Shooting fish deep

headshot · Jun 9, 2005

We usually hunt at 80-130 feet, but i've recently seen some big grouper at 160 plus. How fast can you safely ascend between 200 and 100 feet. I was certified 20 years ago, and haven't taken any advanced courses (except nitrox) even though I dive all the time. I used to think that you could double your ascent rate for every atmosphere you obtained in depth, but recently read somewhere that speedy ascents, even at the 120-140ft range can cause microbubbles that are like the baby bends.

ScubaMarine · Jun 9, 2005

headshot:
We usually hunt at 80-130 feet, but i've recently seen some big grouper at 160 plus. How fast can you safely ascend between 200 and 100 feet. I was certified 20 years ago, and haven't taken any advanced courses (except nitrox) even though I dive all the time. I used to think that you could double your ascent rate for every atmosphere you obtained in depth, but recently read somewhere that speedy ascents, even at the 120-140ft range can cause microbubbles that are like the baby bends.

I recognize you have a lot of experience, especially compared to some of us newer divers. I have taken the advanced course and although Ive only been between 80-130 twice, I would definately recommend it before going deeper. Experience can make up for a lot when diving deep, but wouldn't you feel more comfortable at depth knowing you've been certified for it? Its your life...

Rick Murchison · Jun 9, 2005

headshot:
We usually hunt at 80-130 feet, but i've recently seen some big grouper at 160 plus. How fast can you safely ascend between 200 and 100 feet. I was certified 20 years ago, and haven't taken any advanced courses (except nitrox) even though I dive all the time. I used to think that you could double your ascent rate for every atmosphere you obtained in depth, but recently read somewhere that speedy ascents, even at the 120-140ft range can cause microbubbles that are like the baby bends.

It's a complex question that depends largely on what your nitrogen status is when you start the ascent. Here's my "minimum math" explanation: Since bubble growth is indeed in relation to the change in pressure/total pressure, the growth rate is the same for a 120fpm ascent at 100' as it is for a 25fpm ascent at the surface. But... from an offgassing standpoint there still has to be a mass movement down a gradient from tissue to lung, and that's pretty much a delta P issue alone - so if you want to avoid bubble formation in the first place and move nitrogen out of your system in the dissolved state you need to balance the gradient against the dissolved nitrogen level and that tends to favor a constant ascent rate.
Still, for any nitrogen load/ambient pressure ratio achieved at a specific max depth there is some higher PN2/P ratio that can be tolerated right away without exceeding the ratio that would cause the always present microbubble "seeds" to become growing bubbles, and when that's measured in feet it varies directly with the total pressure - that is, if an immediate ascent of 5' can be tolerated close to the surface, then an immediate ascent of 25' can be tolerated in the 150-125' range.
Bottom line... as a practical matter, a spearfisherman can generally, but not always, get away with some substantial fairly rapid depth changes deep on a short deep dive - but the longer the dive and the more dives in a day the higher the risk.
My philosophy is to try to make good shots and avoid it, and if I do end up making a rapid ascent with a fish then make the rest of the ascent very slow indeed, and, depending on how big an excursion it was, my diving for the day is likely over. (I also won't shoot nearly as big a fish as I would when I was young and immortal, either)
Rick

headshot · Jun 10, 2005

Thanks for the tutorial Rick. I need to read more about the physics, but I appreciate the head start on a more complete understanding.

Rick Murchison:
It's a complex question that depends largely on what your nitrogen status is when you start the ascent. Here's my "minimum math" explanation: Since bubble growth is indeed in relation to the change in pressure/total pressure, the growth rate is the same for a 120fpm ascent at 100' as it is for a 25fpm ascent at the surface. But... from an offgassing standpoint there still has to be a mass movement down a gradient from tissue to lung, and that's pretty much a delta P issue alone - so if you want to avoid bubble formation in the first place and move nitrogen out of your system in the dissolved state you need to balance the gradient against the dissolved nitrogen level and that tends to favor a constant ascent rate.
Still, for any nitrogen load/ambient pressure ratio achieved at a specific max depth there is some higher PN2/P ratio that can be tolerated right away without exceeding the ratio that would cause the always present microbubble "seeds" to become growing bubbles, and when that's measured in feet it varies directly with the total pressure - that is, if an immediate ascent of 5' can be tolerated close to the surface, then an immediate ascent of 25' can be tolerated in the 150-125' range.
Bottom line... as a practical matter, a spearfisherman can generally, but not always, get away with some substantial fairly rapid depth changes deep on a short deep dive - but the longer the dive and the more dives in a day the higher the risk.
My philosophy is to try to make good shots and avoid it, and if I do end up making a rapid ascent with a fish then make the rest of the ascent very slow indeed, and, depending on how big an excursion it was, my diving for the day is likely over. (I also won't shoot nearly as big a fish as I would when I was young and immortal, either)
Rick

Shooting fish deep

Please register or login

headshot

Registered

ScubaMarine

Guest

Rick Murchison

Trusty Shellback

headshot

Registered

Similar threads