KnowYourH2O’s page on drinking water testing for conductivity and total dissolved solids (TDS) explains these two related screening measurements, how they connect, how to interpret them, and why lab confirmation matters.

What they measure

Conductivity is the ability of water to carry an electrical current. That ability comes from dissolved ions (atoms or molecules that have gained or lost electrons), such as sodium (Na+) and chloride (Cl−) from table salt. The measurement does not identify which ions are present or their individual concentrations—only the overall ionic content. Temperature, ion type, mobility, valence, and how fully salts dissociate all affect the reading. Resistance is the inverse of conductivity; convenient units are micromhos per centimeter (µmhos/cm) or microsiemens per centimeter (µS/cm).

TDS is the concentration of dissolved minerals, salts, metals, and other dissolved substances, usually expressed in mg/L. It is commonly estimated from conductivity rather than measured directly every time.

How they relate

Dissolved ions raise both conductivity and TDS, so TDS is often calculated as:

TDS (mg/L) ≈ conductivity (µmhos/cm) × conversion factor

The factor is not universal. The site gives these approximate ratios from lab work and literature and presents data supporting these factors.

• NaCl solutions: ~0.59
• KCl solutions: ~0.61 (certified solutions sometimes ~0.70)
• CaCl2: ~0.76
• CaSO4: ~0.89
• CaCO3: lower and less reliable because of solubility limits (~0.34 in their data)
• Natural/fresh water: commonly 0.55–0.70 (some use 0.65); up to ~0.80 when calcium and sulfate dominate
• Seawater: ~0.75

Practical meaning and limitations

High mineralization can produce mineral taste, contribute to corrosion or scale (encrustation) in pipes and equipment, and restrict some agricultural uses (livestock watering and irrigation). Indirect ecological effects mentioned include loss of certain plants. Conductivity/TDS screening is useful for spotting high ionic content quickly, but it cannot replace ion-specific analysis.

Accurate field measurement requires a conductivity sensor (often with automatic temperature compensation), constant temperature between sensor and sample, and calibration with a known standard such as Potassium chloride (KCl) at the same temperature.

Recommendations on the page

The site advises laboratory testing of both TDS and conductivity on the actual sample so the real ratio and major-ion chemistry can be understood. It lists consumer/professional meters and kits (Hanna HI 9813-6N, Hanna HI98129 combo testers, plus broader drinking-water kits that also cover pH, hardness, etc.) and points readers to contact the site for chemical or biological testing support. No specific numeric drinking-water limits (such as a 500 mg/L TDS aesthetic guideline) are stated on the page itself, and no detailed treatment processes for high TDS are given, but for a typical natural water the concern would be if the conductivity was approaching or over 1000 uS/cm. If the water had a dominant Calcium Chloride or Calcium Sulfate matrix then the concern could be at a conductivity of 500 us/cm.

In short, the page treats conductivity as a fast ionic-strength screen and TDS as an estimated mineralization number that must be interpreted with the correct conversion factor and, preferably, laboratory confirmation.

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Other Websites

Know Your H20
B.F. Environmental Consultants
Carbon County Groundwater Guardians
Keystone Clean Water Team (Donate)

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