I tried a barometric altimeter, seems to work well

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texasbb
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Re: I tried a barometric altimeter, seems to work well

Post by texasbb » May 9th, 2018, 3:51 pm

Bosterson wrote:
May 9th, 2018, 1:34 pm
I also have a Suunto watch, and my understanding is that the watch will display a barometric trend or the current altitude, but not both at once. It's a shame, because I use the altimeter function to assess my location on a map on ridges and such, and it would be nice to also be able to check for pressure changes indicating oncoming storms. I think the Suunto even has a setting where it will detect falling pressure and warn you, but I've never tried it cause I always have the watch set to show the altimeter.
There's really no way a barometric device can show both. When the pressure changes, it has to know somehow whether it was due to an elevation change or a weather change. The only way it can reliably know that is for you to tell it (i.e., by setting the mode). Some of the Suuntos do have an auto mode that tries to intuit whether you're moving so it can set the mode itself (i.e., decide how to account the pressure changes). It works pretty well, but still, it can only be tracking one or the other at any given time.

The storm indicator does work; it's based on the rate of pressure change over some period of time. While interesting, I don't think it's ever told me something I didn't already know. :)

pcg
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Re: I tried a barometric altimeter, seems to work well

Post by pcg » May 9th, 2018, 3:56 pm

Yes, all a Suunto can do is measure atmospheric pressure, the same as the old analog Thommen. The Thommen resembles an analog watch. It has two circular scales and a rotating needle, like a watch hand. The interior circular scale is fixed and always shows you the current atmospheric pressure in inches of mercury, just like the barometer mode of the Suunto. The outer circular scale is rotatable so you can make a specific altitude line up with a specific barometric pressure.
texasbb wrote:
May 9th, 2018, 3:51 pm
Some of the Suuntos do have an auto mode that tries to intuit whether you're moving so it can set the mode itself...
I wasn't aware of that. That seems like an exercise in futility since there is no way for a barometric altimeter can distinguish whether I am stationary in camp (and might want to know barometric pressure) or traveling on a level slope with no change in altitude (and want to see altitude).

You can get a feel for changes in atmospheric pressure due to weather by calibrating often. For example, if you know you are gaining altitude and every time you have the opportunity to calibrate the altimeter you have to adjust downwards a bit because the altimeter keeps showing you higher than you know you are (based on your map), then you know the atmospheric pressure is falling due to weather.

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 10th, 2018, 12:07 pm

pcg wrote:
May 9th, 2018, 6:23 am
Because barometric pressure is constantly changing I calibrate at the trailhead and then every chance I get, at least hourly, using elevation data I've gleaned from topo maps. This isn't necessary for casual hikes, but is sometimes mandatory for more serious bushwhacking, especially in winter, and it has become a habit for me so I do it constantly.
Yeah, Garmin GPS devices theoretically do this constantly (if so toggled), monitoring the pressure as well as elevation readings from the satellites. But manual calibration at the TH can help considerably. Failing to do that can be most apparent when looking at wild differences between EG and EL, especially when the hike starts off with a big change in elevation.
pcg wrote:
May 9th, 2018, 6:23 am
The reading on a barometric altimeter changes due to changes in three things - altitude, temperature, and local pressure changes (weather).
I'm not sure if it's just Garmins that are susceptible, but in extreme wind conditions (presumably "air pressure" by another vector?) the elevation swings get truly wild. Heh, I remember one friend, new to GPS, who was astounded by the EG readings after one particular hike. I was too! So I looked more closely at the track, and found that he had "climbed" nearly 1000' while eating lunch within a summit windbreak. :lol:
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pcg
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Re: I tried a barometric altimeter, seems to work well

Post by pcg » May 10th, 2018, 1:40 pm

kepPNW wrote:
May 10th, 2018, 12:07 pm
I'm not sure if it's just Garmins that are susceptible, but in extreme wind conditions (presumably "air pressure" by another vector?) the elevation swings get truly wild. Heh, I remember one friend, new to GPS, who was astounded by the EG readings after one particular hike. I was too! So I looked more closely at the track, and found that he had "climbed" nearly 1000' while eating lunch within a summit windbreak. :lol:
Wow, that is surprising that it was off by that much, but not surprising that it read high. Atmospheric pressure must have really been dropping - hence the extreme wind. I'm guessing that any GPS that heavily weights barometric pressure data in determining altitude will show faulty altitude readings when windy. The only way I can see that they could remedy this is if the GPS unit could reference elevation data from a stored map and make a correction. Of course, it would have to assume you had your feet on the ground. :lol:

