Altimeter Fenix 5

I have my third Fenix 5 sapphire for a month, now. I had it replaced twice because of several HRM issues that I won't discuss here, but also because of the faulty altimeter. I live in the Netherlands, metric system, and in our country almost everything is between -10 and +150 meters A/B Sea Level. I have been at -18K and at 20K last week. The 4 hour graph looks hilarious. I reset the watch to factory setting last saturday and entered my home's altimeter (6m ASL). I was home ill the last 3 days so the watch hasn't left the house but is now at 24m ASL. It does not jump from - xK to + xK anymore, which implies that the behavior is sofware related (re the reset). But still, I am definitely not at 24 meters, my house is only 10 meters high.

I don't want to send in my watch again for a replacement, all other units have shown similar issues and from what I read in the thread I am not the only one. How can we get Garmin to take this issue to top priority? This is a 700 dollar watch and I would expect better results.

I reset the watch to factory setting last saturday and entered my home's altimeter (6m ASL). I was home ill the last 3 days so the watch hasn't left the house but is now at 24m ASL.

This is exactly as it should be. It is not caused by an error in the watch. It is caused by users not understanding what a barometric altimeter can and cannot do.

A change of 18 meters is equal to an air pressure change of 2 millibar which is next to nothing. Where I am (Denmark, some hundred km from your location), the ambient air pressure has changed by 9 millibars over the last 48 hours.

When the air pressure at your location changes, the watch has to guess if it happened because you went to a different altitude, or it happened because the air pressure at your altitude changed. The watch has no way of knowing this for sure, so it has to guess.

If you want better accuracy, you have to give the watch some input about the actual local air pressure in your area right now - which is exactly what you do when you make a manual calibration. And when the air pressure in your area changes, you will have to do this again. And again the next time. Or accept the guesses which the watch made for you.


(This does not explain your issues with altitude changes of several kilometers. Those are caused by an error in measuring the ambient pressure.)

(This does not explain your issues with altitude changes of several kilometers. Those are caused by an error in measuring the ambient pressure.)

What is a reasonable accuracy for the pressure sensor? My F5, which has had the same issues as others with ambient pressure fluctuations (so I already know the pressure sensor is messed up) when it does become stable, is still off by around 25-30 mbar compared to the pressure at the local station. I'm just wondering so I'll know what to expect for when I get my new F5.

I live about 1.2 miles from the station and my elevation only differs by 60ft for reference.

Thanks!

What is a reasonable accuracy for the pressure sensor? My F5, which has had the same issues as others with ambient pressure fluctuations (so I already know the pressure sensor is messed up) when it does become stable, is still off by around 25-30 mbar compared to the pressure at the local station. I'm just wondering so I'll know what to expect for when I get my new F5.

When you say "ambient pressure", do you really mean ambient pressure, or do you mean the Barometric Pressure reading on the watch?

Ambient pressure should certainly not be off by that much. My F3 is within 1 millibar of the nearest weather stations. However, when you compare to a nearby weather station, it is important to know if that weather station is reporting ambient pressure at the station's altitude, or it is reporting equivalent pressure at Mean Sea Level. In my area, the weather stations report the latter, so I have to do an altitude correction before I compare the weather station to my watch.

Barometric Pressure on the watch is just an artificial number which is calculated from ambient pressure and assumed altitude. It is assumed to be equivalent pressure at Mean Sea Level, but if the assumed altitude is wrong, Barometric Pressure is also wrong. So don't look at Barometric Pressure until you have Ambient Pressure verified.

When you say "ambient pressure", do you really mean ambient pressure, or do you mean the Barometric Pressure reading on the watch?

Ambient pressure should certainly not be off by that much. My F3 is within 1 millibar of the nearest weather stations. However, when you compare to a nearby weather station, it is important to know if that weather station is reporting ambient pressure at the station's altitude, or it is reporting equivalent pressure at Mean Sea Level.

When I say ambient pressure I mean actual ambient pressure on the watch, which I have set to display in one of my activities, not barometric pressure.

I checked and the station was actually reporting MSL pressure on their page but when I checked the station pressure, which was reported elsewhere, it was much closer to my F5 (off by < 1mbar).

So it appears the pressure sensor, when it isn't behaving wildly, is accurate. However there's still the issue of the random wild shifts of ambient pressure for which I'll return the watch.

Thanks for the additional insight.

Have been using my 5S since last Friday. The first few days, without any calibration, the altitude measurement was often about ~40 meters when it should have been about +10.
On Saturday, when I had a 2.5 h hike in the near woods, it showed about -140 meters when it should have been about 25 meters; Today, when I woke up and switched on the watch, I enabled auto calibrate and did the calibration via GPS.
So far, being at work, it shows 10 meters, which is correct according to the available local altitude maps.
All in all, I do not really need for the altitude measurements to be reliable; however, if the numbers start being off by several hundred or more, it could be unpleasant merely because the device is not cheap and should be reliable.

however, if the numbers start being off by several hundred or more, it could be unpleasant merely because the device is not cheap and should be reliable.

Once again: This is not an error of the watch. You need to understand how a barometric altimeter works: The altimeter is chasing a moving target.

No matter how precise the watch can measure ambient air pressure, you will see huge altitude variations because the air pressure at your altitude has changed.

Over the last few weeks I have seen pressures around 980 millibar and 1030 millibar - at the same altitude and location. These pressure changes did not just happen in my watch. I had them confirmed by looking at data from the local weather stations in my area. So these pressure changes are real. Each millibar is equal to an altitude change of 8-9 meter. So the difference between 980 and 1030 millibar is equal to an altitude change of more than 400 meter!

How should the watch know which reference pressure to use this afternoon? You need to tell it, and you do that by calibrating your altimeter.

In other words: When you see the altitude in the watch drifting by some 10s or 100s meter, it is not because some hardware inside the watch is getting inaccurate. It is because your surrounding conditions are drifting.

Thanks for the clarification, greatly appreciated.
So, woudl calibrating the altimeter every day (for example, when I turn the watch on and put it on as soon as I wake up) be normal and appropriate?

EDIT: Read your earlier post about manual calibration, thanks again, it all became much more clear.

Thanks for the clarification, greatly appreciated.
So, woudl calibrating the altimeter every day (for example, when I turn the watch on and put it on as soon as I wake up) be normal and appropriate?

I have setup my watch to autocalibrate whenever I start an activity with GPS. Which is 2-4 days apart.

Inbetween activities the watch does some guesswork. Whenever it sees an ambient pressure change, it tries to decide if that change came from a real altitude change or from a change in pressure at unchanged altitude. In my case, this guesswork is precise enough. I rarely see altitude errors of more than 20 meter. But this will be very dependent on your typical movement pattern. It is probably also dependent on which watch and firmware you are using since the algorithms for doing this guesswork could be polished by the developers forever.


If you use the autocalibration, there are a few tricks you can do to make sure that it is as accurate as possible, see link below. It requires that you know the altitudes of the locations where you will usually start an activity. GPS altitude measurement has a considerable error so just using the GPS altitude for calibration will also cause a lot of variation.

https://forums.garmin.com/forum/on-the-trail/wrist-worn/fenix-5-5s/1288285-how-to-set-the-barometer-as-a-weather-indicator-tool?p=1288756#post1288756

Please note: I use a Fenix 3. The overall working principles should be the some on the F5, but there may be differences.

Since I am getting a chest HRM strap today, I will be doing more outside activities (hopefully, since I am recovering from a broken ankle). Will try to remember to post any interesting observations regarding the issues discussed here.
Once again, thanks for your effort clarifying these things!