The sensor has to be on for one hour before it reports a result and only has an output once an hour.
Post your code in code tags.
Strict warning if more than 3.3V is on the RxTx line the sensor is poo!
Has the sensor format been set before the attempt to communicate the 72H ('r')?
Seems the sensor needs a trailing cr/lf???
Sensor data is in the format so using the < and > as sentence begin ends will work?
See
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