Laser Granulometry for Water Turbidity Measurement

Turbidity is a key indicator of water quality, reflecting the concentration of suspended particles. Traditional turbidimeters measure light attenuation or scattering at a fixed angle, but they provide no information on particle size. Laser diffraction (or laser granulometry) offers an alternative that not only quantifies turbidity but also reveals the particle size distribution, which directly influences optical properties.

Principle

In laser granulometry, a diluted water sample is passed through a laser beam. Particles scatter light at angles inversely proportional to their size. A multi-element detector records the scattered light pattern, and Mie theory (or Fraunhofer approximation) is applied to compute the volume-based particle size distribution. The total scattered light intensity at all angles correlates with the overall turbidity, while the angular pattern decodes size information.

Procedure

Sample Collection and Preservation

Collect representative water samples in clean, dark glass bottles. Avoid settling or flocculation; gentle shaking is recommended before analysis. If needed, dilute highly turbid samples with particle-free deionized water to fall within the instrument’s concentration range.

Instrument Setup

Use a laser diffraction particle analyzer equipped with a recirculation or batch cell. Select an appropriate optical model (Mie theory with real refractive index of typical particles, e.g., 1.33 for water and 1.5–1.6 for suspended solids). Set the pump speed to maintain homogeneity without breaking fragile flocs.

Background Measurement

Run a blank using particle-free water to subtract background scattering from the cuvette and fluid.

Sample Measurement

Introduce the sample into the cell. The instrument automatically records the scattered light intensity over angles. Obtain the particle size distribution (e.g., D10, D50, D90) and the total scattering intensity.

Turbidity Calculation

Calibrate the instrument with a standard turbidity suspension (e.g., formazin). Plot the total scattered light intensity (sum of signals from all detectors, after background subtraction) against known turbidity values (NTU). For unknown samples, convert the integrated intensity to turbidity using the calibration curve. Alternatively, if the particle refractive index and density are known, turbidity can be estimated from the extinction cross-section derived from Mie theory.