the pressure inside the aneroid wafer

taseal · May 3rd, 2008, 01:44

anyone know what it is? I have heard 3 things, which noboby able to give me sources. I heard, vacuum (negative pressure? but what? and where is this?) 29.92, and standardized pressure.

so does anyone know? and do you have a source for it? FAA books don't have it, POH doesn't have it, and the Jepp systems book doesn't either....

I'm ready to call an instrument maker and find out...

if you wonder why I was curious. it was just one of those things that came up and everyone started making their own assumptions.

Sidious · May 3rd, 2008, 01:51

Everything that I have read says the aneroid wafers are "partially evacuated"

However, sources aside it doesn't matter what the pressure inside them is. As long as the entire mechanism is calibrated correctly to sense the change in pressure then it will work no matter what the pressure on the inside is.

taseal · May 3rd, 2008, 01:54

Quote:

Originally Posted by Sidious

Everything that I have read says the aneroid wafers are "partially evacuated"

However, sources aside it doesn't matter what the pressure inside them is. As long as the entire mechanism is calibrated correctly to sense the change in pressure then it will work no matter what the pressure on the inside is.

ok, so if I understand, partially evacuated means means vacuum

so when a fresh new altimeter comes off the factory, its vacuum. question is, where did you see this 'partially evacuated'?

edit: just searching that word with addition of altimeter I came up with this:

http://www.freepatentsonline.com/4255970.html

still looking into it.

bdhill1979 · May 3rd, 2008, 02:01

If I remember rightly it is worded as "partially evacuated" in the instrument flying Handbook. I think it also says that the disk is Bronze or a bronze alloy.

Too tired/lazy to look it up though.

JDMcFly · May 3rd, 2008, 02:07

This is completely off topic, but all I can think of now are those little dry, vanilla/chocolate wafer things.

Thanks.

taseal · May 3rd, 2008, 02:22

Quote:

Originally Posted by bdhill1979

If I remember rightly it is worded as "partially evacuated" in the instrument flying Handbook. I think it also says that the disk is Bronze or a bronze alloy.

Too tired/lazy to look it up though.

yeah, its not in there

well its 10Kpa below standard ath. pressure. 10Kpa converts to 2.95inHG@0c, so that means at 8,000 on a standard day the pressure inside the aneroid wafer is 18.9 which makes it partial vacuum since its lower then the pressure of outside.

damn that was confusing as #### when I had to convert all those figures haha.

however, that is for 'aneroid barometer' and not specifically aneroid wafer for altimeter' but its close enough, and its the same concept. so its def partial vacuum.

taseal · May 3rd, 2008, 02:23

Quote:

Originally Posted by JDMcFly

This is completely off topic, but all I can think of now are those little dry, vanilla/chocolate wafer things.

Thanks.

so go get some

tgrayson · May 3rd, 2008, 02:25

Quote:

Originally Posted by taseal

nobody able to give me sources

My book "Aircraft Systems", by David Lombardo has a drawing with the wafer labeled as "evacuated diaphragm capsules" on page 32. Another book, "Aircraft Systems for Pilots", by Dale Remer, says on p. 391 that the bellows are "evacuated and sealed."

The Instrument Flying Handbook , under Pitot-Static Instruments, Sensitive Altimeter says

A sensitive altimeter is an aneroid barometer that measures the absolute pressure of the ambient air and displays it in terms of feet or meters above a selected pressure level.

Notice is says "absolute pressure"? That means relative to zero pressure, which would be vacuum, again suggesting an evacuated chamber. Anyway, it later says:

Principle of Operation
The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules like those shown in figure 3-3. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.

Later, in the definitions, it says

aneroid. The sensitive component in an altimeter or baro-meter that measures the absolute pressure of the air. It is a sealed, flat capsule made of thin disks of corrugated metal soldered together and evacuated by pumping all of the air out of it.

However, Aviation Weather says

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.

I don't consider "partially evacuated" to contradict "evacuated", because all vacuums are "partial".

That enough references?

taseal · May 3rd, 2008, 02:28

Quote:

Originally Posted by tgrayson

The Instrument Flying Handbook , under Pitot-Static Instruments, Sensitive Altimeter says

Notice is says "absolute pressure"? That means relative to zero pressure, which would be vacuum, again suggesting an evacuated chamber. Anyway, it later says: (taseal: this part I don't understand how its relative to 0 pressure?)

Principle of Operation
The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules like those shown in figure 3-3. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.

that was the best one. going to get my inst book to find it. that was all I neeed

tgrayson · May 3rd, 2008, 02:29

Quote:

Originally Posted by Sidious

sources aside it doesn't matter what the pressure inside them is. As long as the entire mechanism is calibrated correctly to sense the change in pressure then it will work no matter what the pressure on the inside is.

Except that pressure would change with temperature.

taseal · May 3rd, 2008, 02:33

cool. just checked. I guess I missed the word 'evacuated' because I was looking for 'vacuum'.... oh well. thx guys, this is good stuff.

thats why I love this board

taseal · May 3rd, 2008, 02:34

Quote:

Originally Posted by tgrayson

Except that pressure would change with temperature.

if the aneroid barometer concept is the same, i figured out at 0c, the pressure inside is 18.97

dakovich · May 3rd, 2008, 12:20

the answer is 9.

9 beers and the pressure on the aneroid wafer becomes intolerable and must be partially evacuate. 9 beers more and partial turns into complete evacuation of the wafer, along with all other solids and/or liquids accompanying said wafer.

darrenf · May 3rd, 2008, 12:37

Quote:

Originally Posted by JDMcFly

This is completely off topic, but all I can think of now are those little dry, vanilla/chocolate wafer things.

Thanks.

I was going to say the same thing.

OldTownPilot · May 3rd, 2008, 13:15

Quote:

Originally Posted by dakovich

the answer is 9.

9 beers and the pressure on the aneroid wafer becomes intolerable and must be partially evacuate. 9 beers more and partial turns into complete evacuation of the wafer, along with all other solids and/or liquids accompanying said wafer.

18 beers and all the worlds problems are solved!

Drink up everyone!