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Does the sensor have bare wires or a 3.5mm stereo plug? Either way, ensure that the sensor is connected properly to the Mayfly with the sensor power wire going to a 3.3v or 5v constantly powered pin during the address changing sketch. And make sure that the data wire of the sensor is connected to the proper Mayfly pin, which is usually D7 in the examples we write. And of course the sensor ground wire must be connected to a Mayfly GND pin. If you’re using a Mayfly Grove jack, it’s best to use either of the 2 jacks labeled SDI-12, and make sure the header jumper block next to the Grove jack is set to 3v or 5v.
Also make sure that you’re using the latest ModularSensors library files on your computer when you compile and upload the sketch. Changes to the Hydros 21 sensor last year require a slight timing adjustment in one of the library files (line 107 in MeterHydros21.h). You can read more details about it here: https://www.envirodiy.org/topic/hydros-21-decagon-ctd-reading-error/#post-17066