Conductivity vs TDS in Water: What Is the Difference?
Quick answer: are conductivity and TDS the same?
No. Conductivity measures how well water conducts electrical current, while TDS represents the concentration of dissolved material. Many portable TDS meters estimate TDS from conductivity by multiplying it by a conversion factor.
Conductivity
Electrical conductivity reflects the concentration and mobility of dissolved ions. It is commonly reported in microsiemens per centimetre (µS/cm) and is strongly affected by temperature, so temperature compensation matters.
Total dissolved solids
TDS is usually expressed in milligrams per litre (mg/L). A laboratory gravimetric TDS measurement is different from the estimated value displayed by many handheld meters.
Why there is no universal conductivity-to-TDS conversion
The relationship depends on the ionic composition of the water. Different salts conduct electricity differently, so a single conversion factor cannot be accurate for every source. The U.S. Geological Survey has reported that conversion slopes for many natural waters commonly fall within a broad range, rather than one fixed value.
Which measurement is better for an RO plant?
Conductivity is commonly preferred for operational trending because feed and permeate conductivity can be measured directly and consistently. It is useful for monitoring salt rejection and detecting changes, provided instruments are calibrated and temperature effects are handled correctly.
Frequently asked questions
Why do two TDS meters show different values on the same water?
They may use different conductivity-to-TDS conversion factors, calibration or temperature compensation.
Can conductivity identify which salts are present?
No. Laboratory analysis is needed to identify individual ions and contaminants.
Does low conductivity prove water is safe?
No. Microbiological contamination and some chemicals cannot be ruled out from conductivity alone.
Reference
USGS publications describe the relationship between specific conductance and dissolved solids as water-specific rather than universal. See the USGS dissolved-solids and specific-conductance discussion.
See also ABHIRO’s TDS Guide and Water Test Report Guide.
