Protocols for Verifying the Performance of In Situ Turbidity Sensor.

Abstract

As part of our service to the coastal community, ACT Partner Institutions and Stakeholder Council have chosen the performance verification of commercially available, in situ turbidity sensors as the third ACT Technology Evaluation.Turbidity is a property commonly used to describe water clarity in both marine and freshwater environments, providing a gross assessment of light attenuation due to suspended material. However, turbidity is often not a direct measure of the quantity of interest, such as suspended sediment, living particles, and non-living organic matter , but rather a measure of the effect of the desired quantity on the optical properties of the water. At present, there are numerous methods for quantifying turbidity (e.g., light attenuation, surface scatter, side scatter, laser diffraction, acoustic back-scatter, etc.). Differences in methods of measurement and their individual responses to varying types of suspended material have made the measurement of turbidity difficult to perform in a consistent and standardized way. This has necessitated many public-service agencies (e.g., USGS, US EPA, ISO, ASTM, etc.) to define turbidity in very specific terms based on optical methods of measurement, since optically-based appr oaches have been the most conventionally used. Although such standards and definitions were created to be both technically and legally specific ACT Turbidity Protocols PV06-01 5/3/062 (thereby minimizing the ambiguity in interpreting what turbidity is and how it is measured), they still suffer from fundamental deficiencies in their ability create an absolute standard between different natural water types and different instrument designs employing the exact same principles of measurement. Despite these limitations, a variety of in situ instruments that provide some measure of turbidity are commonly and successfully used in many researcher and monitoring settings as at least a relative measure of water clarity. This ACT Technology Evaluation will examine individual sensor performance both in the laboratory and across different field conditions. We will focus specifically on commonly used back- and side- scattering optical instruments that provide values for turbidity in Nephelometric Turbidity Units (NTU). This unit of measurement pertains to a specific concentration of a given standard medium. .