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Tutorial Maximizing Your ROI On Test Equipment How Choosing The Right Instrument Affects Performance And Your Bottom Line by Heather Reskalske I n today's economic climate, it's tempting to want to buy the cheapest test equipment you can now. But, how can you tell what the true cost of test equipment is? You have to consider how widely your water and chemical consumption will vary as the readings on your instrumenta- tion does. And, if you're using that instrumentation as part of your system maintenance or equipment calibration, inac- curate readings can crescendo into system failures over time. Even given the same specifications, not all instruments are equal. How can you tell which instrument to choose? Even when you choose the best instrument for your application, how can you get accurate repeatable readings? This arti- cle addresses design and use issues that affect the accuracy of one of the most significant process control parameters: dissolved solids, as measured by conductivity, resistivity, and TDS measurements. The goal is to help you choose the right instrument for your applica- tion, use it correctly, and get an immediate payoff for your investment in quality. Dissolved Solids Instrumentation Design Several design features affect the accuracy and repeatability of a dissolved solids instrument: the conductivity cell and circuit design and the algorithms used for temperature compensation the current measured is a representation of the types and concentrations of ionized particles in solution. In water qual- ity applications, conductivity is reported in units of micro/ millisiemens (µ/mS). Choose an instrument that holds its accuracy in the conductivity range of the solution you plan to test, considering daily and seasonal fluctuations. The accuracy of a conductivity instrument is heavily dependent on the way it measures this current. The accuracy of this conductivity reading, in turn, affects the accuracy of other measurements derived from it, such as resistivity and TDS. Choose an instrument manufactured with high quality materials designed to withstand the conditions to which it will be subjected. Figure 1: Example error from wrong solution selection The graph illustrates the error that would result from selecting KCl for a solution that should be compensated as NaCl or as 442 in the range of 1000 µS. and TDS conversion. So, it's important to know about your instrument's cell construction and how the temperature com- pensation and TDS conversion are made. You also need to be aware of the range of characteristics and concentrations of the solution the instrument will be testing. Conductivity A conductivity measurement can give you a good indica- tion of the concentration of dissolved solids in solution because any chemical constituent that carries a charge, or ion, can conduct an electric current, and how well or poorly it conducts electricity is a property specific to that constitu- ent. (Pure water is a very poor conductor of electricity.) So 28 Water Online The Magazine, Cleanwater Edition ■ wateronline.com Most instruments measure con- ductivity by applying a potential across two electrodes and measur- ing the current. The cell must be designed to minimize such factors as polarization caused by the accu- mulation of ions near the elec- trodes. Some instruments attempt to resolve polarization issues by coat- ing electrodes with platinum black to increase surface area, thereby increasing current density. But, plat- inum black can be scratched. Don't use a platinized cell if you will be testing viscous samples and suspen- sions that can remove the coating. Also, the cell constant drifts faster, requiring frequent calibration. A high quality, lower-mainte- nance option for reducing the influence of polarization resistance is a 4-wire cell. The 4-wire cell measures the current where it is very small — as it passes between the inner two electrodes, which are close together. Less accurate instruments use a 2-wire cell that measures resistivity by passing an electric current through a solution then determining the difference in volt- age between the two electrodes. A less accurate reading is made because the resistivity of the solution AND the resis- tivity of the electrode are measured due to polarization of the electrodes and the field effect. The stability of the circuitry is an important factor in extreme accuracy. A feedback mechanism that compensates for any nonlinear correlation between the current applied