Forums: Skydiving: Safety and Training:
Your accurate altimeter can be waaay wrong

 


377  (F 666)

Aug 16, 2004, 2:20 PM
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Your accurate altimeter can be waaay wrong Can't Post

At WFFC Alti 2 sets up an altimeter test chamber and offers free testing of all altimeters, whether made by them or not. I Had my Altimaster 2 and Digitude checked and they were both within 100 ft accuracy at all altitudes below 6K. So I decide to wear both of them on the same arm and watch them together, Altimaster 2 on a wrist strap worn like a watch on my wrist and the Digitude worn with a wrist strap and a rubber ring over a couple of fingers to keep it in place, riding on the back of my hand. So here I am going through 3.5 on the Altimaster but the Digitude says 4.5. No time for more in flight testing on this jump but on the next one it's consistent , the Digitude reads almost exactly 1000 ft higher than the Altimaster. Under open canopy they agree and on the ground they are both zero. I figure maybe the Digitude has a rate response problem and cannot respond fast enough to freefall altitude changes, so I take em both back to the Alti 2 booth and ask them to put them both in the chamber and take em down from 15K at about freefall rate. They agree perfectly in the chamber. I am baffled. On the next jump same thing, Digitude is 1K higher but I try rotating my hand so the Digitude is not riding on the back of my hand parallel to the ground. BINGO, the 1 K difference disappears immediately and the two altimeters agree in freefall. Apparently a low pressure area was created on the back of my hand that caused the Digitude to accurately report pressure, but not altitude. 1 K can make a BIG difference at opening time so I thought I'd post this. I never would have known had I not worn two altimeters on the same arm. I thought passing a chamber accuracy check assured accurate altitude readings... WRONG!!!!!


(This post was edited by 377 on Aug 16, 2004, 2:38 PM)


towerrat  (D 28189)

Aug 16, 2004, 2:24 PM
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Re: [377] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

wow, that's great info to have. Thanks for the field test. I'll be doing some testing now, to see what's up with mine.


mr2mk1g  (C 103449)

Aug 17, 2004, 12:57 AM
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Re: [377] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

Which of the two changed when you moved your hand? Did the Digitude come down to the Alti2 or did the Alti2 come up? i.e. which of them was wrong?


LannerFalcon  (A License)

Aug 17, 2004, 4:20 AM
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Re: [377] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

Wink very interesting, because the majority of people wear an alti on the back of their hand, they can't all be deploying 1000ft out?


377  (F 666)

Aug 17, 2004, 9:17 AM
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Re: [LannerFalcon] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

My guess is that my experience was caused by a combination of factors which included surface area of the back of my hand (large) and the angle of attack of my hand (palm perpendicular to the relative wind). I too was very surprised that the error could be so large, but believe me it was there and repeatable on three jumps. If I had been wearing only my Digitude and kept my palm perpendicular to the relative wind, I would have been pulling at an indicated 2.5K and really been at 1.5K. I wonder how Cypres software deals with this kind of error? Surely there is a low pressure area on your back when you are belly flying.


Premier billvon  (D 16479)
Moderator
Aug 17, 2004, 2:28 PM
Post #6 of 8 (1571 views)
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Re: [377] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

Yep. I almost failed the AFF JCC because my digitude was reading 500 feet high. This is an important lesson to learn; your hand-mount altimeter is simply nowhere near as accurate as, say, an aircraft altimeter. What does this mean?

1. If someone says "I'm gonna go out right after you and pull 1000 feet higher" it doesn't mean much. You just can't judge altitude that accurately with a hand-mount. Add a snivel to that equation and you could see him come blazing through your canopy after you open - even if he pulls when his altimeter says he's 1000 feet above your planned pull altitude. I've seen this issue almost result in a fistfight; both people thought the other was lying when it came to

2. Pulling at 1800 feet with a snively canopy may well result in a cypres firing if you combine an inaccurate altimeter with a snivel.

3. Breakoff may not be where you expect if someone else with a different altimeter is keying it. This can be a big deal on bigger ways; you may hear the beep of your audible and become upset when the breakoff key does not come when you expect, but if you leave early, you can mess up the breakoff plan.


firstime  (B 28972)

Aug 17, 2004, 5:42 PM
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Re: [377] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

this is an interesting post, what if the alti is mounted on the bottom side of your wrist. A couple of hundred
feet is ok but 1000k. This might explain a particular
jump about a month ago. We discussed the dive and all agreed on our opening (I am somewhat anal
when it comes to pull time) it turns out that 2 guys told me I was under 3k when I dumped and I disagreed because once my alti just touches the 4k
I reach and open by 34 or so. OH well another thing to ponder on the way to altitude. I will try this with
a 2 way, one alti back of the wrist and one on the bottom and compare in freefall.


FrogNog  (C 34484)

Aug 17, 2004, 6:28 PM
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Re: [firstime] Your accurate altimeter can be waaay wrong [In reply to] Can't Post

I find that if I rotate my wrist so my alti is between my hand and the ground, it reads about 500' lower than if the altimeter is between my hand and the horizon, where it reads about 500' lower than if my altimeter is exactly on the far side of my hand from the ground. It's pretty cool, actually - at pull time I can adjust what altitude my altimeter displays. Smile

But now that I have this figured out, I'm happy with it. It's not a precision instrument for figuring out how high up I am, it's an assistance device to help me deploy at an appropriate altitude.



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