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Do you have the continuous data file from the logger with all the data from the CTD sensor? Your excel chart with weekly reading indicates that something started affecting the depth readings in early January, but if you can graph all 3 CTD parameters with the continuous data, you can probably find a specific event that caused the failure. Being that time of year, it’s likely that either the sensor was subjected to freezing temperatures (which can cause catastrophic failure of one or more of the parameters) or the sensor cable jacket was damaged, which can affect the depth readings by interfering with the sensor’s atmospheric reference (essentially there’s a small air gap through the cable jacket that goes to the goretex vent on the cable end near the logger box that provides the atmospheric compensation). Or it could be that the sensor is just fouled with something on the pressure disk and needs a thorough cleaning. Have you removed it from the stream and examined it carefully and also examined the cable jack along the entire 30-foot length, looking for any nicks or cuts or holes? What depth and conductivity reading do you get on the logger when the sensor is out of the water?