Edge820, latest fw (11.20) => Major issue in Grade calculation (0% on steep climb)

I see this behaviour as well from time to time, with various firmware versions. I've always put it down to the fact that I go so slowly up these grades that it screws up the algorithm. I agree though, it should be able to figure it out from the deltas of elevation and distance. 

Additional info x firmware debug. The 820 was showing VAM and VAM30s, both above 800, for more than 10minutes, and the act. hm was increasing. But the Grade% was constantly 0.

I would like to see an answer from Garmin, or should I open a ticket and as for a device replacement (820 is only one month old); even when to me seems a clear firmware bug, not hardware issue.

In my experience grade data on the 820 has always been useless. I get negative values when on the flats, 0% when climbing, etc. (And like you mention, other metrics like VAM, elevation, distance, etc all seem to be ok.) I contacted customer support about this problem in August 2017, and after a lot of back and forth they told me it was a known software problem that would be fixed in an update in September 2017. It's still broken. If you care about grade data, exchange the 820 for a different unit. I've used lots of different Edge units (500, 800, 810, 820, and 1030), and the 820 is the worst of them all. All the others provided more reasonable grade data.

Ed

In my experience the gradient is almost all the time way off the reality. The value is usually steady at 0% or oscillates. However the recorded activity and total ascend/descend seems fine.

Does the unit use barometer sensor or GPS to calculate the gradient?

Is it wise/necessary to use a speed sensor for more accurate readings?

This is the -bad- answer from Garmin support:

There is a difference in the algorithm the Edge 820 uses to calculate grade due to a minor difference in the baro in the device. This is likely to affect a users experience with the grade readings having a delay but there are no changes planned for the Edge 820 to improve the grade calculation performance.

very poor... 0 is not a delay, is 0 ...

It's been discussed on here last year, compared to previous devices 820 uses data from GPS to calculate the gradient rather than barometer and speed sensor. Therefore if your daily ride isn't from Valloire to the top of Galibier, and you happen to ride under some trees and past tall buildings, the gradient is not going to work. There is a data field IQ app for this that uses altimeter and speed sensor, seems to be working as good as 810 device. Try this:

https://apps.garmin.com/en-US/apps/2be4f241-29c6-4446-8cbb-85837353b040

I downloaded it and tried it on a hilly mtb ride.  It works very well even on relatively short ups and downs.  I have no other device to check it against but seat-of-the-pants says it's reasonably accurate. What's mystifying is that Garmin didn't do this in the first place and hasn't done it after all the complaints about their existing gradient calculation.

I downloaded the app and will try this weekend. But after download do i need to do anything with the data fields to be sure it shows me the altimetric grade? Thanks in advance

Yes.  You need to add the barometric gradient field to your screen.  To add the field to a screen, go to IQ in the list of fields, select that, then select the gradient field under that.   That will be the barometric gradient field.