Optical methods can be used as a “proxy” for turbidity in order to derive real-time information. However, the amount of light scattered is dependent on several variables – particle size and shape, albedo (“colour” or grey-scale of the particles) etc. and the result or “number” from turbidity sensors from different manufactures can vary dramatically. A turbidity sensor features an infra-red LED as a light source (to avoid absorption in the visible range) and a detector (photodiode) to receive the scattered light. Standard ISO 7027 specifies LED wavelength, beam divergence and the angle between beams in order to remove variation between sensors from different manufacturers.
As turbidity increases, multiple scattering events occur and the light penetration depth decreases; the sensing volume is decreased and the response becomes non-linear. This sets an upper limit on the linear range, though can be accommodated using beam geometries with a short light path.
Optical devices cannot be used to accurately derive total suspended solids (TSS) which is a physical parameter. It is possible to perform field calibrations to determine a relationship between the optically derived turbidity/OBS levels and TSS derived by gravimetric analysis (EPA Method 160 https://19january2017snapshot.epa.gov/sites/production/files/2015-06/documents/160_2.pdf accessed 17/9/2020). However, such relationships may change over time due to lunar/seasonal/precipitation-related variations in particle size, size distribution and albedo.
Valeport’s turbidity sensor is essentially 2 sensors in one. It features a single IR LED source at 860 nm with 2 detector configurations. The first features a conventional light path (nephelometry with a 90˚ beam angle compatible with the ISO 7027 standard) for low turbidity levels and a second uses a short light-path backscatter arrangement (~120˚ beam angle) for use at high turbidity levels). Both the Nephelometry and Optical Backscatter (OBS) channels are calibrated in NIST traceable standard polymer microbead suspensions. The nominal units are NTU (Nephelometry Turbidity Units). Care should be taken as these sensors will not give the same reading in Formazine suspensions due to the different particle size and size distribution 1 NTU ≠1 FTU.
Optical monitoring of the source removes lifetime drift and temperature drift allowing a very short warm-up time. All components are potted in optical resin ensuring full ocean depth rating and sapphire windows prevent scratching in aggressive environments. A 24 bit ADC allows use a single gain setting and eliminates gain switching.