Rapid instrument prototyping with open source hardware and software: Application to water quality in hypersaline estuaries.

We describe the design and deployment of a water quality sonde that utilizes mobile phone networks for near-real time data telemetry. The REOL or Realtime Estuary Ocean Logger has the unique and valuable capability of logging data internally and simultaneously relaying the information to a webserver using a cellular modem. The internal circuitry consists of a GSM cellular modem, a microcontroller, and an SD card for data storage – these components are low cost, and backed up with circuit diagrams and programming libraries that are published under open source license. This configuration is versatile and is capable of reading instrument output from a broad spectrum of devices, including serial, TTL, analog voltage (0 - 5V), and analog current (typically 4-20 mA). We find the greatest challenges lie in development of smart software that is capable of handling the conditions brought on by this harsh environment. We have programmed the sonde to first determine whether it is submerged by water, and record the temperature on the electronics before deciding whether to telemeter measurements over the cellular network. The Google App Engine^TM provides an interactive visualization platform. We have tested the REOL with a variety of water quality sensors. In the configuration described here, we use a thermistor, depth gauge and torroidal conductivity sensor to measure water temperature, water level and conductivity up to 200 mS/cm. The latter is necessary for studies in hypersaline estuaries, where porewater salinity can exceed 100 g/kg. We present data from two estuaries in West Africa and from a longer-term deployment in the Narragansett Bay, Rhode Island.