Myron L TH1 - Manual (Page 32)

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XVIII. CONDUCTIVITy CONVERSION to TOTAL
DISSOLVED SOLIDS (TDS) (TPH1 & TP1)
Electrical conductivity indicates solution concentration and ionization
of the dissolved material. Since temperature greatly affects ionization,
conductivity measurements are temperature dependent and are
normally corrected to read what they would be at 25°C (ref. Temperature
Compensation, pg. 25).
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 KCl and NaCl standard solutions.
The TechPro II contains algorithms for these 3 most commonly referenced
compounds. The solution type in use is displayed on the left.
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.