In the quest to develop less expensive, open-source turbidity monitoring instrumentation, a key obstacle has been calibration of the sensor. Calibration generally requires access to expensive standards and solutions, or comparison to an expensive commercial turbidimeter. To address this problem, a low-cost open-source sensor ($350USD) measuring 563nm signal attenuation at 180� incidence (nominally called a �clarity� sensor to distinguish from turbidity as a standard) is being developed. The sensor is designed to be calibrated to local observations of visual clarity and provides a novel measurement technique: four different path lengths. As light attenuation is a function of mineral and organic matter in suspension, and the path length over which attenuation is measured, the use of four different path lengths is proposed to achieve two different goals: (1) a large measurement range; and (2) a method to detect sensor drift due to fouling.This sensor is part of a wider PhD programme investigating the application of clarity sensors to construct dense catchment networks as an indicator of suspended particulate matter (SPM). Research is being undertaken in Aotearoa New Zealand and in Canada to determine how the sensor functions with respect to different sediment transport processes and origins, the effects of large temperature variations, and how useful low-cost sensors can be in distinguishing catchment heterogeneity. Following open-hydrology principles, the research is designed to enable community groups, indigenous communities and citizen scientists to collect SPM time-series data.