their solution, like the KCl 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 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.
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 R.O. Assume it is 45°C and reads 1500 µS
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.
Those who want true TDS readings that will correspond to evaporated
weight will select the correct solution type.
The AR1 contains circuitry for the 3 most commonly referenced
compounds — KCl, NaCl and 442. In the LCD display, the solution type
being used is shown on the left.
XIV. TEMPERATURE COMPENSATION (Tempco)
and TDS DERIVATION
When making 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 happened
over this change of 10°C. The solution is said to have a tempco of 2.2
Another solution would have a different tempco because of its ionization
activity. And, that tempco may be a little different at a different
concentration or temperature. This is why the AR1 uses mathematically
generated models for known salt characteristics that vary with
concentration and temperature.
The AR1 contains circuitry for characteristics of the 3 most commonly
referenced compounds — KCl, NaCl and 442. In the display, the solution
type being used is shown on the left.