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#101484 - 08/20/08 09:05 AM An altitude question.
12Step Offline
member

Registered: 03/04/08
Posts: 89
Loc: Southwest Ohio
My now wife, as a wedding gift, gave me a watch with an altimeter, compass, barometer etc. When we were on our honeymoon in southern Florida, I was playing around with the altimeter an it always said I was at anywhere around -45 to -50 below. I assume that means below sea level???

I checked it at the beach, a little inland, and hiking in the Everglades and it was always around -50 in altitude. I read that the watch determines the altitude by air pressure. So does that mean that I was -50 below sea level? And if so, shouldn't I have been under water. There are no levies off the coast so I am baffled.

Anyone know the answer to help this novice?


Tom
_________________________
"Let's not miss the beauty of the forest by the ugliness of some of its trees." Bill W.

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#101485 - 08/20/08 09:37 AM Re: An altitude question. [Re: 12Step]
ringtail Offline
member

Registered: 08/22/02
Posts: 2296
Loc: Colorado Rockies
Altimeters need to be reset from known elevations fairly often because of high or low pressure weather systems.

When your camp site gets higher over night the weather is likely to turn bad. If you are below sea level it is likely the weather is good.

An altimeter is helpful for navigation in the mountains. No so much in Florida.
_________________________
"In theory, theory and practice are the same. In practice, they are not."
Yogi Berra

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#101486 - 08/20/08 09:49 AM Re: An altitude question. [Re: 12Step]
Dryer Offline
Moderator

Registered: 12/05/02
Posts: 3571
Loc: Texas
What "food" said was dead on. Think of your altimeter as a barometer. Or the barometer as an altimeter....they share the same circuitry but with revered readouts.
I use my trusty Thommen mountaineering altimeter as both a weather predictor as well as a navigation tool. They are only really useful were you might have 200 foot elevation changes on your hike. Mine is in 50ft resolution but in my part of Texas, I'm only at 681 ft....not too useful. In the mountains, I'll hike that elevation change every few hours! Once you know exactly where you are on the map, re-calibrate your altimeter to the correct altitude, based on what the map says....several times during a day's hike. If the altitude is correct, the barometric pressure will be also, and you can tell what kind of weather might be headed your way.
You use your altimeter in concert with map and compass to help identify where you are....in the middle of a long series of switchbacks, for instance.


So, when you see your altitude below where it should be, the barometric pressure of the surround air is higher than normal. Clear or clearing skies. With the big hurricane coming your way, you should be showing higher altitude than where you are. Low pressure, cloudy skies, storm front heading your way.
_________________________
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#101487 - 08/20/08 07:43 PM Re: An altitude question. [Re: Dryer]
scottyb Offline
member

Registered: 05/28/08
Posts: 278
Loc: Texas Hill Country
Does density altitude come into play?

This type of calculator is used for aviation and motorsports because density altitude affects lift, drag, and combustion. Density Altitude Calculator

Right now at my house at 820 FSL, it is 82 deg F, 28.9 in/Hg, and 67 deg dew point. My density altitude is 3947 ft and my absolute pressure is 28.048.

Another question is how does this relate to hiking at altitude. If an internal combustion engine is affected by density altitude, wouldn't the human body also be affected? I assume it would affect combustion at the cellular level, or not?
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Just because you don't take an interest in politics doesn't mean politics won't take an interest in you.... Pericles (430 B.C)

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#101488 - 08/20/08 08:37 PM Re: An altitude question. [Re: Dryer]
12Step Offline
member

Registered: 03/04/08
Posts: 89
Loc: Southwest Ohio
I'm back in Cincinnati now so we missed the tropical storm. The weather in the Marco Island/Naples area was interesting.

In Cincinnati the altimeter says I'm 240 feet, which sounds about right since I live on a hill and surrounded by foot hills, hence the name of my area, "Western Hills."

The manual with the watch is actually a book, so there is a lot I still need to learn about it. The thermometer was accurate. Hiking the Everglades in Auguest, when the watch says it's 98.4 degrees your eyes burning from your sweat is your body telling your brain that the thermometer is correct.

The one thing that's cool about hiking in the Everglades is if there is a 8 foot gator 4 feet from the trail, A) it is perfectly normal. B) You just walk right past it.

Tom
_________________________
"Let's not miss the beauty of the forest by the ugliness of some of its trees." Bill W.

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#101489 - 08/20/08 10:27 PM Re: An altitude question. [Re: scottyb]
Dryer Offline
Moderator

Registered: 12/05/02
Posts: 3571
Loc: Texas
I'm a pilot and have worked plenty of density altitude tables...here in hot Texas. In aviation, density altitude effects engine performance and takeoff roll because you are working with air density.
Hiking altimeters are "temperature compensated" so yes, I believe density altitude is taken into consideration but is not anywhere near as important as in aviation. In hiking, you are looking for the physical altitude, not a 'relative' altitude caused by temperature. Am I saying this right? <img src="/forums/images/graemlins/crazy.gif" alt="" />
_________________________
paul, texas KD5IVP

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#101490 - 08/21/08 04:10 AM Re: An altitude question. [Re: Dryer]
scottyb Offline
member

Registered: 05/28/08
Posts: 278
Loc: Texas Hill Country
My point is, if you are hiking at 820 FSL, but the density altitude is 3900 FSL due to temperature, barometric pressure, and humidity; is it the equivalent to hiking at 3900 FSL? An IC engine aspirates air to burn fuel and our lungs aspirate air to metabolize fuel (food) at the cellular level.
_________________________
Just because you don't take an interest in politics doesn't mean politics won't take an interest in you.... Pericles (430 B.C)

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#101491 - 08/21/08 04:50 AM Re: An altitude question. [Re: scottyb]
Dryer Offline
Moderator

Registered: 12/05/02
Posts: 3571
Loc: Texas
My guess is....yes, that's how high you are. Your body is at 3900 fsl (full service level? or mean sea level...) at standard temp and pressure.
_________________________
paul, texas KD5IVP

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#101492 - 08/21/08 06:48 AM Re: An altitude question. [Re: Dryer]
finallyME Offline
member

Registered: 09/24/07
Posts: 2710
Loc: Utah
I agree with dryer. Since the "combustion" of your body, ICE or even gas turbine are dependent on temperature and pressure, the actual altitude doesn't matter. It is the relative altitude. Maybe you could compare it to the difference between absolute pressure and static pressure? <img src="/forums/images/graemlins/smirk.gif" alt="" /> <img src="/forums/images/graemlins/smirk.gif" alt="" /> <img src="/forums/images/graemlins/wink.gif" alt="" />
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I've taken a vow of poverty. To annoy me, send money.

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