As Sara suggested, the issue is probably related to the SDI-12 address of the sensor. Meter Group ships all of their sensors with a default SDI-12 address of “0” (zero). They do this because it’s the channel their sensors need to be programmed to for working with their own brand of dataloggers. But Meter group sensors don’t respond to SDI-12 commands unless they are programmed to anything other than 0. The DRWI_LTE example sketch you’re using is looking for the sensor on channel 1, so the easiest thing is to use the address changing sketch Sara linked to change the sensor channel to 1. Then reprogram your Mayfly with the logging sketch and it should work.