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Are you sure your serial port monitor is set to the same baud rate as the serial baud rate in your program? Usually when there are nothing but junk characters, it’s a baud rate mis-match. I would suggest using a serial baud rate of 57600. Make sure you have that set in your program and on your serial port monitor. The baud rate between the sensor and the Uno is different than the baud rate between the Uno and your PC.
Do your Uno and sensor have a common ground? If you are powering the sensor from a separate power source than you are powering the Uno from, you need to connect the grounds. No sharing a common ground can mess up the data signaling.
The full specifications for SDI-12 are here: https://www.sdi-12.org/current_specification/SDI-12%20Specification%201.4%20February%2020%202023.pdf Voltage specs are in table 1.
