Practical Considerations for Conductivity and Total Dissolved Solids Measurement
When using a meter to measure either the ppm of total dissolved solids or conductivity of a liquid, it is necessary to periodically calibrate the meter using a calibration standard solution. There are, however, special considerations to be given to each type of calibration. Whereas conductivity calibrations are transferrable from one type of solution to another, ppm total dissolved solids calibrations are very specific to one type of dissolved solids calibrations and these calibrations must not be transferred from one type of dissolved solids solution to the next. Doing this will result in some serious errors in measurement.
Although the basis for testing ppm of total dissolved solids is the conductivity of the solution, it is not correct to assume that these measurements have the same transferability to different types of solutions.
It is always necessary to calibrate all total dissolved solids meters with a parts per million total dissolved solids meters with a parts per million total dissolved solids standard calibration solution that contains the same types of salts or mixtures of salts as the solution to be tested. Failure to do this will result in serious errors in the measurement of total dissolved solids. This is because total dissolved solids meters are calibrated by correlating the conductivity of the solution to the ppm dissolved solids and this correlation varies considerably from one species of dissolved solids to the next.
In the table Figure 1, there are a number of standard curves which correlate the parts per million of total dissolved solids to the conductivity of these solutions. One can note that there is a great deal of variation in the slopes of these curves. According to the Figure 1, if a meter detects a conductivity of 6000 micromhos and is calibrated to read out 1030 parts per million of sodium hydroxide (NaOH) as shown in the curve, the meter would not be able to accurately detect parts per million contents of sodium chloride (NaCl) in the solution. The correct ppm NaCl indication for the detected conductivity of 6000 micromhos would be 3200 ppm, as shown in Figure 1, but the meter would only indicated 1030 ppm, which is clearly unacceptable and so the conclusion is that it is incorrect to use a meter that has been calibrated for ppm NaOH indications for a ppm NaCl solution. A similar conclusion can be made for all types of dissolved solids. Most preformulated parts per million total dissolved solids standard calibrated solutions are formulated with either calcium carbonate (CaCO(3), sodium chloride (NaCl), potassium chloride (KCl) or the 442 (40% sodium sulfate, 40% sodium bicarbonate and 20% sodium chloride) natural water formulation. If your test solution's major dissolved solids components are the same as any of these, you may want to choose the pre-made formulation that best approximates your test solution. Generally, CaCO(3) is used for boiler waters, NaCl is used for brines and the 442 formulation is used for lakes, streams, and wells. Alternatively, if the contents of the ppm standard calibration solution used for calibration are know and there are figures such as Figure 1 or Tables such as Tables 1, 2, and 3 available, it is possible to cross reference from the calibration standard solution's conductivity to ppm total dissolved solids solutions. Other curves and tables are available in various reference books.
The previous discussion and references are based on standard conditions of temperature (25 C). When measuring conductivity or total dissolved solids in other than standard conditions, certain corrections for these variations must be accounted for before going on to determine the final values of conductivity and total dissolved solids. Without some sort of corrections for standard temperature, conductivity or total dissolved solids measurements at various temperatures are meaningless because they cannot be compared. Many meters overcome this by incorporating temperature sensing elements and temperature compensating circuitry into the meter so that the value displayed is corrected for standard temperature. The use of the meter that does not have temperature compensation will require the operator to use lookup table or formulas to correct for the temperature effect or calibrate the meter using a calibration standard that has been brought to the same temperature as the test solution. A good discussion of the effect of temperature on conductivity and total dissolved solids testing can be found on Pages 6 and 7 in the article "e;Theory and Application of Electrolyte Conductivity Measurement"e;, Copyright 1982 by the Foxboro Company.
This discussion should prove useful to all users of conductivity and dissolved solids testing procedures. It is to considered a "e;rule of thumb"e; guideline for using conductivity and dissolved solids testing equipment. Fine tuning of the standard curves and formulas for your specific application is recommended. We hope this discussion helps you to understand the process.