Altimeter inards

[ZUM]

When adjusting the altimeter setting through the kollsman window, the mechanical linkage is decoupled from the aneroid wafer unit. Is this correct?

[FOD]

Not 100% sure, but you are incorrect from the way I understand it. When you are setting an altimeter setting in the kollsman window, you are basically calibrating the altimeter. The altimeter works by the expansion and contraction of these wafers. When you change altimeter settings you are just adjusting (expanding or contracting) the wafers a little bit, to give them another point in which to start from.

Hope that makes sense, someone correct me if i am wrong!

[ZUM]

I was thinking that might be the case at first, but it seems like there's a problem with that scenario. The ambient pressure inside the altimeter case from the static source is opposing the "springiness" of the aneroid wafers, which are trying to expand. It seems like if you could either compress or expand the wafers mechanically by dialing in the setting, there would no longer be equilibrium between the ambient pressure and opposing aneroid wafers. In this case, I would think that there would be a lag in the reading until equilibrium is re-established. What do you think?

[eojohanns]

The link below has a cutaway drawing of an altimeter (page 15 of a .pdf file)

I don't exactly know the answer to the question, either, but it looks like when you adjust the kollsman you adjust the "initial" pressure on the wafer.(?)

http://www.houstoncfi.com/reference/...3/Chapter2.pdf

[eojohanns]

Going further, is there air at a certain pressure inside the wafers? And by turning the knob you increase or decrease the "initial" pressure of the air inside the aneroid, which then affects how it reacts to outside pressure changes?

[kellwolf]

I believe the wafers are sealed and contain air at standard pressure (29.92).

[eojohanns]

Hate when I don't know the answers to this stuff....check this out, from http://www.auf.asn.au/groundschool/umodule3.html

"main component of such instruments is a small, flexible, corrugated metal capsule, from which the air has been partially evacuated, fitted with a metal closure or diaphragm. There is a spring within the capsule, that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure sensing capsule is transferred and magnified, via a mechanical linkage or piezo-quartz component, to a dial pointer or pointers, or a digital display, which indicate the altitude reading

desirable to set the current local surface pressure into the altimeter by setting that reference pressure into a pressure-setting scale (known since the 1930s as the 'Kollsman Window'), which in turn resets the position of the height indicating pointers against the dial." <font color="blue"> </font>

[USMCmech]

Just an FYI,

A mechanic can't open up any flight instrument and repair it. Only certified repair stations can.

You can mark lines on the glass ect, but you can't open them up. They are all R&amp;R items.

[flyguy]

[ QUOTE ]
I believe the wafers are sealed and contain air at standard pressure (29.92).

[/ QUOTE ]
Kellwolf is correct. When you change the altimeter setting all you are doing is moving the pointer on the face of the instrument through mechanical linkage. So the question is does it "decouple" from the wafter unit while it is being set. If the wafers are sealed at standard pressure (and I know for a fact they are) then any adjustment made in the pointer would then transfer through the linkage to the wafers, expand or contract them, then would spring back once you let go as the wafers are trying to keep 29.92 so it must decouple somehow. I have never learned such, nor can I find it in any textbook, but I think what actually happens is the pointer can "freewheel" as it is being set, but does not freewheel as the linkage from the wafers turns it. Movement initiated at the pointer does not transfer to the wafers but movememt from the wafers does transfer to the pointer. Kind of like a bicycle. If you pedal backward it does nothing but if you pedal forward it transfers the movement to the rear wheel. I could be way off base though, but I do know that the wafers are sealed at 29.92 and there is no way to change that.

[kellwolf]

Okay, I'm bored, so I looked it up. There's a diagrahm in the Jepp Instrument/Comm book that is pretty good (page 2-18 for those that have it). The knob turns the needles and that's it. It doesn't look like it does anything to the aneroid wafers. If it expanded or contracted them, then the static air in the instrument would basically just snap it back to whatever the setting was before. Now, this is what I'm able to figure out from the figure, so I'd still like to hear an explantion of someone more in the know. I know my old school had an altimeter that was cracked open to look at, but that is five states away right now.....

[SteveC]

[ QUOTE ]
..... I do know that the wafers are sealed at 29.92 ...

[/ QUOTE ]

Yet, here is a quote from one of the sources listed earlier in this post (bold added by moi):

[ QUOTE ]
The sensitive altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above a pre-set level rather than atmospheric pressure. The main component of such instruments is a small, flexible, corrugated metal capsule, from which the air has been partially evacuated, fitted with a metal closure or diaphragm. There is a spring within the capsule, that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure sensing capsule is transferred and magnified, via a mechanical linkage or piezo-quartz component, to a dial pointer or pointers, or a digital display, which indicate the altitude reading. The static pressure is drawn from the aircraft's static vent, which may induce slight position errors due to aerodynamic effects around the vent.

[/ QUOTE ]

I would think that if the atmosphere has been partially removed, it is not at 29.92"???

[jknight8907]

[ QUOTE ]

I would think that if the atmosphere has been partially removed, it is not at 29.92"???

[/ QUOTE ]

Unless they manufactured it on an above-standard day...

[planejay]

The wafer is set at 29.92 and stays that way...adjusting the pressure setting in the window does nothing more then "calibrate" the altimeter via gears or linkage. Therefore I would say yes....decoupled. Sorry if this was already mentioned.