Sensor readings of -9999 mean the Mayfly logger isn’t receiving valid data from the CTD sensor. If this is a brand new sensor, then it’s probably because the sensor SDI-12 channel number is set to the default channel of 0 (zero) and the Mayfly needs the sensor to be something other than that (because for Meter Group sensors, channel 0 means the sensor is in serial-TTL data output mode and isn’t prepared to communicate with SDI-12 protocol loggers, like what the Mayfly does in our example code). So you’ll need to change the sensor channel number, and if you’re using one of our sample sketches, we usually use channel 1 for the CTD sensor. In the EnviroDIY Monitoring Station Manual in section 6.3 there’s a description of the steps needed for connecting a CTD sensor to the Mayfly logger, including a link to the Github page where there are instructions for how to use the SDI-12 address change example sketch to change the sensor address.
If you’ve already changed the sensor channel, then other reasons for the -9999 signal are usually because the sensor isn’t connected securely into the Grove headphone jack board (if you bought a sensor with a 3.5mm stereo plug on it), or if you’re using a bare-wire sensor, then the wires are in the wrong terminals. Or the Grove cable connecting your sensors adapter board to the Mayfly is plugged into a Grove port other than one of the two jacks labeled SDI-12.