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Choose an instrument that allows you to enter a user- defined temperature compensation based on actual testing if you are working with a known solution. This requires knowing the precise proportioning of chemical constituents in solution and bench testing for variations with temperature. Though this may be the most accurate method if you have a unique solution type, it is not always the most practical. For calibration, choose a solution standard that matches the instrument's selected standard and has a concentra- tion that falls somewhere in the upper two-thirds of the test solution range.
Figure 2: Example temperature compensation
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The KCl values used for temperature compensation for this instrument vary correctly with temperature, in a nonlinear fashion.
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If the water has a very low current, it also has a high resis- tance. In this case, choose an instrument that measures resis- tivity to get the best resolution and meaningful data. You can determine the resistivity yourself by taking the inverse of the conductivity, but many instruments that measure conductiv- ity will convert that reading to resistivity for you. In this case, the accuracy of the resistivity measurement is dependent on the accuracy of the conductivity measurement and the tem- perature compensation. Choose an instrument standardized to a solution that most closely matches your application and calibrate to a standard in the upper 2/3 of the range in which measurements will be taken.
Total Dissolved Solids 714-630-5040 www.filtronics.com 30 Water Online The Magazine, Cleanwater Edition ■ wateronline.com
Conductivity instruments cannot measure particles dissolved in solution that do not conduct electricity. Conductivity is not a good way to determine the concen- tration of dissolved organic compounds, for example. But in most cases, where there is high inorganic salt concen- tration, conductivity can be used to determine dissolved