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texasbb
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Re: I tried a barometric altimeter, seems to work well

Post by texasbb » May 10th, 2018, 4:33 pm

pcg wrote:
May 9th, 2018, 3:56 pm
texasbb wrote:
May 9th, 2018, 3:51 pm
Some of the Suuntos do have an auto mode that tries to intuit whether you're moving so it can set the mode itself...
I wasn't aware of that. That seems like an exercise in futility since there is no way for a barometric altimeter can distinguish whether I am stationary in camp (and might want to know barometric pressure) or traveling on a level slope with no change in altitude (and want to see altitude).
It watches the rate of change in the pressure. If the pressure changes more than some threshold within a 3-minute window, it assumes you're moving and switches itself to altitude mode. If the pressure doesn't change more than some threshold within a 12-minute window, it assumes you're stopped and switches to barometer mode. The thresholds are apparently well enough defined that the storm warning mode still works in camp.

It works surprisingly well. If you ever want to see the other mode, it's easy to switch it quickly for a sneak peek. I frequently would hike with it for several days without recalibrating, and only rarely found it excessively off at the end.

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 11th, 2018, 11:11 am

pcg wrote:
May 10th, 2018, 1:40 pm
kepPNW wrote:
May 10th, 2018, 12:07 pm
I'm not sure if it's just Garmins that are susceptible, but in extreme wind conditions (presumably "air pressure" by another vector?) the elevation swings get truly wild. Heh, I remember one friend, new to GPS, who was astounded by the EG readings after one particular hike. I was too! So I looked more closely at the track, and found that he had "climbed" nearly 1000' while eating lunch within a summit windbreak. :lol:
Wow, that is surprising that it was off by that much, but not surprising that it read high. Atmospheric pressure must have really been dropping - hence the extreme wind. I'm guessing that any GPS that heavily weights barometric pressure data in determining altitude will show faulty altitude readings when windy.
Oh no, sorry, it wasn't the elevation that showed it off by that much, just continuous swings from the gusts. I've seen the same along places like Coyote Wall. Check out the final descent on this track, compared to earlier segments. The wind was absolutely howling there!

Capture.PNG
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aiwetir
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Re: I tried a barometric altimeter, seems to work well

Post by aiwetir » May 11th, 2018, 1:56 pm

Where is that barometer being carried/mounted Karl?
- Michael

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kepPNW
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Re: I tried a barometric altimeter, seems to work well

Post by kepPNW » May 11th, 2018, 4:08 pm

aiwetir wrote:
May 11th, 2018, 1:56 pm
Where is that barometer being carried/mounted Karl?
It was hanging on a packstrap. Actually, the windward packstrap. :?

This video shows both where it was, and just how little (much?) wind it takes for that sort of crazy recording...

https://www.youtube.com/watch?v=OyDJRWhWR94
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Re: I tried a barometric altimeter, seems to work well

Post by raven » May 15th, 2018, 10:16 am

The subject is a bit more complex, since a digital device has to infer altitude and their changes by a sampling procedure, whereas you read an analog altimeter directly.

My old analog Thommens repeated altitudes regularly. Caught topo map elevation changes well every time. In summer, when high pressure established itself reliably I rarely had to reset it over a day or two. However, I checked it against maps when I could.

I replaced it after many years and corrosion or dirt beginning to cause sticking when I would go to higher than usual elevations. I decided I should always wear an altimeter. My first purchase was to a Sony altimeter watch and then Avocet altimeter watches that were marketed to skiers and mountaineers. My memory is that I was happier with the Sony than the Avocets, but neither functioned that well. The tell with the Avocets were that they would report my automobile had climbed or dropped during every hike. More than one Avocet, because I thought the first was a faulty unit.

The I got a Suunto Vertex. Before I took it hiking I set it at field elevation with an aircraft altimeter, and shortly after both read identically at about 3000 feet. It's been almost as good as the Thommens in the hills. Perhaps it is not a coincidence that both Thommens and Suunto have -- or at least had at the time I bought that Vertex -- the same parent company.

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