What Mayfly board version are you using? The ClariVue10 seems to only work reliably with the Mayfly v1.1 RevB since it produces the most stable 12 volt output. Also, we’ve found that at room temperature the ClariVue needs about 8 seconds after the switched power is applied before it will respond to any commands, and then needs another 9 seconds to complete the actual measurement. It could be that the temperature of either the sensor or the logger is causing that interval to be longer than the code expects, causing a mis-communication between the logger and the sensor. We currently have a ClariVue10 sensor installed in a river with a Mayfly v1.1RevB, the water temperature right now is 4C and the air temp is 6C, and all data is normal.