By Sean Lancaster
Updated March 24, 2022
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Dissolved solids can scatter light, making solutions appear cloudy. Turbidity meters measure this scattering in nephelometric turbidity units (NTU). To express those readings in parts‑per‑million (ppm), the instrument must be calibrated with a standardized solution, typically Formazin, where 1 NTU equals 1 mg L⁻¹.
Power on the meter and allow 5–10 minutes for the light source to stabilize, ensuring consistent output.
Select a series of certified Formazin standards that bracket your expected NTU range. For example, Cole‑Parmer sells a 0.2–1 NTU series and a 2–10 NTU series. Some vendors also provide latex‑bead standards that mimic the particle size distribution of your samples.
Record the meter reading for each standard and plot response (NTU) versus concentration (mg L⁻¹). This calibration curve allows interpolation for unknown samples.
Because 1 NTU corresponds to 1 mg L⁻¹ of Formazin, the basic conversion factor is 1 mg L⁻¹ = 1 NTU = 1 ppm. However, particle size and composition vary between solutions, so always use the calibration curve specific to your application.
Accurate ppm‑to‑NTU conversion requires careful calibration, as the instrument’s response depends on particle characteristics.
