We’ve seen that very rarely, usually it happens when the connection between the Mayfly and the sensor is bad, and the fault is either mechanical damage to the sensor cable (rodents chewing, flood damage, etc) or corrosion on the contacts inside the Grove stereo jack due to moisture inside the logger enclosure, or bad connection of the Grove cable between the stereo board and the Mayfly. So whenever we see this sort of pattern in a CTD sensor, we replace the Grove cable and stereo jack, and 95% of the time, that solves the problem. In the rest of the cases, it’s either damage to the sensor cable or the electronics inside the sensor itself are failing (usually due to impact to the sensor body from large debris during a storm).
But if you’ve already replaced the Grove jack (and the cables?) and you’re seeing this on more than one station, then it’s likely a code or voltage issue. What version of the Mayfly board are you using (v0.5b or v1.0)? I don’t know how you’re powering the RS-485 sensors, but if they are being powered using a boost circuit that’s powered from the same voltage regulator on the Mayfly that powers everything else on the board (including the CTD sensor), then it’s possible that a voltage drop during the sampling period is causing the CTD sensor to now be fully powered, thus causing a communication issue between the Mayfly and CTD, resulting in the Mayfly reporting -9999 as the CTD sensor parameters. Have you tried just unplugging the turbidity sensor from your setup (but don’t change the logger code) and see if you’re able to get reliable communication with the CTD sensor again? If so, that would indicate the problem is related to the turbidity sensor being added. Or it could just be a software issue due to timing or other problems with the sketch. You can email me your sketch if you’d like me to take a look at it, once you’ve ruled out all the other obvious physical failures.
Other troubleshooting ideas: If you’ve got one of the ZSC bluetooth sensor interfaces from Meter Group, you could test that the sensor is operating fine independently of the Mayfly, or you could program a spare Mayfly and connect it to only the CTD sensor (first with and then without a cellular board) and have it take frequent readings (like 1 minute) and let it run for a few hours to see if it drops any readings.