Altimeter Calibration Height

In other words, an altimeter indicates altitude at the level of the
instrument itself.

That seems reasonable.

But what if the instrument itself is 100 ft tall? Or maybe just 30 inches
tall? Which point on the altimeter should one pick and why?

To answer that question, one must understand that an altimeter does not
sense altitude, it senses pressure. The pressure would be some sort of
average over the entire height of the sensing surface, whose exact value
would depend on the construction of the device. That pressure would
likely be equal to the ambient pressure at some altitude within that range.

By painting appropriate numbers next to the pointer, one could get the
altimeter to indicate altitude. It is the choice of numbers that is the
issue, as that is arbitrary, and not subject to the laws of physics,
only to the laws of the FAA.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

Recently, Steven P. McNicoll posted:

"Neil Gould" wrote in message
t...

After puzzling the difficulties that several of us have had in
trying to explain the differences between "indicated" (e.g. what the
pilot sees) -- and "senses" (how the altimeter is calibrated), I can
only guess that for you, sitting in the tower, there is no practical
difference because you are always at the same altitude. ;-) You
can review some of the excellent explanations that Jose and others
have provided to answer your question.


What made you think I needed the difference between "indicated" and
"senses" explained to me?

Your usage of the two terms.

Let's take a different tack, maybe this will help.

BTDT, and understand the mechanics involved. I do appreciate your
challenges that make me think about this more thoroughly, though. Also,
consider the distinction between calibration and adjustment, where
calibration is how the altimeter is installed, and adjustment is made via
the Kollsman window/knob. Both of these factors can affect the indicated
altitude regardless of the actual altitude of the altimeter.

Neil

Steven P. McNicoll wrote:

Let's take a different tack, maybe this will help.

Yes, great idea.

Take an altimeter and set it on a table. Don't move it!

Turn the knob that adjusts the pressure scale to one extreme. Record
the indication on the face of the altimeter. Now turn the knob to the
other extreme of travel. Record the indication on the face of the
instrument. Now the big question: which value recorded is the altitude
at the level of the instrument?

Matt

"Matt Whiting" wrote in message
...
Steven P. McNicoll wrote:
"Matt Whiting" wrote in message
news

Right and the altitude indicated has little to do with the altitude of
the instrument.


Right. That's why changing the altitude of the instrument, as is done by
climbing or descending, has so little effect on the indicated altitude.


I can just as easily change the Kollsman setting and change the indicated
altitude without changing the altitude of the instrument at all. Or I can
wait for the ambient pressure to change and watch the indicated altitude
change without moving the instrument. Thus the instrument doesn't
indicate the altitude of itself, it indicates the pressure it is
experiencing.


Do you have a webcam? I'd like to see you typing with your foot in your
mouth.

"Jose" wrote in message
t...

This tack has been taken, and shows our understanding of the issue. Yes,
the indicated altitude will change when the altimeter is moved up to the
top of the tower, despite the static port being at the base.


Eureka!



The physics of the instrument is not at issue. Rather, the calibration
rules are, and
those are made by the FAA.


I've said nothing about calibration rules.

"Jim Logajan" wrote in message
.. .

That seems reasonable.

But what if the instrument itself is 100 ft tall?


Then it wouldn't fit in the airplane.

"Neil Gould" wrote in message
...

Your usage of the two terms.


I used them correctly. Perhaps your understanding of them is incorrect.



BTDT, and understand the mechanics involved. I do appreciate your
challenges that make me think about this more thoroughly, though. Also,
consider the distinction between calibration and adjustment, where
calibration is how the altimeter is installed, and adjustment is made via
the Kollsman window/knob. Both of these factors can affect the indicated
altitude regardless of the actual altitude of the altimeter.


One step at a time. Let's see where you are in your understanding of
altimetry before we move on. Answer the questions.

"Matt Whiting" wrote in message
...

Yes, great idea.


I thought so.



Take an altimeter and set it on a table. Don't move it!

Turn the knob that adjusts the pressure scale to one extreme. Record the
indication on the face of the altimeter. Now turn the knob to the other
extreme of travel. Record the indication on the face of the instrument.
Now the big question: which value recorded is the altitude at the level of
the instrument?


One step at a time. Let's see where you are in your understanding of
altimetry before we move on. Answer the questions I posed to Neil Gould.

Steven P. McNicoll wrote:
"Matt Whiting" wrote in message
...
Yes, great idea.


I thought so.


Take an altimeter and set it on a table. Don't move it!

Turn the knob that adjusts the pressure scale to one extreme. Record the
indication on the face of the altimeter. Now turn the knob to the other
extreme of travel. Record the indication on the face of the instrument.
Now the big question: which value recorded is the altitude at the level of
the instrument?


One step at a time. Let's see where you are in your understanding of
altimetry before we move on. Answer the questions I posed to Neil Gould.

Yes, one step at a time. Tell me how an instrument that indicates its
own altitude can indicate two vastly different values without changing
its altitude? It is a very simple question.

Matt

"Matt Whiting" wrote in message
news

Yes, one step at a time. Tell me how an instrument that indicates its own
altitude can indicate two vastly different values without changing its
altitude? It is a very simple question.


Let's answer the questions in the order they were asked.