Myron L 9PTK - Manual (Page 65)

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A. 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 tempco, 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
tempco of 2.2 %/°C.
Tempco always varies among solutions because it is dependent on their
individual ionization activity, temperature and concentration. This is why
the Ultrameter III features mathematically generated models for known
salt characteristics that also vary with concentration and temperature.
B. Finding the Tempco of an Unknown Solution
One may need to measure compensated conductivity of some solution
unlike any of the 3 standard salts. In order to enter a custom fixed tempco
for a limited measurement range, enter a specific value through the User
function. The tempco can be determined by 2 different methods:
1. Heat or cool a sample of the solution to 25°C, and measure its
conductivity. Heat or cool the solution to a typical temperature
where it is normally measured. After selecting User function,
set the tempco to 0 %/°C as in Disabling Temperature
Compensation, pg. 24 (No compensation). Measure the new
conductivity and the new temperature. Divide the % decrease
or increase by the 25°C value. Divide that difference by the
temperature difference.
2. Heat or cool a sample of the solution to 25°C, and measure
its conductivity. Change the temperature to a typical measuring
temperature. Set the tempco to an expected value as in User
Programmable Temperature Compensation, pg. 24. See if the
compensated value is the same as the 25°C value. If not, raise
or lower the tempco and measure again until the 25°C value is
read.
C. Finding the TDS Ratio of an Unknown Solution
Once the effect of temperature is removed, the compensated
conductivity is a function of the concentration (TDS). There is a ratio
of TDS to compensated conductivity for any solution, which varies
with concentration. The ratio is set during calibration in User mode
as in User Programmable Conductivity to TDS Ratio, pg. 25. A truly
unknown solution has to have its TDS determined by evaporation
and weighing. Then the solution whose TDS is now known can be
measured for conductivity and the ratio calculated. Next time the same
solution is to be measured, the ratio is known.