Altimeter Question

Sidious

Sidious

Well-Known Member
Alright I've got two questions that Ive got my mind bent around right now.....

What is the pressure of the aneroid wafers in the altimeter? Ive heard 29.92, Ive heard it doesnt matter.

And.... How does the altimeter know how far above sea level we are?

Because to me, when we put the local altimeter setting in the kollsman window it should read zero since we are at that pressure level. But it knows.

Is it just the gears, linkage or equations that allow it to know? Or is it the fact that the wafers have a standard pressure on which to base the calculations.

Thanks and please list sources where you got the info!
 
Think of the aneroid wafers like a Kit Kat bar. A layer of chocolate then cookie, chocolate.... By changing the Baro pressure in the Kollsman window, you are chaging how much the cookie is squished when referenced to the standard datum plane or 29.92

Like the magic 8 ball, the altimeter knows all because of....magic!

My source was my brain. I wish I could tell you where I derived the source, but I am at home, sleepy and have no resources available but the internet and lord knows when I google something I generally get a topic totally unrelated
 
Sidious said:
Because to me, when we put the local altimeter setting in the kollsman window it should read zero since we are at that pressure level. But it knows.
Click to expand...

What you're inputting is not the pressure at the station, but the pressure at sea level, regardless of where the station is. If you were to input the actual barometric pressure of where the airplane is, it would read zero.
 
tgrayson said:
What you're inputting is not the pressure at the station, but the pressure at sea level, regardless of where the station is. If you were to input the actual barometric pressure of where the airplane is, it would read zero.
Click to expand...


Ahh I just read this info and it says something to the effect that the altimeter setting in the ATIS, AWOS etc. it pressure altitude corrected to sea level pressure like you've said.

Thats where the hang up was! Now about those wafers...
 
Sidious said:
Ahh I just read this info and it says something to the effect that the altimeter setting in the ATIS, AWOS etc. it pressure altitude corrected to sea level pressure like you've said.

Thats where the hang up was! Now about those wafers...
Click to expand...

seriously, think of it like a kit kat bar accordion. it expands and contracts as a result of ambient pressure. when it expands with a decrease of pressure because of a climb, the link arm moves, much like the push rod in the aileron. That push just moves a dial and the face pointer of the altimeter reading.
 
mmmmmm, KitKaat

HomerDrool.gif
 
Sidious said:
Ahh I just read this info and it says something to the effect that the altimeter setting in the ATIS, AWOS etc. it pressure altitude corrected to sea level pressure like you've said.

Thats where the hang up was! Now about those wafers...
Click to expand...

"I THINK"
Aneroid wafers don't measure the air pressure, they just expand and contract as the pressure around them (inside the instrument case) change! so.. they don't have to be containing 29.92 air pressure.
They're calibrated to show zero at sea level in standard condition. so whatever pressure the wafers had to have to get zero.
 
AC00-6A- Aviation Weather
Chapter 3
The Aneroid Barometer
Essential features of an aneroid barometer illustrated in figure 9 are a flexible metal cell and the registering mechanism. The cell is partially evacuated and contracts or expands as pressure changes. One end of the cell is fixed, while the other end moves the registering mechanism. The coupling mechanism magnifies movement of the cell driving an indicator hand along a scale graduated in pressure units.

Figure 9. The aneroid barometer. The aneroid consists of a partially evacuated metal cell, a coupling mechanism, and an indicator scale. The cell contracts and expands with changing pressure. The coupling mechanism drives the indicator along a scale graduated in pressure units.
Click to expand...
Partial vacuum like TG is saying
 
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