Hi Scott, interesting article! Regarding turbidity, Rob Davies-Colley, an academic here in NZ has been doing some research comparing nephelometers, as turbidity thresholds are often written into environmental consent documents for discharges into freshwater bodies. His findings are similar to what you found, i.e., there can be up to two-fold difference in magnitude between sensors, even for sensors with identical ISO standards. I have also heard Rob speak at conferences and he has stated that even two identical turbidity sensors from the same manufacturer can have very different NTU/FNU responses. However there will generally be an overall relationship between sensors.
In short, this means that turbidity can’t be reliably compared between sensors (or across sites) in it’s raw form. However, you can develop a relationship between the sensor output and an SI unit measurement (i.e., Total Suspended Solids) and then compare that.
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