Altimeters: Difference in readings due to position

[brewman 0]

I've got a pro-dytter in my helment and a Neptune that I wear on the back of my left hand. I've noticed the pro-dytter alarms about 400-500 feet higher based on the Neptune reading.

Saturday, I was playing around with the location of my hands in a tracking dive. I noticed that when I placed my hands up against my belly, the Neptune was reading 800-900 feet lower than when my hands were out in front of me in a standard boxman position.

Does anybody know in what position an altimeter will give the most accurate reading of actual altitude?

[Travman 6]

From memory the pro-track suggests wearing it on your leg, don't have my manual handy to double check that though.

[darkwing 4]

There are significant pressure differences around your body. Everyone seems comfortable with the idea of a partial vacuum behind you. This partial vacuum means the pressure there is lower than the pressure characteristic at that actual altitude. The pressure below you (all of this assumes belly to earth position) is similarly higher than the true pressure at your actual altitude. This is largely true in smaller scale situations, such as above and below your hand.

The net result is that you need to put it where there is neither higher nor lower pressure, which translates to an airflow on the side. Hard to see on your leg though, so the hand is an OK compromise.

-- Jeff
My Skydiving History

[Reginald 0]

Jim,

The Cypres has an interesting bit of information built into its code. The air pressure difference in a person’s burble is equal to 200 feet of altitude. There is some sophisticated logic built to cover if a person is tumbling and to compensate for that, however. Basically that “lower” reading of 200 feet translates into a Cypres firing at 950 feet in a test chamber, this is equal to 750 feet in the sky with a 200 foot difference for the burble. So a couple of hundred feet difference is pretty well established. I’m surprised you got up to 900 feet difference based on position around your body and 500 between your Protrack and Neptune. Have you thought of putting an analogue altimeter on your wrist with your Neptune? I know I’m going to get flamed here but I‘ve heard a number of people (that you know) complain about the accuracy of a Neptune.

"We've been looking for the enemy for some time now. We've finally found him. We're surrounded. That simplifies things." CP

[brewman 0]

Ron,

I'll try that next time I see you at the DZ!