Ultra-low-cost optical pH sensors for high-density and community-assisted coastal/estuarine monitoring

Optical dye techniques have become the preferred method for measuring pH in aquatic systems. However, these methods conventionally use spectrophotometers for measuring specific-wavelength light absorbance in samples. Although this instrumentation provides excellent accuracy and precision, the cost of such instruments (thousands of $s) prevents the deployment of large numbers of instruments for high areal density measurements and for use by community and educational groups. This lack of detailed areal coverage is becoming a limiting factor in understanding fine-scale pH temporal and spatial variability in dynamic coastal and estuarine systems. This presentation examines the use of low-cost optical devices (i.e., narrow-band LEDs and light sensors developed for consumer products) to construct inexpensive (<$100) pH sensors suitable for coastal and estuarine monitoring. In addition to reducing costs relative to spectrophotometer-based systems, the new systems are physically robust and compact, easily lending themselves to in-situ monitoring applications, and can readily be constructed by non-specialists. Although the new sensors do not match the accuracy and resolution of spectrophotometer-based instruments, their data quality is sufficient to provide meaningful advances in the understanding of coastal and estuarine systems. In particular, the low-cost sensors allow deployment of large numbers of instruments by research projects and community/educational groups with only moderate financial resources.