Myron L 6Pfc and 4P - Manual (Page 44)

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A. How it’s Done
Once the effect of temperature is removed, the compensated conductivity
is a function of the concentration (TDS). Temperature compensation of
the conductivity of a solution is performed automatically by the internal
processor with data derived from chemical tables. Any dissolved salt at
a known temperature has a known ratio of conductivity to concentration.
Tables of conversion ratios referenced to 25°C have been published by
chemists for decades.
B. Solution Characteristics
Real world applications have to measure a wide range of materials and
mixtures of electrolyte solutions. To address this problem, industrial
users commonly use the characteristics of a standard material as a
model for their solution, such as KCl, which is favored by chemists for
its stability.
Users dealing with sea water, etc., use NaCl as the model for their
concentration calculations. Users dealing with freshwater work with
mixtures including sulfates, carbonates and chlorides, the three
predominant components (anions) in freshwater that the Myron L
Company calls “natural water”. These are modeled in a mixture called
“442™” which the Myron L Company markets for use as a calibration
standard, as it does standard KCl and NaCl solutions.
The Ultrameter II contains algorithms for these 3 most commonly
referenced compounds. The solution type in use is displayed on the
left. Besides KCl, NaCl, and 442, there is the User choice. The benefit
of the User solution type is that one may enter the temperature
compensation and TDS ratio by hand, greatly increasing accuracy of
readings for a specific solution. That value remains a constant for all
measurements and should be reset for different dilutions or temperatures.
C. When does it make a lot of difference?
First, the accuracy of temperature compensation to 25°C determines the
accuracy of any TDS conversion. Assume we have industrial process
water to be pretreated by RO. assume it is 45°C and reads 1500 µS
uncompensated.
1.
If NaCl compensation is used, an instrument would report 1035
µS compensated, which corresponds to 510 ppm NaCl.
2. If 442 compensation is used, an instrument would report 1024
µS compensated, which corresponds to 713 ppm 442.
The difference in values is 40%.
In spite of such large error, some users will continue to take data in
the NaCl mode because their previous data gathering and process
monitoring was done with an older NaCl referenced device.
Selecting the correct Solution Type on the Ultrameter II will allow the
user to attain true TDS readings that correspond to evaporated weight.