I'm working with a CitSci group in Eastern OH trying to develop a water quality sensor network to monitor changes in TDS in peoples well water and in local springs. We are the new dumping ground for the waste brine from fracking in PA and OH. Massive amounts being pumped into injection wells in our area so we are trying to get ahead of the curve on monitoring change. Need to keep the cost of the sensor/logger package under $20 as we are working with a very small budget. Planning to use the promini logger from the Cave Pearl Project as Ed seems to have worked out all the issues with that brilliantly. I've seen some threads here for different sensors but they seem to stop a couple of years ago. Anyone still working on this problem, or have any guidance for development of a low cost option?