Am I under weighted?

[Splitlip]

Hi all.
Think I pretty much got my weight dialed in w/ my BP/W.
Today I did a safety stop at 15' with 900# plus or minus. At 15' with 900# in my tank and no air in my wing I felt dead nuts on neutral. I was wondering what 500 # less psi would do to my buoyancy during the hang.
I am about 190-195 pounds. Was in a 3/2 one piece wetsuit diving a 98 cf lp steel OMS tank and an Al (about 2#) back plate with 4 # on a weight belt in sea water.
Thanks in advance.

 

[Cacia]

close I bet. I had a "fun" little experience a couple weeks ago, that I would not want to repeat. I went in with two pounds less than usual, thinking "oh, its a deep dive, won't matter". Then, on my ascent, I noticed I had a tiny bit of deco...I was not on a line. I had a bit of a time trying to stay down with a low tank. Anyway, it impressed me and I realized I had been too casual about being underweighted. Had I been on an ascent line, it would not have mattered..trying to hold a stop when you really need to and beiing underweighted was unsettling....I am such a genius, aren't I?

I dive with 2 (2#s) on a belt, with a 3mm and a steel BP, 135 lbs, Al 80. I blew one weight off on the boat, cause I couldn't thread it, and was in a hurry. You must be lean, diving with 4#'s, with the given and 195 lbs?

 

[spectrum]

Each cubic foot of air weighs .08 pounds

Your cylinder is 98 CF at 2640 PSI so each CF = 26.93 PSI

500 PSI less / 26.96 = 18.56 cubic feet of air

18.56 X .08 LB?CF = 1.48 LB

You will be 1-1/2 pounds lighter if you breathe that cylinder down to 400 PSI

Pete

 

[lowwall]

The specs on that tank are 2640psi = -7.7lbs buoyancy (compared to empty) or .2917lbs/100psi. Assuming you want to be neutral at 500psi, you'll want to add weight equivalent to the buoyancy of 400psi (i.e. 900psi where you were neutral minus the 500psi of where you want to be). So you need another 1.2lbs of negative buoyancy.

Let's check this by calculating from another direction. A quick google search suggests that a cubic foot of air weighs .0807lbs. Which would mean 7.9lbs of air in a 98cubic foot tank rather than 7.7, but remember a reg is not a pump, so you can never get the tank below ambient pressure. 400 psi = 15.15% of the tank's capacity = 14.85 cubic feet of air = 1.2lbs.

So that's my answer until MikeFerrara comes along and tells me otherwise

Edit - Whoops, reread the question. You were asking about 500psi less (not 500 psi left). Following the logic above, I get 1.5lbs (rounded to the nearest .1) from both calculations.

 

[lowwall]

You must be lean, diving with 4#'s, with the given and 195 lbs?

Or strong. Fat has a density of .9g/cc, muscle averages 1.1g/cc. So you can offset every pound of extra fat with a pound of muscle. I would use around the same weight with that setup and no one is likely to call me lean.

 

[cool_hardware52]

If you find yourself slightly "light" at a shallow stop, swim down a little, stay a little longer, assuming you have the gas. Your exposure suit will compress a bit, you will become a little less buoyant.

This of course works best with thick neoprene, but handy none the less.


Tobin

 

[Splitlip]

Thanks All.
I was very comfortable weighted as I was. Based on the math and Catherine's heads up on the Deco, I think I am going to add 1#, maybe look for a different STA.

 

[dumpsterDiver]

If you find yourself slightly "light" at a shallow stop, swim down a little, stay a little longer, assuming you have the gas. Your exposure suit will compress a bit, you will become a little less buoyant.

This of course works best with thick neoprene, but handy none the less.


Tobin

This is important. If you suspect that you are too light on ascent, completely stop at 40 or so and check your bouyancy. If you are still negative, then come to 30 and STOP. It is much safer to do a safety stop at a deeper than optimal depth than to realize at 12 feet that you are pretty much outta control.

I have done many stops with very bouyant dead fish, and have decided to stop at a little deeper depth and to breath a little shallower rather than let the fish float up or punch holes in their bodies and make even more blood.

 

[Splitlip]

This is important. If you suspect that you are too light on ascent, completely stop at 40 or so and check your bouyancy. If you are still negative, then come to 30 and STOP. It is much safer to do a safety stop at a deeper than optimal depth than to realize at 12 feet that you are pretty much outta control.

I have done many stops with very bouyant dead fish, and have decided to stop at a little deeper depth and to breath a little shallower rather than let the fish float up or punch holes in their bodies and make even more blood.

agreed. Thanks