WKenny
Contributor
I have encountered problem on occcasions with venting too much gas from my wing too early on some ascents and slipping into negative buoyancy which makes the ascent more difficult. I'm trying to get a better conceptual understanding of how large a volume of gas I should be retaining in my wing, (and dry suit), during the ascent to avoid slipping into negative buoyancy.
I'm diving in cold water, (the Great Lakes), with a DUI TLS 350 trilaminant material dry suit, a BARE "Hi- Loft" thinsulate undergarment, a doubles rig with two 85 cu ft, low pressure Worthington steel tanks, and an 80 cu ft aluminium deco stage tank, and no "extra weight" beyond a few miscellaneous things like a small hand-held light, small shears, and a DSMB with a small spool. That rig is about 30 pounds negatively buoyant at the end of a normal dive with a reserve of about 1/3 gas in the double tanks, and the deco tank is full, determined as follows:
1) Negative buoyancy of the doubles tank when empty, (-0.7 lbs each), so @ -1.5 lbs
2) Weight of the gas with the tanks about 1/3 full @ -4.5 lbs
-(the capacity if each tank is about 82 cu ft, so total capacity of both
tanks is about 164 cu ft;
-(1/3 of 164 cu ft is about 55 cu ft)
-(55 cu ft of gas times .08 pounds per cu ft = @ 4.5 lbs of gas.
3) Aluminium BP, harness, wing, 2 regs, SPG, tank, manifold & bands @ -20 lbs
4) Negative bouyancy of full 80 cu ft aluminium deco tank. @ -3.5 lbs
TOTAL Negative Buoyancy of the tanks/rig on a normal ascent: about -30.0 lbs
I am about 6 ft, 2 inches tall, and I weigh about 210 lbs. I don't know how I could get the rig much lighter. I'm not carrying any extraneous weight as I see it.
Before starting an ascent, I check my buoyancy and try to get neutrally buoyant with my lungs about half full of gas. I ascend in a horizontal position. I inhale to establish slight positive buoyancy to start the ascend. As soon as I feel upward movement, I vent a small puff of gas from the wing, and exhale to move back toward neutral buoyancy. I conduct the ascent generally by repeating this procedure, unless I slip into negative buoyancy which throws the ascent out of whack. The dry suit pretty much self vents as the depth gets shallower.
What is the approximate volume of gas that I should have in the wing, (and dry suit), to be neutrally buoyant when I start the ascent at a depth of 100 feet? Using the old adage "A pint is a pound", (i.e. 1 U.S. pint of water weighs about 1 lb), should I have about 30 pints of gas in the wing, (and dry suit), to offset the 30 pound negative buoyancy of those tanks and rig? 8 U.S. pints equals 1 gallon, so 30 pints is equivalent to about 3.75 gallons. Really? That seems like a great deal of gas to have in the wing, (and dry suit) for buoyancy control. Am I analyzing this correctly, or am I missing something?
Then as I ascend and that gas expands, do I need to vent it so that I maintain about that same volume, 3.75 gallons of gas, during the entire ascent to remain close to general neutrally buoyancy during the ascent? At the surface, wouldn't the 30 pints of gas, (i.e. 3.75 gallons), still be necessary to offset the 30 pounds of negative buoyancy of the rig for neutral buoyancy at the surface?
I understand that I should vent a series of small puffs of gas from the wing "early and often" during the ascent to maintain a balance of venting sufficient gas to avoid an uncontrolled ascent, while not venting too much gas too soon which can cause me to slip back into negative buoyancy. However, it is the total volume of gas that I need to retain in the wing, (and dry suit), during the ascent that has me somewhat confused. With a rig that is about 30 pounds negatively buoyant as described above, do I reallly need to retain about 3.75 gallons of gas in the wing, (and dry suit), over the course of the entire ascent to avoid slipping into negative buoyancy during the ascent?
If I really do need that much gas in the wing, (and dry suit), for proper buoyancy control, then I need to be diligent about not venting too much gas too soon on the ascent. But having that much gas in the wing seems like it might be courting disaster if I ever failed to vent it quickly enough to prevent it from expanding too much and causing an out-of-control ascent.
