Bench-top Suspended Solids Meters: Principles of Operation

In water quality analysis, suspended solids (SS) represent one of the most fundamental parameters, indicating the mass of insoluble particulate matter dispersed in a liquid sample. Bench-top suspended solids meters are laboratory instruments designed to quantify this parameter with precision and reliability. Their operating principles fall into two major categories: optical methods and the gravimetric (weight-based) method. Each approach serves distinct purposes, balancing speed against accuracy.

Optical Methods – Light Scattering and Transmission

The most widely adopted principle in modern bench-top meters is optical scattering. In this method, a light source—typically a near-infrared LED emitting at 860 nm wavelength—illuminates the sample. As the beam passes through the water, suspended particles intercept the light and scatter it in various directions. A photodetector, usually positioned at a fixed angle such as 90° relative to the incident beam, captures the scattered light. 

According to Rayleigh and Mie scattering theories, the intensity of this scattered light is directly proportional to the concentration of suspended matter in the sample. The instrument's internal microprocessor converts the optical signal into a concentration reading, typically displayed in milligrams per liter (mg/L).

Some advanced instruments employ a dual-detector configuration, combining a 90° scattered-light detector with a 180° transmitted-light detector. The ratio between scattered and transmitted intensities extends the dynamic measurement range and improves accuracy across varying particle size distributions.

 Others utilize a dual-source, eight-beam measurement system, where two infrared sources operate simultaneously to validate results and minimize interference from contamination, temperature fluctuations, and dissolved color. These optical configurations often comply with ISO 7027, the international standard for nephelometric turbidity determination.

An alternative optical approach is the transmission method, which functions similarly to a spectrophotometer. A parallel beam of light passes through the sample, and suspended particles absorb and scatter the light, attenuating the transmitted intensity. The instrument measures the ratio of transmitted to incident light and, applying the Lambert-Beer law, derives the suspended solids concentration. While this method offers good sensitivity in low-concentration ranges, it is generally more susceptible to interference from particle color and shape than scattering-based techniques.