Question 1

What is TDS and how is it measured?

Accepted Answer

TDS (Total Dissolved Solids) is the total concentration of all dissolved inorganic and organic substances in water - minerals, salts, metals, and organic matter. It is typically measured with a conductivity meter (EC meter), which measures how well water conducts electricity. The conductivity reading is then multiplied by an empirical factor (typically 0.5–0.7) to estimate TDS in mg/L.

Question 2

Are ppm and mg/L the same?

Accepted Answer

For dilute aqueous solutions, ppm by mass (mg/kg) ≈ ppm by volume (mg/L) because water density ≈ 1 kg/L at 25°C. For pure water or dilute solutions under ~10,000 mg/L, the difference is negligible. For seawater (35,000 mg/L), density ≈ 1.025 kg/L, so there is a ~2.5% discrepancy. This tool treats ppm = mg/L as a common practical approximation.

Question 3

What is PSU and how does it differ from ppt?

Accepted Answer

PSU (Practical Salinity Units) is defined by the PSS-78 standard as a dimensionless ratio based on electrical conductivity, calibrated to match the mass fraction scale. Numerically, PSU ≈ ppt (g/kg) for seawater in the range 2–42. The key difference: ppt is a mass fraction (g per kg of seawater), while PSU is technically unitless. In practice, 35 PSU ≈ 35 ppt ≈ 35,000 mg/L for open ocean water.

Question 4

Why is the conductivity conversion approximate?

Accepted Answer

There is no universal exact formula from conductivity to TDS because the relationship depends on the specific ions in the water. The TDS factor (commonly 0.5–0.7) varies: freshwater streams ~0.55, groundwater ~0.67, seawater ~0.53. This tool uses 0.64 (a common middle estimate). Your actual TDS meter uses a fixed factor too, so always check which factor your meter is calibrated to.

Question 5

What is the salinity of seawater?

Accepted Answer

Average open ocean salinity is approximately 35 PSU (35 ppt = 35 g/kg = ~35,000 mg/L TDS). The Mediterranean is slightly higher (~38 ppt) and the Baltic Sea much lower (~7 ppt). The Dead Sea reaches ~300 ppt - nearly 10× seawater - making it one of the saltiest natural bodies of water on Earth.

Question 6

What TDS level is safe for drinking water?

Accepted Answer

WHO guidelines suggest drinking water TDS should be below 600 mg/L for palatability, with a maximum recommended level of 1000 mg/L. Most tap water is between 50–400 mg/L. Reverse osmosis water is typically 10–50 mg/L. Water above 1000 mg/L may taste salty or bitter and can affect plumbing in high-hardness areas.

Salinity / TDS Converter

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