# Altimeter - Barometer relationship explanation please

I recently purchased a new gee-whiz watch that has, among other functions, an altimeter and a barometer. I need this so I can tell how much pressure I can deal with when I’m high.

Anyway, I calibrated the barometric pressure to reflect today’s weather conditions, then went to the altimeter to set it, and found that it had adjusted automatically from about 1600 feet to the proper altitude for this area.

I can’t seem to get my mind around this concept. How does setting the barometric pressure correlate to an automatic change in altitude?

Hey, I just got one of these too. A Christmas present from my girlfriend, bless her for knowing what a gadget freak I am… Cool, aren’t they.

Anyway I have been reading up on this subject a bit lately, so I know whereof I speak. Or type.

The watch has a pressure sensor that measures the current absolute pressure.

Say it is 995 millibars. (You might be using inches of mercury as your units, but the principle is the same.)

You check your local weather report and find that the pressure is, say, 1004 millibars. The weather stations correct their readings for altitude, so what they are giving is the sea level pressure.

So, if you tell the watch that the sea level pressure is 1004 millibars, and it’s sensor tells it that the local pressure is 995 millibars, it thinks “Hey, the pressure is 9 millibars less than at sea level… let’s plug that into the ol’ height equation” and will come up with (rough guess… 1 millibar = 30 feet height difference) a height of 90 feet. Or whatever.

If you are interested, the equation is:

z = (RT/gM).log[sub]e/sub

where z is the height difference between the starting height and the measurement height, R is the gas constant, T is temperature of the air in degrees Kelvin, g is the acceleration due to gravity, M is the molar mass of the gas (in this case air), p[sub]o[/sub] is the atmospheric pressure at the starting height and p is the atmospheric pressure at the measurement height.

This is affected somewhat by the temperature of the air - the greater the difference between actual temperature and “standard temperature” at a given height, the greater the difference in altitude will be.

I don’t know if you have the same watch as me - mine is a Suunto Observer, and it gives an altitude readout to 1 metre… enough to track your progress as you walk up and down stairs. It seems to be unbelievably accurate too, once calibrated. Obviously you need to do this regularly, as changes in air pressure due to weather systems will affect your apparent altitude.

Err, sorry, in my example the calculated altitude would be approx 270 feet, not 90 feet, obviously. That’ll teach me for thinking in metres!

It’s the old “how to measure the height of a building with a barometer” problem. Which, of course, does not involve string to lower the barometer or a stopwatch to time dropping the barometer.
You said you used the current weather conditions to set the air pressure. The current conditions reported by weather agencies and air traffic control towers are not the actual conditions at that spot on the ground, they have been corrected for their relation to nominal sea-level pressure (29.92 inHg, 14.7 psi). Therefore, when you input the barometric pressure, using the standard lapse rate of about 1 inHg = 1,000’ altitude, you have your height above sea level. Or, vice versa.

What r_k said.

Okay, that all makes sense and thanks. By the way, I bought this one from Weather Affects; it’s made by High Gear.

r-k is right. It may help to think about what a barometer/altimeter is measuring: the pressure exerted by (and thus, indirectly, the height of) the column of air above you.

Since that pressure varies, the device is “settable”: If you know and set the altitude, it will tell you the barometric pressure (which is usually the sea-level-equivalent pressure, rather than the true pressure measured by the instrument). If you know and set the pressure, it will tell you your altitude.

If your watch is like mine, you can set it to act either as an altimeter (pressure changes interpreted as changes of height) or as a barometer (pressure changes interpreted as changes of ambient pressure).

The way to use it is then to set the altitude at a known height (eg your house or the trail car park), use the altimeter mode, and then when you are back home or at a camp, switch it to barometer mode, seeing as your altitude will not then change. (And if it does, you have bigger problems than checking your height.) The watch will then monitor atmospheric pressure changes overnight, giving you an warning if a weather front is moving in.

r_k (currently 73 metres above sea level )

My dad had that type of question set when he was at uni, in an important exam…“given a barometer, how would you measure the height of the university tower?” (apparently a local landmark).

His friend wrote “Take the barometer to the top of the tower, drop it, time how long it took to hit the ground, and calculate…”

After failing him, and after much argument, they let him resit that one question, and he wrote “I’d take the barometer to the porter at the bottom of the tower, and say ‘You can have this barometer if you can find out how tall the tower is.’”

[highjack]
Quote by ChefGuy: “I recently purchased a new gee-whiz watch that has, among other functions, an altimeter and a barometer. I need this so I can tell how much pressure I can deal with when I’m high.”

ROFLMAO!! That’s going on my list of interchangeable signature lines!! Thanks!
[end highjack]