Conductivity from Total Dissolved Solids
The inverse of the TDS estimate: EC (uS/cm at 25 C) = TDS (mg/L) / k, k commonly 0.55-0.75 (default 0.65). 650 mg/L -> 1000 uS/cm, with an 867-1182 band from the k range so it is not read as exact. Predicts a conductivity-meter reading from a target/permit TDS or sets an EC alarm setpoint. An estimate, not a measurement; calibrate k to a lab result. The operator of record governs.
Formula and source
conductivity_us_cm = tds_mgl / k_factor; k commonly 0.55-0.75 (default 0.65); band EC = TDS/0.75 to TDS/0.55.
Electrical conductivity from total dissolved solids (Standard Methods 2510), the inverse of the TDS estimate, by name.
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