Can anyone tell where I can find my power meter’s serial number in intervals?
In the Data Tab of an activity at the bottom. 
External ID is not your PM serial!
The info regarding power meter written to the FIT file depends on the connection it is using. ANT+ gives you all info available, BLE often not…
The data from the fit files isn’t always correct either. It reads Garmin Rally 200, but I’m driving one-sided, so I’m driving 100. The serial number is also incorrect.
What is strange is that my edge reads exactly the same number, but Garmin says on its website that it is the wrong serial number - now I’m confused 
when doing research on how to get PM Serial Number, ([SOLVED] PM SN + Status in the Data tab - #5 by app4g) i found out that the SN is diff between when it’s connected as BT or via ANT+
Not only that, the Data that is printed physically on the sensor vs what is reported by querying the device (via BT) is also different.
Then, to add insult to injury, it seems that Garmin FIT file only accepts Digits in the SN and my H10 has alphabets on the SN and my Assioma has dots… so…
i have no idea…
welcome to the club 
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