Comparison of portable nephelometric turbidimeters on natural waters and effluents

Five different cuvette-style portable turbidimeters using a nephelometric (side scattering of light) principle were assessed simultaneously to determine between-meter variability. The assessment was conducted using primary formazin standards, and a variety of different effluent types and receiving waters encompassing a wide range of turbidities and optical properties. For the formazin standards, the coefficient of variation (CV) ranged from 1.5% to 6.8% providing an indication of precision. For the effluents and receiving waters, the CV ranged from 6.6% to 44.1 %, which compares favourably with previous interlaboratory studies reporting CVs ranging from 23% to 144%; however, the previous studies were not direct side-by-side comparisons of meters. A power analysis showed that, with the exception of very clear headwater streams, single turbidity samples are sufficient to detect changes in clarity recommended in the New Zealand guidelines for the management of water colour and clarity.