I am building a data logger for a very small public water utility. The logger reads a 0-5VDC residual chlorine signal, filters it, and writes the value to a CSV file daily. The monthly files are retrieved, imported into an Excel spreadsheet and sent to a WA state agency. I have added a battery backed RTC to the system, and an HC-06 Bluetooth interface for configuration and observing real time behaviour. Prototyping is done, all the code is finished, and final software testing is this week as well as board construction. Some much easier and cheaper than the commercial hardware that I used to use as an engineer before I retired. And fun!