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Conductivity/Resistivity Analyzer/Controller
9782 Series Conductivity/Resistivity Analyzer/Controller - Operator’s Manual 7/998-8
8.6 Determining and Entering the TDS Conversion Factor
Introduction
The 9782 always measures conductivity. However, the process value can be displayed in terms
of total dissolved solids (TDS). If a TDS solution temperature compensation type was specified
during I/O setup, then the calibration menu will contain items used to enter the TDS conversion
factor for each cell.
Determining TDS conversion factor
To determine the TDS conversion factor, it is first necessary to establish the total dissolved
solids in a representative sample of the process. The formal determination of TDS is a laboratory
standard method performed on a weighed grab sample of the process fluid. To summarize how a
obtain a TDS value:
• Suspended solids, if present, are filtered out.
• All water is evaporated.
• The residue is dried and weighed.
• The result is divided by the original sample weight to obtain ppm TDS.
For detailed guidance in determining the official TDS, see “Standard Methods for the
Examination of Water and Wastewater,” jointly published by the American Public Health
Association, American Water Works Association and Water Pollution Control Federation,
Washington, DC.
To determine the conversion factor needed by the 9782 Analyzer/Controller, first use the
laboratory procedure summarized above to give an official TDS value. Next divide the TDS
value by the conductivity of the sample to yield the conversion factor for that particular process
fluid. The conversion factor is then entered into the 9782 to normalize the TDS readout.
With power plant cation conductivity measurements, ion chromatography results may be used to
establish the conversion factor for readout in ppb chloride or sulfate ion. Nominal values are 83
ppb per µS/cm for chloride ion and 111 ppb per µS/cm for sulfate ion. The 9782 does not
provide temperature compensation in TDS for chloride or sulfate ions.
Out-of range-values forced to closest limit
As long as the entered TDS value is within the acceptable limits for a given cell constant, the
value is accepted by the Analyzer/Controller. If a value is outside the accepted range, the unit
will not display an error message; instead it will force the value to either the high or low limit of
the range of the cell constant. See Table 5-10 for TDS conversion factor defaults.
Calibrate the Analyzer/Controller before entering TDS conversion factor
If you intend to enter a cal factor or use calibration trim, do so before entering the TDS
conversion factor as described here.
If you use calibration trim, first set the solution temperature compensation in I/O setup to the
non-TDS choice for your process. For example, if you plan to use “NaCl (TDS)” set the solution
temperature compensation type to “NaCl” temporarily for calibration purposes. (Solution
temperature compensation type is the one I/O setup parameter that can be changed without
triggering a cold reset.)