Altimeter accuracy

[riddler 0]

I'm reading an old skydiving book from the 60s, and the author states that the altimeters aren't very accurate for the first 500 feet.

Book is "Skydiving - the Art and Science of Sport Parachuting", by Bud Sellick, 1961.

"The altimeter won't tell the correct altitude for the first five or ten seconds of acceleration. By the time you reach terminal velocity it should read correctly."

Is this true of older altimeters, but not true of modern ones? Or is it something that still holds for modern altimeters? If so, why?

Trapped on the surface of a sphere. XKCD

[trigger 0]

.CHOP WOOD COLLECT WATER.

[trigger 0]

How did that happen?
Altimeters,i guess with time things only get better[progress]
Not really sure about the anologue alti's but speaking to a friend/mentor today i'm virtually sold on the idea of using either a Suunto or Digitude with regards to canopy swooping.

.CHOP WOOD COLLECT WATER.

[Chrisky 2]

Well, as the normal standard alti works barometric, the problem should still be there. if you look at you Pro-Track records, it takes 5-7 second to measure the data correctly, too.
The cause might be the sudden change in pressure?
Also, barometric altimeters tend to have a certain amount of mis-showing, that's why they usually come with a testing certificate showing the precision and misshowing at different pressure levels. I.e., my baro shows 4200 at 4000 meters, while beeing exact at 1500 and being low -100 at minus 1000 (whoever needs that, i wonder). My Suunto is pretty precise, much like the Pro-Track.

The mind is like a parachute - it only works once it's open.
From the edge you just see more.
... Not every Swooper hooks & not every Hooker swoops ...

[riddler 0]

Quote

barometric altimeters tend to have a certain amount of mis-showing

True - they can easily be off by as much as 500 feet. Some also have a tendency to read high, and others read low.

But this author is specifically saying that altimeters - at least from the 1960s - don't read accurately until you reach terminal velocity. I'm wondering if this is still true of modern ones. I'm pretty sure older skydiving altis were barometric as well.

Trapped on the surface of a sphere. XKCD

[ianmdrennan 2]

You should check out the neptune (Altitude in 10 ft increments in canopy mode). It's incredible for helping with swoops. Most notably going to dropzones where your visual references are significantly different.

Normal Disclaimer: Use it as an aid to your eyes, not in place of them
Blue skies
Ian

Performance Designs Factory Team

[ryoder 1,590]

I would expect any mechanical device to have some amount of lag, but to say an altimeter reads incorrectly during the part of the jump when your vertical velocity is the *lowest*, sounds bass-ackwards to me.

My theory is that the author is confusing characteristics of the Vertical Speed Indicator with those of the altimeter. When setting up a climb or descent, the VSI has a *very* noticeable lag you can measure in seconds before it settles down and shows an accurate display. The author may have been thinking about this.

BTW, Riddler; If you are making dives on which it is critical that you get accurate altitude measurements in the first 500 feet, I think I'll stay on the ground and shoot video.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[Chrisky 2]

Basically the principle of function of a barometric altimeter is very simple, so i doubt that there were any significant improvements made since the 60s...
Simple is just best, low tech rules.
It's a metal pressure bubble expanding(on way up) and pushing up a small bar being translated into the needle rotation. So the deciding factor on accuracy is the manufacture and quality of the pressure bubble (sorry, i'm german and only know the german term for it).
So it might be that they are somewhat more precise than back then, due to more precise manufacturing, but not decisively.
And if you have the manufacturers certificate, you can calculate the exact altitude (except for when you get the result, you're already at a different level...).

The mind is like a parachute - it only works once it's open.
From the edge you just see more.
... Not every Swooper hooks & not every Hooker swoops ...

[sdctlc 0]

I recently bought a Free Fall Accessories altimeter and it is very accurate. I watched it note small altitude differences both in a car going up a hill and in the airplane during the first 1000' just to see how close it is compared to my previous altimeter. It is more accurate then my old Alti III by a long shot and it suprised me actually. it also suprised me that up high it read better then other altimeters when comparing it to the airplane altimeter. Just my 2cents...

Scott C.

"He who Hesitates Shall Inherit the Earth!"