Altimeter behaviour

[djro 0]

I baught a recently serviced second hand alti 2, it arrived reading 11,500ft so I reset it (and it was very stiff because of the o-ring) then a day later its reading 190ft...

So my two queries are about the o-ring, I remember during my training it was veery easy to adjust the dial and the altimeters are the same model... and the movement of the dial keeping in mind the unit hasn't been touched and my house hasn't risen almost 200ft!

[likestojump 3]

your house may not have risen 200ft, but the ambient pressure may have :)

[djro 0]

Instead of a day later I should have put "later in the day" the only thing that stands out in my mind was a rise in temperature. It was out of direct sun light so is it plausable for the pressure change in an ~4-6 hour period to relate to a 190ft difference?

[bodypilot1 0]

barometric pressure

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[billvon 2,396]

>then a day later its reading 190ft...

Looks like it's going to rain!

[Hellis 0]

The altimeter basicly only tells you how much air is stacked above you right now, and the numbers on the altimeter is just to make them work in aviation.

So what happend was air moved away from you making the air less "heavy", that made the needle move.
If you look on a weathermap you will see H and L, that means highpressure and lowpressure.
Highpressure means you have a high stack of air, and to create balance it flows (wind) towards the lowpressure where there is less air.

So that day you where in the highpressure and the airpressure lowerd because of "things needed to be balanced out", this means your altimeter reacted as if you in the airplane going up (lower airpressure).

And that is the reason you ALWAYS should check your altimeter before geting in the plane

[shropshire 0]

Did you wind it backwards from 11500 to zero or forwards ...... I just can't take all this pressure

(.)Y(.)
Chivalry is not dead; it only sleeps for want of work to do. - Jerome K Jerome

[popsjumper 2]

Where are you located?

Search out an Alti-2 rep and have him/her point you in the direction of someone with a test chamber.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[popsjumper 2]

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Did you wind it backwards from 11500 to zero or forwards ...... I just can't take all this pressure

She likes wearin' lipstick, she likes French cuisine
But she won't let me use my passion unless it's in a limousine.

She got me under pressure,
She got me under pressure.

My reality and yours are quite different.
I think we're all Bozos on this bus.
Falcon5232, SCS8170, SCSA353, POPS9398, DS239

[theonlyski 3]

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I baught a recently serviced second hand alti 2, it arrived reading 11,500ft so I reset it (and it was very stiff because of the o-ring) then a day later its reading 190ft...

So my two queries are about the o-ring, I remember during my training it was veery easy to adjust the dial and the altimeters are the same model... and the movement of the dial keeping in mind the unit hasn't been touched and my house hasn't risen almost 200ft!

That's WHY you can adjust it. Just make sure its on 0 before you get on the plane every time.

As for the adjustment being stiff, the ones you were using as a student probably were a little more 'experienced' than your newly restored one.

"I may be a dirty pirate hooker...but I'm not about to go stand on the corner." iluvtofly
DPH -7, TDS 578, Muff 5153, SCR 14890
I'm an asshole, and I approve this message

[IanHarrop 37]

My Alti-2 is from the 70's and works great both as an altimeter and as a barometer.

When its home, it sits on my kitchen table and I get to see what happens with the air pressure every day.

If it didn't go up and down with air pressure that's when I would be worried about it.

"Where troubles melt like lemon drops, away above the chimney tops, that's where you'll find me" Dorothy

[djro 0]

Thanks all

[DaVincisEnvy 0]

You wouldn't even need to have a low pressure system move toward your area to cause the air pressure to drop. Changes in temperature will do the same thing. Warm air is less dense than cold air, so if the temperature rose during that 4-6 hour time frame, your alti would sense the lower air pressure and would read a higher altitude (lower pressure).

Was it outside or inside during this time period?

[djro 0]

[DaVincisEnvy 0]

Weird. Well, unless a strong storm system was approaching your area, you might want to have it tested. Temperature changes and large-scale atmospheric pressure changes (e.g. approach of a strong low or high pressure system) are usually the only factors that will cause an altimeter to change on that time scale.

