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14. Addendum 3: Calculating TDS Conversion Factors
You can calibrate your meter using TDS calibration standard solutions. The calibra-
tion standard only needs to give the TDS value at a standard temperature such as
25°C. To determine to the conductivity-to-TDS conversion factor use the following
formula:
Factor = Actual TDS ÷ Actual Conductivity @ 25°C
Definitions:
Actual TDS:
Value from the solution bottle label or as a standard you make using
high purity water and precisely weighed salts.
Actual Conductivity:
Value measured using a properly calibrated Conductivity/
Temperature meter.
Both the Actual TDS and the Actual Conductivity values must be in the same
magnitude of units. For example, if the TDS value is in ppm the conductivity value
must be in µS; if the TDS value is in ppt the conductivity value must be in mS.
Check your factor by multiplying the conductivity reading by the factor in the above
formula. The result should be the TDS value.