May 14, 2001, 8:17 AM
Post #1 of 9
Chest Altimeter's and Sit-flying
I have often been told to be aware that chest altimeters can be effected when freeflying in the sit position because the burble creates a low pressure area. Does anyone know how much it is likely to effect it by? I have just started sit-flying and haven't really noticed any difference although I am still wary and have my TimeOut set for 5,000ft so I have time to slow off the speed for deployment.
I have noticed very little difference between my chest mounted and my audible altimeters when sitflying. That said, I usually glance at my chest mount at around 6,000 feet AGL and listen carefully for my audible at 4,000 feet AGL, and at ~160 mph any discrepancy would be easy to miss. I imagine that the burble around one's chest altimeter in a good, back straight up and down sit would be a lot less than the burble in a flatter back to earth position, but then, aren't our altimeters always subject somewhat to the vagaries of the wind currents around our bodies? Perhaps Altimaster or some other altimeter manufacturer would be kind enough to do some air-flow tests in a wind tunnel, and recommend optimal altimeter placement for various disciplines.
Why don't you borrow a wrist/hand mount and find out for yourself? You could see what different body positions have what effect on the difference between the two. This would be like a calibration for your individual burble.
You sound like me. I wear 3 altimeters, one on the wrist, one on the chest, and an audible (PRO-Dytter). I've checked my wrist mounted Altimaster-3 against my chest mounted ft-50. I've noticed that the ft-50 reads higher at altitude (+400 at 14k), I've cross-checked with the altimeter in the plane and found that the Alti-3 is dead on.
It is hard to read the wrist mount from a sit, which is why I got the chest mount. I still always wear both even when doing RW. I think you should always have an audible when you sitfly.
Don't RELY on your audible. Use your eyes and get to know how your pull heights/breakoff heights look. It's a good clue when everyone disappears, tracking off into the distance that you've missed break off....
I knew someone would pick me up on the fact that I used the word rely. What I meant is that I am aware that an audible alti in my helmet would not be effected by sit flying whereas my chest alti might.
If I waited till I saw people tracking off on the last few jumps I fear I would not be here to tell the tale...have only done solos recently...hee hee hee. And I do look at the ground as a gauge of height but I'd probably end up pulling abvove 5,000ft if I relied on that (...sorry, I'm feeling a bit cheeky at the mo ).
Thanks for your 0.5 pence...i'll put it in a jar and save it up for a cup of tea at the DZ...