Weak numerical comparability of ISO‐7027‐compliant nephelometers. Ramifications for turbidity measurement applications

Nephelometric turbidity, a measure of light scattering by particles suspended in water, is commonly used for indicating water clarity or suspended particulate matter (SPM) concentration. Different turbidity sensors have long been known to respond differently to the same suspensions. Design standards have been introduced to improve comparability of turbidity sensors, notably the ISO‐7027 standard adopted by a number of manufacturers. We compared six ISO‐7027‐compliant nephelometers in river silt, kaolinite (layer clay) and algae‐laden pond water, with rigorous tank experiments over a wide (100‐fold) concentration range. The responses of four different field‐type (in situ) and two cuvette instruments, all calibrated to the same freshly made formazin standards, were very strongly linearly correlated, but ranged about twofold in magnitude. Apparently, even sensors meeting the same design standard (ISO‐7027) cannot be relied on to output numerically similar formazin nephelometric unit (FNU) values. This weak numerical comparability highlights the futility of treating turbidity as an absolute quantity, for example in environmental standards or studies of fine SPM effects on aquatic life. Indeed, reporting of turbidity in informal units such as FNU is best avoided. Turbidity records should be converted, by site‐specific calibrations, to quantities of ultimate interest such as SPM concentration or total phosphorus. For performance monitoring of field nephelometers, we advocate routine site‐specific calibration, not to formazin, but to the light beam attenuation coefficient (beam‐c; units: m−1). Beam‐c is a proper (SI) physical quantity that can be precisely measured by beam transmissometry, as in our experiments, and is accurately convertible to visual clarity. Six different nephelometric turbidity sensors, all‐compliant with the international design standard (ISO‐7027), varied two‐fold in response (FNUs) in tank tests with three optically contrasting suspended materials over a wide range of concentrations. This ‘numerical ambiguity’ of turbidity reveals it to be a flawed metric. A superior metric, the light beam attenuation coefficient (in SI units: m−1), was used as the optical reference in our experiments—and has the major practical advantage that it controls the visual clarity of waters.