Turbidity is a measure of the lack of clarity or “cloudiness” of a fluid caused by undissolved particles. Turbidity is an optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample.

As light passes through a sample containing suspended solids, the particles absorb the light energy and re-radiate the energy in all directions. Particle shape, size, colour and refractive index determine the spatial distribution of the scattered light by the particle. Particles smaller than the wavelength of light (e.g. bacteria) scatter light in equal intensities in all directions, while particles larger than the wavelength of light result in a greater forward scattering.

As light passes through a sample containing suspended solids, the particles absorb the light energy and re-radiate the energy in all directions. Particle shape, size, color and refractive index determine the spatial distribution of the scattered light by the particle. Particles smaller than the wavelength of light (e.g. bacteria) scatter light in equal intensities in all directions, while particles larger than the wavelength of light result in a greater forward scattering.

As the light scattered in the forward direction is variable depending upon the particle size, the measurement of light transmitted through the sample will yield variable results. This problem is eliminated when turbidity is measured at right angles to the incident light beam - an angle considered to be very sensitive to light scatter by particles in the sample. Both the International Standard (ISO7027:1999(E)) and North American EPA Method (1801.1) defining the measurement of turbidity requires scattered light measurement at 90°.

Modern turbidimeters use a technique called nephelometry, which measures the amount of light scattered at right angles to an incident light beam. The Kemtrak TC007 turbidimeter complies to International Standard ISO7027:1999(E), a European standard primarily defining the allowable turbidity in drinking water.

Both transmitted light and scattered light at 90° is measured and mathematically combined using a ratio algorithm to calculate the turbidity of the sample. The dual detection system and ratio algorithm provides a turbidity measurement that has significantly better performance with colored and/or light absorbing samples.

A 880nm long life LED light source is used to provide reliable turbidity measurements from 1 to well over 10000 NTU depending upon the configuration of the optical flowcell or probe. The Kemtrak TC007 is an instrument of high accuracy with long-term calibration stability.

TC007 turbidity meter measurement principle

Symetric scattering

Scattering concentrated in the forward direction

Light         Lens

source

Sample flow cell

Transmitted

light

detector

Scattered

light

detector

Small particles <1/10th wavelength of the light

Medium particles around <1/4 wavelength of the light

Large particles larger than wavelength of the light

Extreme scattering  in the forward direction