YSI ProDSS [626901] Turbidity Sensor. The ProDSS turbidity sensor is a digital smart sensor featuring welded titanium construction for use with the ProDIGITAL family of instruments. Compatible instruments include the ProDSS Meter, ProSwap Meter, and ProSwap Logger. Turbidity is the indirect measurement of the suspended solid concentration in water and is typically determined by shining a light beam into the sample solution and then measuring the light that is scattered off of the particles which are present. The suspended solid concentration is an important water quality factor and is a fundamental measure of environmental change. The source of the suspended solids varies in nature (examples include silt, clay, sand, algae and organic matter), but all particles will impact the light transmittance and result in a turbidity signal. The ProDSS Turbidity sensor employs a near-infrared light source and detects scattering at 90 degrees of the incident light beam. According to ASTM D7315 method, this type of turbidity sensor has been characterized as a nephelometric near-IR turbidimeter, non-ratiometric. This method calls for this sensor type to report values in formazin nephelometric units (FNU). FNU is the default calibration unit for the ProDSS sensor, but users are able to change calibration units to nephelometric turbidity units (NTU), raw sensor signal (RAW), or total suspended solids (TSS).