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The TDS code currently posted by DFROBOT for the Gravity: Analog TDS Sensor/Meter for Arduino is a total re-write since I downloaded it. The code I downloaded about 6 months ago had an error in it (applied temperature correction to the raw analog voltage) The updated code is much more modular (a bunch of functions) and seems to have the TDS calculation correct now, with temperature correction applied after calculating electrical conductivity. I haven’t tried the new code but it seems to use the same multi-nomial equation to convert from voltage to electrical conductivity. I’m not sure where they got the coefficients for the equation but I developed my own equation from running the DFROBOT sensor side-by-side with a Onset HOBO U24 logger for about 3 months. I’m going to run my DFROBOT sensors side-by-side with Onset loggers for the foreseeable future because I’m not sure if there is variation in response between individual DFROBOT sensors.
BTW, the DFROBOT TDS sensor seems like it might be stable over long periods. I’ve been sampling the same one at 15 minute intervals since June and haven’t seen anything weird. I won’t know for sure about stability ’till I download the Onset logger in the spring to compare to the DFROBOT sensor data.