Altimeter?

[ryanb914]

How does altimeter change? One day its 3024 the next day its something totally different. Please answer this for me, Thanks! :)

[TopBunk]

Maybe it's something to do with the earth's rotation -or is that too high a level for you?

[Mad (Flt) Scientist]

An altimeter is basically a pressure sensing device. So is a barometer, which can often be found in houses marked with various expected weather according to the sensed pressure. You'll have heard weather forecasters talking about low pressure and high pressure systems, and the change in weather that results. So, atmospheric pressure changes as the weather changes. (Why it does so is another topic in itself).

Back to the altimeter. We want to know our altitude, but what we are measuring is pressure. Pressure varies with altitude, and there is a "standard" atmosphere definition for the relationship. But that only works on a "standard" day. So if it's a non-standard day - due to weather - then you have to in effect re-calibrate your altimeter for today's weather. That's what the pressure setting is that you are seeing as changing from day to day - it's today's calibration setting so that the alimeter gives an altitude which agrees with your "real" altitude.

[RAC/OPS]

Just remember ryanb914, there are no such things as stupid questions, just stupid answers. Admirably demonstrated by TopBunk.

[ryanb914]

Ok thanks, I know alot of stuff, just didn't understand how that worked, thanks!

[desmondnnk]

what Mad (Flt) Scientist said was exactly right.

here is my 2 cents

the static pressure is fed to a container, surrounding a cannister with constant internal pressure, so as the static pressure varies with altitude , the cannister contract or expand to generate the altitude reading of the aircraft.

pressure can varies on ground as well(due to weather, locations etc.) , that is why you could get different readings even on ground and pilots needs to zero the instrument on ground.

other technologies might have been used, but i hope this can help you visualise the basic mechanism a little bit.

cheers
D

[de facto]

Slight practical drift:8
.HIGH to LOW watch out below as your true altitude will be lower than your indicated if your altimeter setting is not changed.
.low pressure at take off,switching to standard pressure, your aircraft altitude will increase ,this with a low altitude level off shortly after transition altitude and here comes the LEVEL BUST.:E

Fully agree, no stupid questions,only stupid answers.

[eckhard]

Hi ryanb914.

I also subscribe to the 'no stupid questions' camp, so well done for asking this one!

Judging by your wording of the question, and your location in Chatanooga, I'm guessing that by 'altimeter' you actually mean 'altimeter setting'.

This is aften heard in the USA, as in: "Cessna 457TG, taxi to runway 35, the Chatanooga altimeter 2995".

This means that the altimeter should be set to 29.95 inches of mercury (inHg) in order to read correctly at Chatanooga airport.

As you point out, the 'altimeter' can change from day to day, or even from hour to hour. It is a measure of the atmospheric pressure (reduced to sea level) at the airport in question. If correctly set, the altimeter will indicate the altitude above sea level when on the airport, i.e. the airport elevation. When flying in the vicinity of the airport, the altimeter will continue to indicate the aircraft's altitude above sea level, which means you can fly at the appropriate pattern altitude, or at the desired altitude for a cross-country flight.

Of course, this indicated altitude will be subject to errors (as previously explained) due to instrument, static source, air temperature and other variables. The most important error is usually the setting of the sub-scale (barometric error). This can be due to pilot mis-hearing, mis-setting, mis-reading, or can be due to the fact that the local atmospheric pressure has changed and the altimeter setting has not been adjusted to reflect this.

So, to get back to your question: "How does altimeter change?" The answer is that the atmospheric pressure patterns will change due to solar heating, seasonal variation, terrain and wind effects. As these patterns flow across the surface of the earth, the atmospheric pressure at any given airport will change. You can see these patterns of isobars on most weather maps.

Hope this helps.
Eck