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 is an absolute measurement with calibrations that are transferrable from
one type of solution to another, ppm total dissolved solids calibrations are specific to one
type of dissolved solids solution and 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 this measurement is absolute. It is always necessary to
calibrate all total dissolved solids meters with a parts per million total dissolved solids
standard calibration solution that contains the same type 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.
One similar conclusion can be made for all types of dissolved solids. Most pre-formulated parts
per million total dissolved solids standard calibrated solutions are formulated with calcium
carbonate (CaCO3), 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, CaCO3
is used for boiler waters, NaCl is used for brines, and the 442 formulation is used for lakes,
streams, wells, and boilers. Alternatively, if the contents of the ppm standard calibration
solution used for calibration are known, it is possible to cross reference from existing
calibration curves to curves for different types of dissolved solids solutions. 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 correction 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 sensing circuitry into the meter so that the value
given is corrected for standard temperature. Using a meter that does not have temperature
compensation will require the operator to use look-up tables or formulas to correct for the
temperature effect. 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 "Theory and Application
of Electrolyte Conductivity Measurement", Copyright 1982 by the Foxboro Company.
This discussion should prove useful to all users of conductivity and dissolved solids testing
procedures. It is to be considered a "rule-of-thumb" 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 better understand the
process.
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