Handheld meters work by measuring the electrical conductivity of the sample and applying a multiplier to estimate the TDS. This multiplier is usually in the range of 0.6-0.7x. While this factor is fairly accurate at low TDS of surface waters, high recovery and high TDS waters will not give accurate TDS values using this method.
Electrical conductivity is related to ionic activity. As waters become increasingly concentrated with salts, activity coefficients will also change in a nonlinear fashion. Ions tend to become less active as the ionic strength increases in the system, which in turn will change the electrical conductivity.
AdvisorCi calculated electrical conductivity using the following formula:
EC= Σ(Λ°m m)
Where m is the molar concentration of a specific ion and Λ°m is the molar conductivity of that ion. The total values are summed and then adjusted for activity and temperature.
TDS is calculated using the complex ion speciation engine and then the total value of ions is summed from there. Since EC and TDS are calculated separately, it is possible depending on the ionic strength and composition, to have TDS higher than EC or not in a ratio (TDS/EC) that is seen with handheld meters.