Hach MP-4, User Manual

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Appendix B Conductivity conversion
B.1 How conductivity conversion works
When the effect of temperature is removed, the corrected conductivity depends on
the concentration (TDS). Temperature compensation of the conductivity of a
solution is performed automatically by the meter’s 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 ºC have been published by chemists for decades.
B.2 Solution characteristics
Real-world applications have to measure a wide range of materials and mixtures of
electrolyte solutions. To address this problem, industry applications tend to 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 who deal with sea water, etc., use NaCl as the model for their concentration
calculations. Users who deal with freshwater work with mixtures including sulfates,
carbonates and chlorides. These are modeled in the 442 standard solutions.
The meter contains algorithms for these three most commonly referenced
compounds. The solution type in use is shown on the left side 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 TDS ratio by hand. This
increases the accuracy of readings for a specific solution. That value remains a
constant for all measurements, and should be reset for different dilutions or
temperatures.