Emily Eidam, Theodore Langhorst , Evan B Goldstein , McKenzie McLean

Optical backscatter sensors (OBSs) are commonly used to measure the turbidity, or light obscuration, of water in fresh and marine environments and various industrial applications. These turbidity measurements are commonly calibrated to yield total suspended solids (TSS) or suspended sediment concentration (SSC) measurements for water quality, sediment transport, and diverse other research and environmental management applications. Commercial sensors generally cost >$1000-3000. Here we leveraged simple, low-cost microprocessors, electronics, and housing components to design and construct open-source OBSs for <$150 per unit. The circuit relies on a photodiode to sense the backscattered light, two stages of signal amplification, and a high resolution analogue-to-digital convert to read the detected value. The instrument and logger utilize inexpensive, custom-printed circuit boards with through-hole soldering mounts; micro-SD card reader and real-time clock modules; and PVC housings with commercial end caps and epoxy-potted diode emitter and receiver. All parts are readily and publicly available, and minimal experience in soldering and coding is required to build and deploy the sensor. In lab and field tests, standard deviations were comparable to those measured by commercial sensors (~2-3% of the mean for suspended muds and 20-30% for suspended sands). These open-source sensors represent a useful advance in inexpensive sensing technology with broad applications across scientific and environmental management disciplines.