Most instruments and sensors drift away from their calibrated signal-response over time and use. In the lab, especially instruments that can have carry-over contamination from previous samples, it's easy to run a blank sample at adequate intervals and subtract the blank from subsequent samples. It's not really possible to run a sensor blank in the field, so what could be other options? For something like conductivity, would it be feasible to take conductivity calibration standards into the field and test the sensor's response to those standards, recalibrate, and possibly make corrections to previous measurements? How have you addressed signal drift with low-cost sensors?