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#1823197
They may well have temp compensation built in but the basic principle is the same, the altimeter reads/measures the difference in pressure between inside and outside.
If I’ve got that wrong then I really am going to give up on this one!
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By VRB_20kt
FLYER Club Member  FLYER Club Member
#1823207
The altimeter is just connected to the static port and measures the air pressure by allowing that pressure to compress a set of sealed bellows. It’s a sealed instrument. As the bellows compress and expand they cause the needle to move.

It measures the absolute pressure, not a differential one such as the air speed indicator does.

Essentially it is a plain old barometer.
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#1823209
Ok it doesn’t “measure” anything, it “uses” the pressure difference to move the needle.
If the subscale is set at ISA 1013 at 15c and you climb to an indicated 90 ft AMSL the outside pressure will be 1010, thereby measuring the outside pressure.
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By T6Harvard
FLYER Club Member  FLYER Club Member
#1823400
Thanks @VRB_20kt , got it! I know it was just a barometer and I know cold air is denser but I had also got confused with over/under reading.

Silly really but seems others have had the same initial problem. In the end I drew a diagram to go with the words :)

(I'd also mis-remembered the 'difference' comparing pressures is how ASI works) doh.
#1823425
T6 Harvard.
Don’t worry about it.
I once was so convinced with my theory that I was invited to come up and stand at the front of the class of students and prove on the blackboard that Pythagoras was wrong. Half way through my calculations I saw the light and admitted he was right.
Our teacher congratulated me with “don’t believe it just because I said so, believe it because you understand it”.
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#1867637
Apologies for resurrecting this topic again!

I am a bit confused as well. Somewhere I seem to be making an error. Could I request some help?

For starts: When the question says, the altimeter will under-read/ over-read, what are they referring to? Meaning the altimeter will display a lower/ higher altitude?

Then, as a premise:

    Altimeter (barometer) measures the pressure.
    Pressure: The amount of "weight" exerted, in this case, towards the earth by the air.
    Density: How closely the air molecules are packed together.

    : Higher the temperature, higher the pressure = lower density (?)
    : Colder the temperature, lower the pressure = lower density (?)
Right?

So, going from high temperature (i.e. same as higher pressure/ lower density) to a lower temperature (with higher density, lower pressure), what does under-read mean? Meaning the altimeter will under-read the altitude? How does it under read? That's what I am confused about!

In theory this should cause the barometer wafers to shrink and indicate a lower altitude. But the aircraft may be higher than that, no? Lower pressure should ideally indicate a higher altitude
#1867702
VRB_20kt wrote:At a given altitude an over reading altimeter will read higher than it should. If under reading it will read lower.


Confusion here usually stems from the fact that altimeters work backwards. I mean when the aneroid capsule is experiencing a lower pressure, the number it displays is greater (ie more feet).
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By GrahamB
FLYER Club Member  FLYER Club Member
#1867710
@alexben

Don't confuse yourself by bringing density into your thinking. It comes into play with aircraft performance, but you can ignore it for altimetry purposes.

As in the diagram above, think of pressure values as surfaces in the air, rather like contour lines on a map. The pressure (and volume) of a parcel of air of a given number of molecules at any point is determined by the weight of the column of air above it; the volume it occupies also varies with temperature - the lower the temperature, the less space that parcel occupies as the molecules are less energetic. As a result, on a cold day the pressure surfaces are compressed; on a warmer day they expand upwards.

When you fly at a constant altitude with a constant setting in your altimeter, you are actually flying along one those pressure surfaces, undulating up or down in true altitude terms as the temperature changes.

Your altimeter is calibrated against a standard model of the atmosphere - ISA . If the temperature is lower than ISA, the pressure surfaces you fly along using the altimeter will be closer together than in ISA conditions, hence closer to the ground in true terms than the altimeter is indicating.
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