Garmin and Newton/Isaac different power readings

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David Lloyd
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Garmin and Newton/Isaac different power readings

Post by David Lloyd »

Something unusual today - I was doing a 30min TT for testing FTP and noticed that the power output being displayed on the Newton was different to what my Garmin Edge 500 right next to it was displaying (I use the GT firmware to send the Newton power to Garmin). Sure enough when I got home and uploaded the Garmin Edge to Strava and the Newton to Isaac, the Garmin averaged 280w and the Newton 208w. Any advice please? Should I be pressing "yes" when my Garmin asks me on startup if I want to calibrate the detected powermeter (can't see why because the output from the Newton is already at zero)?
KenS
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Re: Garmin and Newton/Isaac different power readings

Post by KenS »

Check your recording parameters on the Garmin. Are you using Smart recording which drops readings? Is zero averaging off? This causes the Garmin to ignore zero readings.
No need to calibrate the power meter. I never get that prompt although could be because I'm using an old Edge 705
-- Ken
Velocomp
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Re: Garmin and Newton/Isaac different power readings

Post by Velocomp »

David Lloyd wrote:Something unusual today - I was doing a 30min TT for testing FTP and noticed that the power output being displayed on the Newton was different to what my Garmin Edge 500 right next to it was displaying (I use the GT firmware to send the Newton power to Garmin). Sure enough when I got home and uploaded the Garmin Edge to Strava and the Newton to Isaac, the Garmin averaged 280w and the Newton 208w. Any advice please? Should I be pressing "yes" when my Garmin asks me on startup if I want to calibrate the detected powermeter (can't see why because the output from the Newton is already at zero)?
Please post your ride file.
John Hamann
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racerfern
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Re: Garmin and Newton/Isaac different power readings

Post by racerfern »

Check your zero recording option on the Garmin. It affects both power and cadence averages. Note that the non-zero average which is what it appears you have, will approximately equal NP on the Newton. It could be higher or lower but will generally be in the range of NP not AP.

BTW, if your NP is substantially higher than AP then you have quite a bit of variation in power output. The formula is VI=NP/AP. So, +/-280/208=1.34 which is a substantial disparity. Read up on it, for a TT type effort or FTP test, you would like a number much closer to 1.10.
Fernando
David Lloyd
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Re: Garmin and Newton/Isaac different power readings

Post by David Lloyd »

Thank you for the replies. The problem was the Garmin was lower (282w), not higher than the Newton (308w), so the Garmin does not appear to be dropping the zero readings. The Garmin also lost the satellite during the ride and didn't re-acquire so only recorded part of the ride - I am thinking it just lost some of the data stream from the Newton. Today's ride it all seems to match, so I will just pass this off as an anomoly and won't post files unless this problem occurs again.
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racerfern
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Re: Garmin and Newton/Isaac different power readings

Post by racerfern »

You had us all worried as we were digging deep within our resources to figure out what was going on. Who woulda thunk it was a typo, 308 vs. 208. Sheez, wouldn't I love that kind of typo.
Fernando
Davis6022
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Re: Garmin and Newton/Isaac different power readings

Post by Davis6022 »

I have the same set up as you, David, and I noticed a slight variation between the Garmin power graph and the Newton power graph in Isaac when I import the Garmin ride file into Isaac. The Garmin power graph tends to be smoother than the Newton, and the Normalized Power is usually slightly lower by 2 or 3 watts. I chalk it up to the way each unit processes the data from the DFPM, so it's not a concern. One thing that would be helpful is if the Newton would display a calibration value like the Garmin does. It doesn't have to be the same scale as the Garmin. It would just be useful to see if it's changing from calibration to calibration. #wishlist.

Mike
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