Water Online

June 2012

Water Online the Magazine gives Water & Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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Tutorial and the conductance of the sample will offset polarization effects and electrode fouling. Most conductivity instruments include a temperature com- pensation feature to account for the profound effects of temperature on the activity of ions. Compensation is made to some standard, generally 25ºC, to create a basis for com- parison when solution temperature fluctuates. Temperature affects the conductivity of particular chemical constituents in solution in a nonlinear fashion. This is com- pounded by the fact that each chem- ical constituent responds to tem- perature changes in varying degrees, affecting the total conductivity of the solution disproportionately. To account for solution type characteristics, choose an instru- ment standardized to a solution type that closely matches the solu- tion you are trying to measure. If measurements of the solution have historically been made using another standard, KCl, for example, choose an instrument calibrated to that standard to determine relative changes in concentration. Users dealing with seawater, however, achieve the best results using NaCl. Users dealing with fresh- water should consider choosing 442™. (442 is a proprietary for- mula originally developed by the Myron L Company to model fresh water. It contains the three predom- inant components of natural water: sulfates, carbonates and chlorides. 442 is formulated with these anions in naturally occurring proportions.) If you are going to use the same instrument in diverse applications, choose one that gives you the flexibility to choose from multiple solution types. Using an instru- ment standardized to the wrong solution type results in increased error or reduced accuracy because the incorrect temperature compen- sation algorithm is applied (see Figure 1 on the prior page). The accuracy of temperature-com- pensated readings is also depen- dent on how well the instrument's temperature conversion algorithms model the behavior of the solution at varying temperatures. Lower quality instruments use a generic temperature compensation slope, for example 2%/ºC for naturally occurring water, that assumes changes in conductivity are directly proportional to changes in temperature. This is actually true only for a very narrow range of temperatures. Choose an instrument that uses corrections that change with concentration and temperature instead of single average values (see Figure 2 on the next page). wateronline.com ■ Water Online The Magazine, Cleanwater Edition 29

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