Questions regarding altimeter settings and density

J

jmartinezm

Well-Known Member
Ok so I've been trying to grasp the concept of how the altimeter settings work and everything, and I've come to figure out the altimeter is the amount of pressure the outside environment is putting out(heeheehee) which makes mercury either rise or fall, and (not sure how it's actually implemented into the altimeter in the aircraft, but I'm guessing it's the same concept) that's why we need to set the altimeter every time we get into the cockpit. The thing I don't get is the standard 29.92 and how you know when you need to change it..

Let's assume denver is exactly 5,000 feet above sea level, and pressure decreases approximately one inch every thousand feet, so if the conditions are standard at sea level, the altimeter should be 29.92. If the conditions are standard in denver, the altimeter should be 5 inches less, which takes it down to 24.92. I'm assuming that you don't set the altimeter to 24.92 in standard conditions in denver, so it has to be adjusted somehow. Is 29.92 the standard altimeter for every location in standard conditions? Say you were in Denver, and the AWOS is saying that the altimeter is 29.92, and we know the altitude of denver is 5,000 feet. What causes the altimeter to read something different at denver than somewhere at sea level at 29.92? My theory is that the actual Hg level in denver is 24.92, but the standard is 29.92, and the 5 inches is assumed to be taken into account already, thus, calling the altimeter to be 29.92 in denver and represented (somehow) on the altitude indicator.

Also, if my theory is true, say the altimeter in denver would be 30.00, which would be 8 above standard, so would the actual pressure level be 25?

Also! When does air get dense or less dense? Hot makes air less dense, and cold weather makes air more dense? Or is it the other way around?
 
The ATIS will have the altimeter setting (if the airport has one). I usually set it by field elevation before I take off and verify with ATIS info. I haven't had it be different yet. ;) The controllers will also give you the new local setting if you use flight following or IFR as you fly along. If you stay in Cessna type planes you will not get to the point that you use 29.92. If unsure you can ask ATC for a current setting.
 
I'm familiar with the setting of the ATIS' altimeter setting and putting it in and everything, but what I don't get (and I'm pretty sure I didn't explain it correctly!) is if Denver if 5,000 ft above sea level, why isn't the altimeter 24.92 instead of 29.92 in standard conditions, given the fact that the pressure goes down an inch per 1,000 ft in altitude. :D
 
You're thinking too much. Denver isn't at sea level, therefore 29.92 wouldn't be standard for an airport at 5,000 feet. Standard conditions at Denver would have your altimeter indicate 5,000 MSL. Know what I mean?

Hot air is less dense by the way. Think of the cold air molecules trying to huddle together for warmth, i.e. more dense. That's why youll notice better engine performance when it's cooler out. More air for the propeller to rip into and more dense air getting into the engine for combustion and cooling.
 
Right! But if the current altimeter is 30.11 in denver, wouldn't that only increase the altitude by 190 feet? I guess my question is, how does the altimeter know to indicate 5,000 feet with the altimeter, if it should be at 24.92 to make up for that 5,000 gain and pressure decrease? Sorry if I'm thinking too much, I get OCD about things I want to figure out and just keep trying to figure it out!

Edit: sorry, it should've been decrease by 190 ft!
 
Read the Pilot's handbook of aeronautical knowledge. It has it all explained with pretty pictures.
 
jmartinezm said:
Right! But if the current altimeter is 30.11 in denver, wouldn't that only increase the altitude by 190 feet? I guess my question is, how does the altimeter know to indicate 5,000 feet with the altimeter, if it should be at 24.92 to make up for that 5,000 gain and pressure decrease? Sorry if I'm thinking too much, I get OCD about things I want to figure out and just keep trying to figure it out!

Edit: sorry, it should've been decrease by 190 ft!
Click to expand...

Assuming you subtracted 30.11 from 29.92 and got 190....that would be added to 5,000....not sea level.

haha I think you're confusing me now. Practical application: on the ground at denver set it to 29.92 when the temp is 15C, you're gonna get 5,000 msl. This is what you want it to show.

The altimeter only "knows" by the altimeter setting you set in the Kohlsman window...spelling?

Basically, in denver you dont give a shiz about what's going on at sea level. The area around you is 5,000 feet above sea level. You're still at 0 AGL and that's where the importance of alt. settings comes in: Keeping you away from the ground (not sea level).

p.s. feel free to ask me more....I'm working on my CFI so it's good practice. Just listen to your actual instructor if he tells you anything different from what I've said.
 
Alright, so now my question is why would an altimeter show a different altitude at a sea level airport, if set to 29.92, than it would at denver, if it too is set to 29.92? Like, what causes this drastic change? lol ;)

Ohhhhh, ok, I think I get it, the altimeter if 15C at Denver, set to 29.92 wouldn't show 5,000? It would show you AGL? or this completely wrong?
 
Alright, so now my question is why would an altimeter show a different altitude at a sea level airport, if set to 29.92, than it would at denver, if it too is set to 29.92? Like, what causes this drastic change?

The differences in pressure and temperature act on the instrument. At 5,000 MSL there's likely less pressure that there would be at the beach. You're alt. will detect that difference.

Altimeters always indicate MSL. 15C at denver with 29.92 would indicate 5,000 MSL. You're on the ground though...So, 0 AGL. If the traffic pattern is 1000 AGL, you're flying 1000 feet over terrain, yet at 6,000 MSL.
 
Ok nevermind, back to the old concept, ok, so the altimeter does actually pick up the change in pressure and temperature? Well of course it would, why else would it need the static ports which do just that... silly me! -_-
 
jmartinezm said:
Ok nevermind, back to the old concept, ok, so the altimeter does actually pick up the change in pressure and temperature? Well of course it would, why else would it need the static ports which do just that... silly me! -_-
Click to expand...

Most definitely. You just fine tune the instrument for nonstandard temp and pressure with the setting.
 
No prob. It can be pretty abstract thinking since we can't see how the density changes in relation to alt and temp.
 
Here's my take on it, maybe it'll clear things up for you.

Your altimeter has aneroid capsules that basically measure the outside pressure, and display it on your altimeter. The knob you twist simply adjust for today's pressure, so you get the correct altitude on your altimeter.

Here's an example: you takeoff from Miami (approximately sea level) with altimeter set to 29.92. Think of your altimeter when you get to 5,000 ft. It shows 5,000 but did the settings (29.92) change? Of course it didn't.

Hope that help
 
hahah Dude, your avatar is sooo ugly. I just pictured it teaching what you just wrote....of alllll of the possible avatars...really???
 
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