Comparison of nephelometric turbidity measurements using wavelengths 400-600 and 860 nm

Two sets of turbidimeters were compared: one had a spectral peak response of the detection system in the 400-600 nm range and the other measured scattered light with a wavelength of 860 nm according to ISO 7027. Samples ranged in turbidity between 0 and 40 NTU and consisted of formazine standards, or natural and synthetic waters with a variety of particulate matter and humic acid concentrations. Water colour reduced readings of formazine suspension with instruments not meeting the ISO 7027 specification. These instruments also gave lower readings for natural water and drinking water regardless of colour. The results were confirmed by an interlaboratory comparison in Norway.