Am I descending mountains in my sleep?

Mine is not perfect to,  wrong altitude and pressure. But if a put down my watch at home. Altimeter dosent move when in automode. My old fenix 5 could move sometimes when it was realy stormy outside

Yes, it's worth remembering that there is just one pressure sensor that is used to estimate altitude as well as record atmospheric pressure. There isn't a separate altimeter and barometer. The other element in the estimation is temperature, since cooler air is denser than warm air. The watch should have some way of compensating for this although it's built in thermometer does not give an accurate reading of the surrounding ambient air.

trux said:

perhaps I just insufficiently explained what I meant:

I believe this may be the case for both of us.  What I was trying to say about auto mode is it can be fooled and doesn't work properly all the time.  We're both in agreement that in auto mode the watch switches between altimeter and barometer based on what the watch "thinks" is happening.  Altimeter mode doesn't completely rely on GPS, it also uses the accelerometer.  You can wake in the morning and calibrate your elevation and barometer to the correct readings while in auto mode as I did this morning.  Just moving around your house, never changing elevation, the watch can show a increase or decrease in elevation as the pressure changes.  The watch was fooled because it detected a pressure change and detected movement and assumed a change in elevation occurred.  I've been awake for 3 hrs now and as I'm typing this my watch is displaying a .02 inHg drop in pressure and an increase of 17 feet in elevation just from moving around while vacuuming the house.  Had auto mode "guessed" correctly, it should have been in barometer mode and shown just a pressure change, instead it went to altimeter mode showing me going from 470' to 487'.  I know this wasn't the case for the OP as they said they left theirs in a drawer.  I was pointing this out in general to explain how auto mode can and will give you false readings.

tess said:

it also uses the accelerometer

The accelerometer alone cannot really be easily used for this purpose. Instinct has no gyro sensors, hence the data from the 3D accelrometer are useless for detecting the vector of the acceleration. The device does not know whether it accelerates vertically or horizontally (or diagonally). All it knows is that it moves, and cannot detect whether you are climbing stairs, or just scratching your head in the bed, riding an elevator, or walking around in the house staying on the same floor, or hiking in the mountains with the GPS disabled.

Still, I agree that the detection in auto-mode is very far from being perfect, and could be certainly done much more intelligent, and should recognize some common patterns like the slow and steady barometric change over several hours with no acceleration at all, but I am afraid we won't see it any soon.

trux said:

The accelerometer alone cannot really be easily used for this purpose.

My point exactly, but it does nonetheless causing inaccurate measurements like the example I gave earlier. 

trux said:

certainly done much more intelligent, and should recognize some common patterns like the slow and steady barometric change over several hours with no acceleration at all

We'd still be subject to wind gust while not moving outdoors or entering pressurized buildings within mere feet from our location.  Even perspiring as I understand this is an issue with the Instinct in particular because of the barometer port location. 

trux said:

but I am afraid we won't see it any soon.

I don't think we'll ever see the day when a watch would be smart enough to correctly detect every different scenario one could be in.  If that day ever came, there wouldn't be a need for any of the modes. You'd have a one time calibration at the manufacturing site and then be on your way.