Thank you for your advice and assistance with this issue.
I'm diving in cold water, (the Great Lakes), with a DUI TLS 350 trilaminant material dry suit, a BARE "Hi- Loft" thinsulate undergarment, a doubles rig with two 85 cu ft, low pressure Worthington steel tanks, and an 80 cu ft aluminium deco stage tank, and no "extra weight" beyond a few miscellaneous things like a small hand-held light, small shears, and a DSMB with a small spool. That rig is about 30 pounds negatively buoyant at the end of a normal dive with a reserve of about 1/3 gas in the double tanks, and the deco tank is full, determined as follows:
1) Negative buoyancy of the doubles tank when empty, (-0.7 lbs each), so @ -1.5 lbs
2) Weight of the gas with the tanks about 1/3 full @ -4.5 lbs
-(the capacity if each tank is about 82 cu ft, so total capacity of both
tanks is about 164 cu ft;
-(1/3 of 164 cu ft is about 55 cu ft)
-(55 cu ft of gas times .08 pounds per cu ft = @ 4.5 lbs of gas.
3) Aluminium BP, harness, wing, 2 regs, SPG, tank, manifold & bands @ -20 lbs
4) Negative bouyancy of full 80 cu ft aluminium deco tank. @ -3.5 lbs
TOTAL Negative Buoyancy of the tanks/rig on a normal ascent: about -30.0 lbs
I am about 6 ft, 2 inches tall, and I weigh about 210 lbs. I don't know how I could get the rig much lighter. I'm not carrying any extraneous weight as I see it.
Before starting an ascent, I check my buoyancy and try to get neutrally buoyant with my lungs about half full of gas. I ascend in a horizontal position. I inhale to establish slight positive buoyancy to start the ascend. As soon as I feel upward movement, I vent a small puff of gas from the wing, and exhale to move back toward neutral buoyancy. I conduct the ascent generally by repeating this procedure, unless I slip into negative buoyancy which throws the ascent out of whack. The dry suit pretty much self vents as the depth gets shallower.
What is the approximate volume of gas that I should have in the wing, (and dry suit), to be neutrally buoyant when I start the ascent at a depth of 100 feet? Using the old adage "A pint is a pound", (i.e. 1 U.S. pint of water weighs about 1 lb), should I have about 30 pints of gas in the wing, (and dry suit), to offset the 30 pound negative buoyancy of those tanks and rig? 8 U.S. pints equals 1 gallon, so 30 pints is equivalent to about 3.75 gallons. Really? That seems like a great deal of gas to have in the wing, (and dry suit) for buoyancy control. Am I analyzing this correctly, or am I missing something?
Then as I ascend and that gas expands, do I need to vent it so that I maintain about that same volume, 3.75 gallons of gas, during the entire ascent to remain close to general neutrally buoyancy during the ascent? At the surface, wouldn't the 30 pints of gas, (i.e. 3.75 gallons), still be necessary to offset the 30 pounds of negative buoyancy of the rig for neutral buoyancy at the surface?
I understand that I should vent a series of small puffs of gas from the wing "early and often" during the ascent to maintain a balance of venting sufficient gas to avoid an uncontrolled ascent, while not venting too much gas too soon which can cause me to slip back into negative buoyancy. However, it is the total volume of gas that I need to retain in the wing, (and dry suit), during the ascent that has me somewhat confused. With a rig that is about 30 pounds negatively buoyant as described above, do I reallly need to retain about 3.75 gallons of gas in the wing, (and dry suit), over the course of the entire ascent to avoid slipping into negative buoyancy during the ascent?
If I really do need that much gas in the wing, (and dry suit), for proper buoyancy control, then I need to be diligent about not venting too much gas too soon on the ascent. But having that much gas in the wing seems like it might be courting disaster if I ever failed to vent it quickly enough to prevent it from expanding too much and causing an out-of-control ascent.
Thank you for your advice and assistance with this issue.