Head unit impact on power recording?

Searched this forum and some others and couldn’t find the answer, sorry if this is a FAQ…

Can a head unit impact the power that’s recorded for an activity and if so in what way?

Could two different head units, connected to the same power meter (theoretically), read power meaningfully differently, not counting very short timescales (due to different recording intervals/cadences)?

Context: I triple recorded an activity and two out of three power meters were very closely aligned (almost suspiciously so…) while the third was a bit off. Thing is, that third power meter has seemed to be reliable in the past, and in this case I had to dig out an ancient Garmin watch (Forerunner 935) to record it. Could that old watch be to blame in any way?

One could be set to count 0s the other one isnt.

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Make sure the outlier is recording once per second. Garmin used to have a recording option called smart recording. This option will for sure change the numbers.

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Garmin actually forces to 1s anytime a power meter is connected. So there’s no differences there, and I’ve never seen any differences between different Garmin units (e.g. watch vs bike computer/etc) in a very very very long time.

Not sure if you did any hard sprints/etc if this was just installed, or a zero offset/etc as well.

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there might be a difference between bluetooth and ant+, due to drop-outs

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Thanks, this is the info I needed. I can scratch one explanation off the list then.

(fwiw I’m inclined to think the two closely aligned files were actually reading from the same PM, one over BT and one ANT, because their alignment beggars belief. Unfortunately zwift FIT files appear not to record that metadata.)

True they do this now. But this was not always the case. I know this for a fact because I have files from riders where the smart recording is seen in the timestamps in the fit file.

I’m not sure when this change was made

It’s been the case for at least a decade, since their first Vector pedals came out, and even before that too.

Good to know