power meter drift issues?

hey all,

unfortunately i've fallen into the power meter accuracy snake pit.

recently i've been seeing what looks to me like power meter drift. you
can see a pretty good example of this from my last trainerroad workout
here (done on a pre-2017 wahoo kickr snap):

https://analyze.dcrainmaker.com/#/public/ade655fb-936b-462b-6037-ef06a2775681

one power meter is a set of garmin vector 3 pedals with a claimed
accuracy of +/- 1.0%. (i've followed the service advisory notes for the
vector and verified the spindle torque. after installing the vectors i
reset the install angles and i also did a static torque test, as
described on the official garmin support pages, using a rice lake
calibrated 20 kg weight). the second power meter is a powertap g3 hub
with a claimed accuracy of +/- 1.5%. both power meters were calibrated
(zero'd) before the workout, which was done indoors at a consistent
temperature of around 70 F.

at the start of the ride, the powertap numbers are a few watts below the
vector numbers, which makes sense given some drivetrain power loss. but
by the end of the ride the power meters have reversed, with the powertap
giving numbers about 10% above the vector numbers, which seems wrong.
(aside from the drift, the difference between the two power values seems
out of spec for claimed accuracy of the devices, especially when you
consider that there should be some drivetrain power losses).

has anyone else seen something like this before? to me it seems like
one of the power meters is drifting out of spec? any ideas what else
might be going on or how to root cause the problem (short of buying
a tacx neo or a third power meter ;)?

cheers,
ed
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  • But that's exactly my point Browner40 I don't know who to trust?

    I have done a static weight test - with both Vector and Assioma - both beeing very good. And yesterday V3 was the one closest to NEO (Assioma very close to too). Both V3 and Assioma agreed on NEO showing to much. Maybe they're right - I can't tell...? What disturbs me here is that V3 the day after just drops another 2%.... WTf...? How can that be reliable? Well it isn't! And this is my issue. Call it settling in. I'm not sure what I call it...

    Check out this video - ALERT - 40 min tech stuff about powermeter principles. What is solid priciples and what is less good. I think some of the explanation lies here....!

    https://www.youtube.com/watch?v=zZ9fh6srmnk

    I have found out a way to test my NEO and I will do so in a few days (currently I'm out). I'll post the result here.

    GP Lama in comparing powermeters:

    https://www.youtube.com/watch?v=JhheDLXRM7I
Reply
  • But that's exactly my point Browner40 I don't know who to trust?

    I have done a static weight test - with both Vector and Assioma - both beeing very good. And yesterday V3 was the one closest to NEO (Assioma very close to too). Both V3 and Assioma agreed on NEO showing to much. Maybe they're right - I can't tell...? What disturbs me here is that V3 the day after just drops another 2%.... WTf...? How can that be reliable? Well it isn't! And this is my issue. Call it settling in. I'm not sure what I call it...

    Check out this video - ALERT - 40 min tech stuff about powermeter principles. What is solid priciples and what is less good. I think some of the explanation lies here....!

    https://www.youtube.com/watch?v=zZ9fh6srmnk

    I have found out a way to test my NEO and I will do so in a few days (currently I'm out). I'll post the result here.

    GP Lama in comparing powermeters:

    https://www.youtube.com/watch?v=JhheDLXRM7I
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