Qnh ?

Perhaps I am telling you something you already know, but just to be sure:
The Barometric Pressure shown on the watch is actually equal to QNH (though I do not know if the watch uses standard ISA for the altitude correction).
So if you are really patient, you could do a "reverse" calibration from QNH by inputting different altitudes until the watch shows a Barometric Pressure which is equal to your known QNH.

However, I think that the 5X also has a more direct way of inputting QNH. Some weeks ago, I tried to help an F5 user who had problems with his altimeter, and he claimed that his watch had asked for a sea level pressure when he calibrated the altimeter. But since my F3 doesn't have this option, I don't know what the setting does.

Try this?

https://www8.garmin.com/manuals/webhelp/fenix5x/EN-US/GUID-BC734846-01A7-4F33-86D4-DFBDBC06CDB4.html

Your link describes how one calibrates the altimeter from a known altitude. It does not describe what the OP is asking for: Calibrating the altimeter from a known air pressure at sea level.

Start the barometer widget -> start button -> calibrate -> enter current elevation? -> Yes -> Set elevation -> Enter sea level pressure? -> Yes -> Set pressure

Allan, thanks for your reply.

I am a bit puzzled.

For the watch to show the QNH, it would have to know the local pressure (QFE) and the present altitude (via GPS). Then calculate the QNH from those,
If the altitude is already accuired via GPS why would I need a QNH?

I live in an area at sea level so no problem for me, but when I am at a location at 2000 feet, I would like to see the barometer indicate the pressure at my location!

Your second reply nails it, a QNH can be obtained from the nearest station with a METAR. Set the QNH in the watch and the altimiter is calibrated (just like a pilot does in his cockpit).

Meanwhile I found the entry for QNH: calibrate barometer, set altitude manually is followed by pressure at sea level.
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Now I have to find a tall building to test the barometer and altimeter (no mountains in Holland ;-)), and some quiet weather with a constant air pressure.

Nico

Thanks MrProper,
While typing my reply I found the setting as well.

I am a bit puzzled.

For the watch to show the QNH, it would have to know the local pressure (QFE) and the present altitude (via GPS). Then calculate the QNH from those,

Yes, that is what the watch does when you calibrate the altimeter/barometer using a known altitude (which is not necessarily coming from the GPS):

• The watch measures the ambient pressure, using its pressure sensor.
  
• You input the known altitude (or the watch retrieves the GPS altitude, or retrieves the 2D GPS coordinates and looks them up in its internal table of earlier manually input altitudes).
  
• The watch uses the measured ambient pressure and the known altitude to calculate a QNH (shown as Barometric Pressure).
  

After the calibration, whenever the ambient pressure changes, the watch will do one of two things:

• If the watch is in Altimeter Mode, it will assume that the pressure change was caused by you climbing to another altitude. So it will calculate a new altitude based on the measured ambient pressure and the latest QNH.
  
• If the watch is in Barometer mode, it will assume that the pressure change was caused by a changed QNH at your location. So it will calculate a new QNH based on the measured ambient pressure and the latest altitude.
  

If the watch is in Auto mode, it will try to make a best guess about the reason for the pressure change and switch between Altimeter Mode and Barometer Mode as it sees fit
.
I see nothing strange about this, but if you are puzzled, there must something in the above which collides with your expectations?

If the altitude is already accuired via GPS why would I need a QNH?

Now it is my turn to be puzzled, and I have to ask:
Why did you ask the question in the first place? If your watch already has an altitude from the GPS, and you trust this altitude, why are you looking for a way to input QNH into the watch instead of using the GPS altitude for calibration?

I thought you asked because you would prefer to use a QNH input instead of an altitude input. But then you would not ask this new question.

I live in an area at sea level so no problem for me, but when I am at a location at 2000 feet, I would like to see the barometer indicate the pressure at my location!

This is called Ambient Pressure in a Garmin watch. There is no built-in widget showing ambient pressure, at least not in my F3. But there is an ambient pressure data field which you can enable in activities. And there are at least two downloadable widgets which show ambient pressure:
https://apps.garmin.com/en-IE/apps/1...f-b1c053fd917d
https://apps.garmin.com/en-US/apps/0...2-c9aa00e43df0

Your second reply nails it, a QNH can be obtained from the nearest station with a METAR. Set the QNH in the watch and the altimiter is calibrated (just like a pilot does in his cockpit).

Meanwhile I found the entry for QNH: calibrate barometer, set altitude manually is followed by pressure at sea level.
e

Just to be sure: Does the watch prompt you to input either a known altitude or a QNH? Or does it prompt you to input both of them in succession? The latter option would seem redundant.