Strange behaviour of the Altimeter

The pressure the sensor detects, is influenced by two factors in the same time - the elevation and the atmospheric pressure. So when you keep the watch in the Altimeter mode, the elevation will necessarily change as the the barometric pressure changes.

trux said:

when you keep the watch in the Altimeter mode, the elevation will necessarily change as the the barometric pressure changes

It should. But as you can see, elevation does not change and Instinct show barometric pressure changes instead

(In this case the mode setting was changed to "Altimeter" only during movement).

Are you sure you have the auto-calibration turned off (both during and not-during an activity)? That could perhaps explain why the elevation was suddenly fixed, and control given over to the barometer at the point ~-3h. Although, it does not sound like the correct behaviour for the forced altimeter mode, anyway.

I'd recommend sending the information to Garmin Support, but I am afraid it may take couple of years more before they get it right (if it ever happens).

I have observed this sort of behaviour when I wash my hands (which is very often during covid times). Either take off the watch before or make sure to help the moisture out the sensor holes afterwards. I usually take off the watch, hold it by the strap and gently slam it on my thigh a couple of times to drive the water out. A recalibration is usually required. I do the same when I train if I'm sweating a lot...

Also during winter I light my fireplace a lot, this causes the pressure inside the house to change 

Water and temperature change certainly influence the pressure sensor reading, but the pressure change should still show up as altitude change, and not starting to trigger the barometer, when the watch is forcefully set to the Altimeter Mode. That's the problem described by the OP, not the reason of the pressure change.

trux said:

Are you sure you have the auto-calibration turned off (both during and not-during an activity)?

Hm, I have never noticed this setting. It is turned on, it's default I suppose. What should this setting do? The Manual says it uses calibration of altitude from the GPS, but the GPS is off during all my experiments.

6619804 said:

What should this setting do?

Yes, it calibrates the elevation with the help of GPS and optionally also using the DEM (Digital Elevation Model). The exact algorithm was never disclosed, and it is unclear how often, or under which conditions exactly the calibration happens, adn it also is not clear whether the watch attempts to get the location by temporarily turning the GPS on, or using the last acquired position, or perhaps using other means. I would definitely try turning it off for the testing, though as I already wrote, even if the calibration was on, it would not justify switching from the altimeter mode to the barometer mode without your consent.

Both pressure and altitude seem to be affected simultaneously. That's what I usually observe when the sensor is blocked. 

darnich said:

Both pressure and altitude seem to be affected simultaneously.

Of course, but if you explicitely set the watch into the manual "Altimeter mode" instead of the "Auto mode", any pressure change, regardless of its origin, must translate into the change of the elevation. Barometric pressure must stay fixed, in the Altimeter mode. That's the exact purpose of the "altimeter" vs the "barometer" manual mode, in contrary to the default "auto" mode that attempts to guess the origin of the pressure change.