Differences in power-values between .ibr- & .fit-files

[RolfK]

Hello, has anyone else seen this?

I'm getting significantly different power-values for intervals between the AeroPod .ibr-file and the data in the .fit-files with powerdata retransmitted from the AeroPod to my Garmin Edge 520:

The attachment Screenshot 2018-11-15_21-44-12-428.jpg is no longer available

The attachment Screenshot 2018-11-15_21-46-26-091.jpg is no longer available

If I plot the two datastreams, the pattern is very similar, but I'm actually not able to recognise data sequences between the two streams:

ibr power in Isaac

Screenshot 2018-11-15_21-44-12-428.jpg (130.63 KiB) Viewed 2847 times

I hope there's an explanation - perhaps a AeroPod-setting that should be changed?

[RolfK]

fit-data

Screenshot 2018-11-15_21-46-26-091.jpg (96.65 KiB) Viewed 2846 times

[RolfK]

line plot

Screenshot 2018-11-15_21-52-35-120.jpg (76.15 KiB) Viewed 2845 times

[Velocomp]

The numbers recorded in the .fit file come from the DFPM; the numbers in AeroPod are both AP power and DFPM power.

In your graph the "View DFPM" box is unchecked, so you're looking at AP power. The .fit file shows DFPM power. The difference is likely caused by the default value of CdA used in AP. If you increase your default CdA a little bit, you will get the same power numbers.

If you check the "View DFPM" box in Isaac, you should see a number close to the .fit recording.

[RolfK]

Got it, thank you. When I check the box, the power numbers align. It also seems that the power numbers exported in TCX-files follow the checkbox setting; that's good.

Of course, only my opinion, but for me the estimated power numbers aren't of much value, and I would expect that they aren't for most other AeroPod users. I'd bet that if you buy an AeroPod, you already have a DFPM. A whole other matter for PowerPod users, of course. So, for me, separating the two products would make sense, instead of AeroPod being a (much) improved PowerPod with better CdA capabilities. Make one a powermeter, the other a CdA-meter. I think you could strengthen AeroPod that way. I also realize that it would take a significant amount of ressources to have more separate product lines.

[Velocomp]

If there is a 20W difference, then either the AP cal ride did not yield a good measurement of default CdA, or your subsequent test riding was in circumstances significantly different from your default CdA used in your Cal Ride.

AP measures your "default" CdA value during the cal ride. We're not yet 100% satisfied with this default CdA measurement algorithm. So, it is possible that cal-ride-measured, default CdA isn't as good as it should be. You can tweak your default CdA value in your profile; after that, your AP watts and DFPM watts will be very close whenever you ride in your default position.

But as you point out, AP does not actually use the power numbers shown in the main window to measure CdA. So, for CdA measurement it is not at all important that the AP power numbers are "off". If you don't want to see AP numbers, simply check the "View DFPM" box and you'll see your other power meter numbers.

Note that CdA is computed only in profile 4. So, you have 3 other profiles where AP power measurement is possible.

When you set up your bike in any of the other 3 profiles AP makes very solid estimates of your default CdA based on the parameters you enter. Therefore, AeroPod functions as a very good power meter on the other 3 profiles--without requiring a DFPM. In our view this is a meaningful plus; AP is an aero sensor, plus a power meter.