Perfect calibration with a i-magic & Powermeter

Anything about Power (watts)

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mcorn
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Post by mcorn » Wed Nov 04, 2009 5:38 am

My experience, based on PowerTap comparisons, is you are not likely to get a lot more accurate than plus or minus 5%. There are just too many variables to try to control. If you want to be a perfectionist, you have to check tire pressure, clean the tire and roller, do a 20 minute warmup, then recalibrate each time. For me, it's not worth the effort, but for others who need this, that may be what is required. If you are religious about all of this, then perhaps you can get to less than 5%. To be honest, I've never really figured out what accounts for the variations. I'm not even sure whether ambient and component temperature makes a difference or not, but if it does, that would be a very hard variable to control. I also don't think the curve is all that linear either. One factor that will introduce a lot of error into the system is the percentage of downhills in the course. I think downhill wattage accuracy is not very good, so a course with a lot of steep downhills may show a lot more variation from your SRM.

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Stanifortius
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Re: Perfect calibration with a i-magic & Powermeter

Post by Stanifortius » Mon Oct 28, 2013 10:09 pm

Well I've just begun using a power tap and the difference in power readings is ridiculas, I rode a tempo ride on calliso this evening afterwards I compared the two files, the forties recorded an ave power of 268, where as the power tap recorded just 163 that's a massive difference....any suggestions?
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mcorn
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Re: Perfect calibration with a i-magic & Powermeter

Post by mcorn » Tue Oct 29, 2013 12:24 am

See if using Scale Factor at 80 helps.

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Stanifortius
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Re: Perfect calibration with a i-magic & Powermeter

Post by Stanifortius » Tue Oct 29, 2013 5:38 am

mcorn wrote:See if using Scale Factor at 80 helps.
Scale factor 100
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If you take all the bumps and loops out of the rollercoaster, you end up with a shopping trolley on rails

mcorn
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Re: Perfect calibration with a i-magic & Powermeter

Post by mcorn » Tue Oct 29, 2013 6:03 am

Set it at 80.

wheelz
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Re: Perfect calibration with a i-magic & Powermeter

Post by wheelz » Thu Oct 31, 2013 6:40 pm

I compared two files (Quarq and Fortius) this week using Dolomite. 25 threshold + 15 threshold intervals. I ended with a 10w NP difference over an hour. The first 25 minutes average was pretty spot on. The Fortius usually reads low when cold(first few minutes) and then drifts. Then for the 2nd it drifted and the Fortius went 20w higher then the Quarq. It's not that much of a problem, 5 TSS difference between the two. On the other side, it's not the ideal measuring tool to compare progression.

The downhills are the worst.

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Re: Perfect calibration with a i-magic & Powermeter

Post by mcorn » Thu Oct 31, 2013 10:46 pm

wheelz wrote:I compared two files (Quarq and Fortius) this week using Dolomite. 25 threshold + 15 threshold intervals. I ended with a 10w NP difference over an hour. The first 25 minutes average was pretty spot on. The Fortius usually reads low when cold(first few minutes) and then drifts. Then for the 2nd it drifted and the Fortius went 20w higher then the Quarq. It's not that much of a problem, 5 TSS difference between the two. On the other side, it's not the ideal measuring tool to compare progression.

The downhills are the worst.
I think your observations are pretty consistent with what I reported a few years back when I had a wired PowerTap hub I could compare my Fortius with. In fact, I think your results are a little better than what I observed. I've always said that people need to be realistic about the accuracy of these trainers. While they can be very effective trainers, they are not intended to be laboratory-grade instruments and they lack the high-end electronics (as well as the very high cost) of the various torque/wattage measurement systems. For the self-propelled systems like the Fortius and Genius, downhills also interject another level of errors in power measurement as well. The systems are much better suited for long-term relative comparisons than absolute measurements of progression over a short period.

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Re: Perfect calibration with a i-magic & Powermeter

Post by wheelz » Mon Jan 19, 2015 5:48 pm

Updaing since I notcied lately with this years setup that so far I'm pretty much on par with my Quarq when I do a RLV video. I did the full Glandon and was spot on. Same for Monte Baldo. I didn't do anything special. Maybe I would see more of a difference if I did an ergo training in Catalyst. To push it further, I should put the two files in a software(gc or trainingpeaks) and compare parts of the file where I rode tempo or where there was a downhill, etc. This is on TTS3, still with a Quarq (that has been calibrated with a known weight).

I guess it approves what mcorn said.

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