Optical sensors
Optical sensors are designed in many ways to detect light scattering and attenuation in order to measure sediment and turbidity in water samples. Some of them are shown on the sketch below, but despite their diversity, they all work in the same basic way. A water sample, the red-shaded region, is illuminated by a light source, shown by the red light bulb, and one or more photodetectors convert the light radiated from the sample to photocurrent. The amount of photocurrent depends mainly on the area of the illuminated particles but also on particle size, shape and reflectivity. Since the area of the illuminated particles is directly proportional to the suspended-sediment concentration, SSC, measurements of light scattering provide a way to estimate SSC. Certain conditions, explained in the answers to other FAQs (size, color, disaggregation effects), must be met to do this accurately. Light absorption by the sample will also affect the photodetectors and the indicated turbidity of the water. While laboratory turbidity meters can have several photodetectors, submersible sensors usually have only one. This is because it is difficult to design submersible sensors with multiple detectors. OBS sensors detect light scattered over a range of angles that depends on the model and particle concentration in the sample. The OBS-3 detects light scattered between 140 and 160°, the OBS-4 sensor (70-160°), the OBS-5 (88-170°); see sketch.

SSC
SSC is a physical quantity with units mass per unit volume (mg l^-1, kgm^-3). So knowing its value at a particular time and location enables calculations of such things as sediment loads of streams, background SSC levels for managing waterside construction, dredging, and logging to minimize adverse impact, and operations complying with NPDES permits. Calibration for turbidity measurements establishes a numerical relationship between OBS signals and corresponding standard NTU values. The resulting mathematical formula is used for assigning NTU values to water samples from OBS signals. Unlike SSC, turbidity values are relative units that have no direct physical significance.