[djro 0]

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>then a day later its reading 190ft...

Looks like it's going to rain!

Well it's back to 0 and it did indeed rain

[bofh 0]

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Was it outside or inside during this time period?

Unless the original poster lives in a air-sealed house, or used heavy fans during one of the readings, that does not matter for the air pressure.

[DaVincisEnvy 0]

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Was it outside or inside during this time period?

Unless the original poster lives in a air-sealed house, or used heavy fans during one of the readings, that does not matter for the air pressure.

It matters for temperature. Higher temperature = lower density = lower air pressure. The reverse for colder temperatures is also true.

In a quiescent weather pattern, the surface air pressure can certainly change by the amount described by the OP over a 4-6 hour period due to changes in temperature alone (typically decreasing during the afternoon as temperatures rise). Of course, the approach of a low pressure system could cause a similar (and more profound) change in surface air pressure.

I asked whether his alti was inside or outside because inside, the temperature is rather constant, eliminating that variable as a cause for the observed decrease in pressure.

[Remster 24]

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It matters for temperature. Higher temperature = lower density = lower air pressure.

You think the temperature, and density of 10 feet worth of air (what would be inside) has any significant impact on the pressure coming from 330,000 feet of air on the outside?

Remster

[sundevil777 93]

The temp can of course be different inside vs outside, but the pressure will not be (unless it is a super sealed house).

People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am

[davelepka 4]

Actually sounds like your altimeter is working fine. Keep in mind that 11,500ft, on a 12k altimeter is the same thing as -500ft. The needle didn't rotate clockwise all the way to 11,500 between the shipper and your house, it moved slightly counter-clockwise during the trip as the pressure increased for one reason or another.

As for the stiff adjuster knob, take it the DZ and ask an instructor to look at it for you. If they say it's good, you can take another altimeter with you on the first jump. Compare the two on the way up to see if they 'agree', and do the same in freefall. Make sure you are clear on which is the new one, and which is the 'experienced' one, so in case they read differently in freefall, you know which one to believe (that would be the 'experienced' one).

Also, keep in mind that none of these are 'precision' altimeters. It's common for there to be differences between two good altimeters, with those differences typically becoming greater the higher up you go. So if one is at 12,000, and the other at 12,500, that's normal and you should not adjust them to be equal. As long as they are equal on the ground (both at 'zero') then you're in good shape.

[DaVincisEnvy 0]

You think that a geopotential height increase of 200ft is a significant change in pressure? It's not. That represents a final/initial pressure ratio of ~0.993. That is easily achievable by simple daytime heating and cooling.

And while homes are not air tight, any forced-air heating or cooling system can create temporary positive and negative pressures in the home relative to the outside air. An alti inside such a home and with a constant climate-controlled temperature would not read true (outside) atmospheric pressure. In homes that are designed for constant positive pressure, the buffering against outside pressure fluctuations can be even more pronounced.

Conversely, an alti left out in a sunny spot that heats up to 10-20 degrees above the standard (2m above the ground, in the shade) temperature could indeed experience a slight decrease in pressure due to the formation of a micro-scale low pressure area.

It's the same process that forms the vertical thermals that bounce us around under canopy as we cross paved and other dark land surfaces. Small changes (decreases, in this example) in pressure over a small area due to differential heating of the surface can create enough buoyancy to bounce you you right up to the pucker factor.

And it can certainly deflect an altimeter needle by 200ft.

[BrianM 1]

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in case they read differently in freefall, you know which one to believe (that would be the 'experienced' one).

I'd believe whichever one said I was the lowest, unless it was obvious that it was wrong.

"It's amazing what you can learn while you're not talking." - Skydivesg

[nt8us 0]

if the calibration knob is stiff on an altimaster 2 it is not the O ring at fault. the gear that is pressed on to the knob may have been forced too far onto theknob shaft. however tighter is FAR better than loose,because it is less likely to move the needle if you accidently bump the knob in the aircraft(thus giving a faulty reading)