The CTD sensor you have is a digital output sensor, meaning all of the measurement happens onboard the sensor and it outputs a string of 3 numbers, all the Mayfly logger board is doing is capturing those numbers and storing them. If there was a problem with the Mayfly board, then you wouldn’t get the numbers at all, and there’s no calibration or conversion needed with those sensors, so the numbers being recorded on the Mayfly memory card are exactly the same numbers that are being sent directly from the sensor. If the data from the sensor doesn’t match up to your QA/QC measurements, then the sensor is either fouled or broken. There are a variety of ways those sensors can fail, sometimes it’s pretty obvious, and other times it’s more subtle and you need to look carefully at all the data and compare it to the real-world, hand-held measurements.
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?