Myron-l TH1 User Manual download pdf (Page 32)

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.

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Myron-l TH1 User Manual Page 32