentration. This is why the
MP meters feature mathematically generated models for known salt characteristics
that also vary with concentration and temperature.
C.2 Temperature compensation of unknown solutions
The user may need to find the corrected conductivity of a solution that differs from
the three standard salts. In order to enter a custom fixed temperature compensation
for a limited measurement range, enter a specific value through the User function.
The temperature compensation can be determined by two different methods.
C.2.1 Find temperature compensation by calculation
1.
Heat or cool a sample of the solution to 25 ºC, and measure its conductivity.
2.
Heat or cool the solution to a typical temperature where it is normally
measured.
3.
Select the
User
function.
4.
Set the temperature compensation to 0%/ºC (see section 3.12.1 on page 20 ).
5.
Measure the new conductivity and the new temperature.
6.
Divide the percentage decrease or percentage increase by the 25 ºC value.
7.
Divide that result by the temperature difference.
C.2.2 Find temperature compensation by adjustment
1.
Heat or cool a sample of the solution to 25 ºC, and measure its conductivity.
2.
Heat or cool the solution to a typical temperature where it is normally
measured.