Achieving Accuracy

June 01, 2010

Design features and calibration tips for accurate dissolved solids measurements

There are many portable handheld devices that measure dissolved solids, but not all instruments are equal. Beyond the product specifications, how can you tell which instrument to choose? Even when you choose the best instrument, how can you get accurate repeatable readings? This article addresses design and use issues that affect the accuracy of conductivity, resistivity and total dissolved solids (TDS) measurements so that you choose the right instrument for your application and use it correctly.

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 and TDS conversion. So it is important to know about your instrument’s cell construction and how the temperature compensation 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 indication 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 constituent (pure water is a very poor conductor of electricity). So the current measured is a representation of the types and concentrations of ionized particles in solution. In water quality 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.

Most instruments measure conductivity by applying a potential across two electrodes and measuring the current. The cell must be designed to minimize such factors as polarization caused by the accumulation of ions near the electrodes. Some instruments attempt to resolve polarization issues by coating electrodes with platinum black to increase surface area, thereby increasing current density. But platinum black can be scratched. Don’t use a platinized cell if you will be testing viscous samples and suspensions that can remove the coating. Also, the cell constant drifts faster, requiring frequent calibration.

A high-quality, lower maintenance 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 voltage between the two electrodes. A less accurate reading is made because the resistivity of the solution and the resistivity 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. Nonlinearity between the current applied and the conductance of the sample can occur due to polarization effects or electrode fouling. High-end instruments will take this into account in circuitry design, employing some compensation method, such as a feedback mechanism. Ask the manufacturer to explain the design features that contribute to the accuracy of the conductivity measurement specifically.

Most conductivity instruments include a temperature compensation 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 comparison when solution temperature fluctuates. Temperature affects the conductivity of particular chemical constituents in solution in a nonlinear fashion. This is compounded by the fact that each chemical constituent responds to temperature changes in varying degrees, affecting the total conductivity of the solution disproportionately.

To account for solution type characteristics, choose an instrument standardized to a solution type that closely matches the solution 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 freshwater should choose 442, a proprietary formula originally developed by the Myron L Co. to model freshwater. If you are going to use the same instrument in diverse applications, pick one that gives you the flexibility to choose from multiple solution types. Using an instrument standardized to the wrong solution type results in increased error or reduced accuracy because the incorrect temperature compensation algorithm is applied.

The accuracy of the temperature-compensated readings is also dependent on how well the instrument’s temperature compensation algorithms model the behavior of the correct solution at varying temperatures. Lower quality instruments use a generic temperature compensation slope; for example, 2% per degree Celsius for naturally occurring water, assuming 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.

Opt for 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 performing 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 concentration that falls somewhere in the upper two-thirds of the test solution range.

Resistivity

If the water has a very low current, it also has a high resistance. In this case, choose an instrument that measures resistivity 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 conductivity will convert that reading to resistivityfor you. In this case, the accuracy of the resistivity measurement is dependent on the accuracy of the conductivity measurement and the temperature compensation. Choose an instrument standardized to a solution that most closely matches your application and calibrate to a standard in the upper two-thirds of the range in which measurements will be taken.

TDS

Conductivity instruments cannot measure particles dissolved in solution that do not conduct electricity. Conductivity is not a good way to determine the concentration of dissolved organic compounds, for example. But in most cases, where there is high inorganic salt concentration, conductivity can be used to determine dissolved solids with a high level of accuracy.

A TDS instrument uses temperature compensated conductivity measurements, taking into account solution type to determine the actual amount of dissolved solids in solution. Proper solution modeling is critical to accurate TDS readings, as well as the accuracy of the original conductivity reading. Again, this is because each kind of chemical constituent conducts electricity to varying degrees and will affect the total conductivity disproportionately with variations in temperature. Select and calibrate a TDS meter by the solution standard that most closely matches the application in which the TDS measurement will be made. A meter that has a conductivity-to-TDS conversion algorithm that accounts for critical points in changes of the behavior of the solution based on actual testing is more accurate. Ask the manufacturer about the conversion algorithms if you are unsure. A TDS meter that allows you to specify a custom solution TDS conversion based on your own testing will yield the greatest accuracy in situations where the characteristics of the sample are known. Some manufacturers will make calibration solutions to order. Calibrate with a solution in the upper two-thirds of the range of concentrations that will be measured.

Routine Maintenance

Choose an instrument that can stand up to the conditions to which it will be subjected. Familiarize yourself with the operation and maintenance of your instrument. Clean your conductivity cell and calibrate conductivity, resistivity and TDS parameters to the proper standard solutions per your operation manual. Inspect the instrument for wear and tear and store it properly to prevent unnecessary damage.

Finally, an instrument is only as good as its user. The dissolved solids meter is designed to tell you how much of a known substance is in the water, not what is in the water. You must know what chemical constituents exist in what concentrations in the solution to be tested and choose an instrument standardized to that solution type with accuracy specifications that match the range of possible measurements. This means that, ultimately, the accuracy of the instrument depends on you.

Heather Petrig Rekalske is technical writer for Myron L Co. Rekalske can be reached at 760.438.2021 x1223 or by e-mail at hrekalske@myronl.com.

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