Turbidity is a measure of the degree to which suspended matter in water obstructs the transmission of light. Suspended matter and colloids in water, such as mud, dust, fine organic matter, zooplankton, and other microorganisms, all contribute to turbidity. This turbidimeter (turbidity meter) utilizes the principle of 90° light scattering. When a parallel beam of light emitted from a light source passes through a solution, part of it is absorbed and scattered, while the rest is transmitted through the solution. The intensity of scattered light at a 90° angle to the incident light conforms to the Reylai formula:
Is = ((KNV²)/λ) × I₀ Where: I₀-incident light intensity; Is-scattered light intensity; N-number of particles per unit solution;
V-particle volume; λ-incident light wavelength; K-coefficient
Under constant incident light conditions, within a certain turbidity range, the intensity of scattered light is directly proportional to the turbidity of the solution.
The above formula can be expressed as: Is/I₀ = K′N (K′ is a constant)
Based on this formula, the turbidity of a water sample can be measured by measuring the intensity of scattered light from particles in the sample.
Modern turbidimeters primarily employ the 90° scattered light method (scattering method) for measurement, often using near-infrared LEDs of specific wavelengths (such as 880nm or 860nm) as their light source. To improve measurement accuracy, some instruments utilize ratio measurement technology, employing a dual-detector system to compensate for sample colorimetry, light fluctuations, and stray light. Additionally, some turbidimeters feature a 360° × 90° optical path design, where scattered light, after passing through a conical mirror, surrounds the sample 360°, ensuring uniform light signal capture and eliminating errors caused by local concentration differences; this is particularly suitable for low-turbidity measurements. For high-turbidity measurements, surface scattering technology is used, where the light beam is directed towards the liquid surface, and the detector detects the scattered light at a 90° angle above the liquid surface, with the optical elements not contacting the water sample. The transmitted light scattering method (ratio method) calculates turbidity by simultaneously measuring the ratio of the intensity of transmitted and scattered light, thus reducing interference. The instrument design must comply with relevant international and domestic standards, such as ISO 7027, US EPA 180.1, and HJ1075-2019.
