Calibrate local barometer

TobiasLj said:

Regarding the pressure and calibration you can check if it is the barometer that is wrong or the algorithm to detect elevation changes over time that is wrong as long as your eTrex can show the ambient pressure. If you have the same difference at start between your eTrex and F6 as you have on the top the barometer is working. Even if it is not calibrated to perfect accuracy it should still work.

I think I got your point.

I just returned from a brief walk, starting at 243 m and ending at the same point at 246. I know the difference is small, but it always happens: I always finish my walks/runs/rides at a higher altitude than the one set at the start.

The Fenix local barometer seems to have the same difference with the professional calibrated one I have at work as before (989.9 vs 989.1), but the sea level value is overestimated by half a hectopascal.

I calculated the sea level pressure at 1015.75 and calibrated the pressure. I just went down a flight of stairs, 3 m correctly detected, but the sea level pressure jumped at 1016.1. 

I really don't know what is going on here: shouldn't the sea level pressure stay constant 10 second after you calibrated it? Otherwise, why would one calibrate it in the first place?

2572376 said:

You may need to look at what altimeter modes there are because if during the activity the weather, or temperature or humidity changes then the barometer will change and hence the altimeter will. Over a short period of time I turn off all auto calibration and put the barometer in altimeter mode. Then all changes in pressure will contribute to an altimeter change.

If you could do this between 2 points of known altitude like land marks, or map contour lines or trig points, then you could check whether it was working properly. Of course your change of altitude should be done within a few minutes and on a calm weather day.

Thanks for your input, I will try to do that. Of course it's quite inconvenient having to change each time from auto to altimeter mode: I never had to change this input in the old eTrex and it always worked like a charm.

Of course, I know no one with a second F6... I know a guy with an Instinct, I may try and compare the two...

Thank you! I understand now the issue here and how the watch barometer works now. 

I got here a bit late, and read through some of the posts, but I don't think you got an answer to your question on adjusting the absolute pressure value provided by the watch.

I don't think its possible to do what you're asking, but I wrote an app to compensate for the difference between my Garmin and another device.

I don't have access to lab grade equipment like you do, but I have other barometers that read to 0.1mb resolution.  I chose one of those as my reference, and adjusted my fenix to that using an app I wrote for the Fenix.  Ever since I've done it whenever I check the two against each other they're usually either exactly matching, or within 0.1mb or 0.2mb at the worst.

The app I wrote allows you to select your elevation reference, and displays the absolute pressure along with the sea-level corrected pressure.  I also have an experimental mode, where I display the pressure to 0.01mb resolution, and provide both the filtered and unfiltered (raw) data from the pressure sensor.  I never use that for much, I just wanted to see how it tracked as I moved the watch up and down a few meters, and how much filtering the watch applies to the data.

If you are interested you could PM me with your e-mail and I could send you the file for the app and you should be able to install it on your watch.

5693709 said:

I'll try to play around with the menu system to see if I can figure out what I did

I know about two "hidden" menus.

Developer menu: Go into System settings->About press the light button 8 times.

System test: Power off, power on while pressing the down button, keep pressing down until the test menu appears. If you press lap button it will cycle through different tests and one of them is the barometer. Don't know if you can calibrate though...

see my edited reply above, about 5 minutes after I posted it I remembered how I did it, and it wasn't native to the watch.  I wrote an app to do it.  I don't think its possible to change the setting in the watch itself.

gspinoza said:

I calculated the sea level pressure at 1015.75 and calibrated the pressure. I just went down a flight of stairs, 3 m correctly detected, but the sea level pressure jumped at 1016.1. 

I really don't know what is going on here: shouldn't the sea level pressure stay constant 10 second after you calibrated it? Otherwise, why would one calibrate it in the first place?

You didn't specify in this post if you had the watch in barometer mode, altimeter mode, or auto mode.  If it was in barometer mode then the sea level pressure would be expected to change by the amount you saw, but the altitude would be expected to remain constant at wherever you had last set it.  If it was in altimeter mode then you'd expect the altitude to change by 3m, which sounds like what happened, but the sea level pressure shouldn't change like it did.  I never use the auto mode, so I don't know what weirdness it might add, but it does seem wrong that both the altitude and sea level pressure readings changed at the same time.

gspinoza said:

I still can't get why my eTrex 30 is so vastly superior to the Fenix when it comes to elevation. Few days ago i went for a hike in the mountains, with both Fenix and eTrex. I started walking at 1900 m, and when I came to a pass at 2706 m the eTrex read - guess what - 2706 m, while the Fenix was reading 2669. 37 m of difference (~ 120 ft), way beyond the accuracy stated by Garmin. I recalibrated the Fenix there, and at the top of the mountain, the eTrex still gave the exact measurement to the meter, while Fenix already was lagging 15 m behind.

I remember having this discussion with you a year ago or so, you're still not understanding how barometric altimeters are designed to work.  The fact that your eTrex was spot on after an 800m climb this time of year means that it was probably using GPS for the elevation information, and it happens to be very good at determining altitude with GPS.  If it wasn't using GPS, then its barometric altimeter is not working as designed, and just happened to be correct by pure luck.

The standard atmosphere used to calibrate all baro altimeters would expect the air temperature at 1900m to be 2.65°C.  That's based on a sea level temp of 15°C and a temperature decrease of 6.5°C per 1000m of elevation gain.  At 2700m the expected temp would be -2.55°C.  I'm guessing when you were hiking a few days ago the temperature was actually higher than that, correct?

If the air temp is higher than the standard atmosphere model expects, then the air is less dense than it expects.  In that case, pressure changes less as you change elevation than it would if the conditions matched the standard atmosphere model.  Let's say the actual temp when you were hiking was 15°C, and the average temp for your altitude range based on the standard atmosphere is 0°C.  A rough calculation in degrees Kelvin will tell us that the density of the air was 273/288 of what the standard atmosphere expected because air density is proportional to absolute temperature (ignoring minor influences like humidity).  Absolute temperature in °K is °C + 273.15.

Now, multiplying the fraction 273/288 by the 800m you climbed, says the altimeter should have interpreted that pressure change to be 758m rather than 800m because of the reduced air density.  That's a difference of 42m, which is very close to your observed 37m difference.  If the air temp were 14°C the difference would be 39m and at 13°C would be 36m.  Your altimeter is working perfectly, it was designed to work under standard atmosphere conditions, but when you go hiking those conditions don't actually exist and you have to know how to compensate for the differences.

5693709 said:

You didn't specify in this post if you had the watch in barometer mode, altimeter mode, or auto mode.  If it was in barometer mode then the sea level pressure would be expected to change by the amount you saw, but the altitude would be expected to remain constant at wherever you had last set it.  If it was in altimeter mode then you'd expect the altitude to change by 3m, which sounds like what happened, but the sea level pressure shouldn't change like it did.  I never use the auto mode, so I don't know what weirdness it might add, but it does seem wrong that both the altitude and sea level pressure readings changed at the same time.

The altimeter was set in auto mode. As a matter of fact, after few moments the sea level altimeter readjusted itself to the value I set before. So what I think happened is that the watch reacted immediately to a change in pressure (when I went downstairs), but after a little while the internal algorithm decided it was not due to a weather change, rather to a change in altitude and adjusted itself.