How altimeter works?

[johnask]

Hello

Can someone answer on this 2 questions:

1)
Knob moves the pointers and numbers in kollsman window together but when you climb or descend,only pointers rotate but the pressure setting in the window does not move.
Can someone describe in detail mechanism when we rotate knob,what happening inside altimeter?
If you better look altimeter is calculator,he allways sustract pressure in kollsman window with pressure where airplane fly,then this pressure indicate in feets..How this "mechanical calculator" do that?

2)
"QFE pressure"
Usually in kollsman window we have 946-1050 hPa scale.What happend if airport is on 4000m and you ask airport for QFE pressure.Airport is very high so pressure on airport will be approx 600hPa..How you will set kollsman window to 600hPa if scale lowest pressure is 946hPa?
Or same question just ask on different way, airplane stay on airport which is on 4000m,if you set altimeter to 0 feet,what kollsman window will show?

[jtt]

Take a tin can and put and cover the top with an rubber foil, sealing it air-tight. If the pressure in the tin can is equal to the outside pressure the foil will be flat. If you take it to higher altitudes where the external pressure is lower, it will start to bulge out more and more..

Next glue a stick perpendicular onto the foil. Take a stick with teeth that can rotate a gear when the stick moves in or out. Glue a hand (like on a clock) to the gear. If the pressur in the tin can is equal to the outside pressure the hand is in the zero position. Go to higher altitudes and the hand moves in one direction, go to lower altitudes and it moves in the other. Here's your (very) basic barometric altimeter.

Now, to adjust for changing external pressures also have a hole in the base of the tin foil, into which a cylinder fits very tightly (so it's all still air-tight). Adjust everything so that the cylinder is half in when you're at 0 feet and with an external pressure of 1000 hPa.

Now, if the pressure on the outside is below 1000 hPa move the cylinder out a bit. This reduces the pressure in the tin can and if you move it out by just the right amount the rubber foil again is flat and the altimeter shows 0 feet. If the outside pressure is higher than 1000 hPA move the cylinder in a bit to increase the pressure in the can by the correct amout. Moving this cylinder in and out is what you do when adjust the altimeter for the current local pressure.

Of course, an aviation grade altimeter will be a bit more complicated, but the basic principle is the same.

Concerning your question 2): You never set the altimeter to show 0 feet at an airport that's at an elevation of 4000 feet, you want it to show you the actual elevation of the airfield above sea-level. Thus the pressure you adjust for on your altimeter is the pressure at an elevation of 0 feet, calculated from the pressure at the airport and its elevation, using the Boltzmann barmetric equation. This pressure will be somewhere in the range between 950 and 1050 hPa - or the weather there is so ****ty that you won't want to be anywhere near to it anyway;-)

[johnask]

jtt;

I know basics how barometers/altimeters works.But I dont understand mechanic relation between pointers and barometric scale,if you rotate knob then both is moving but when flying only pointers moving.So must be some mechanic which allow this separted movement.Also if you set pointers on 0 feet kollsman window will show you local pressure,and if you set sea level pressure(QNH) then pointers will show your elevation.This is amazing,but what is mehanics behind this separated moves!

[jimjim1]

Originally Posted by johnask

1)
Knob moves the pointers and numbers in kollsman window together but when you climb or descend,only pointers rotate but the pressure setting in the window does not move.
Can someone describe in detail mechanism when we rotate knob,what happening inside altimeter?
If you better look altimeter is calculator,he allways sustract pressure in kollsman window with pressure where airplane fly,then this pressure indicate in feets..How this "mechanical calculator" do that?


https://youtu.be/kdFGbUouE_4?t=270

Adjusting altimeter
4m 30s in.
Pprune does not honour the youtube t=270 parameter

You Tube

FLY8MA.com Flight Training
Ep. 58: How an Altimeter Works | Inner Workings
View from start

I thought this video to be very, very good. There are about 100 more episodes in the youtube channel playlist.

TM-55-6610-247-40 Sensitive Altimeter Manual
repair manual for altimeter - very detailed

TM-55-6610-247-40 Sensitive Altimeter Manual

You could buy an old altimeter on ebay.

There will be a friction device on the adjusting knob or somewhere on the gear train.
It is possible that some designs might use a drive that is mostly not reversible e.g. a worm gear.

Here are some other fun mechanical calculators.
https://en.wikipedia.org/wiki/Planimeter
Used to determine the area of an arbitrary two-dimensional shape.

https://en.wikipedia.org/wiki/Difference_engine
Designed early 1800's finally build about 2000 funded by the late Paul Allen.

https://en.wikipedia.org/wiki/Analytical_Engine
Programmable digital computer, described 1837, never built.

[Uplinker]

It is really simple: The pressure sensing vessel is physically connected at one end to the altimeter pointer, and the other end is connected to the setting knob.

The setting knob is geared to move both the zero point of the altimeter, (to set pointer on zero feet), and the pressure reading of the altimeter.

Thus, moving the setting knob changes both the altimeter readout and the pressure readout, but changes in pressure affect only the pressure vessel which only moves the altimeter pointer.

Hope this helps. If I could draw a diagram on here I would.

[meleagertoo]

I am grateful to the OP for introducing me to the Kollsman window, an expression I had never heard until now - after nearly 40 years of commercial flying!