Messed up and ascended like a missile

[Storker]

It is not true that "Your tank actually holds 15L of gas what ever the pressure"....A 15L tank is measured to hold 15L of water (or 15L of gas at 1bar)....at 200bar a 15L tank holds 3000L of gas (air).

No. It's still 15L. At 200 bar. Which becomes approximately¹ 3000L at 1 bar. If you had said "3000 surface liters" or "3000 bar·L", I would've agreed with you.

¹ "Approximately" because only an ideal gas would behave exactly like that. Now, for our purposes air is close enough to ideal gas behavior at pressures up to a little above 200 bar, so we usually don't bother with non-ideality for those pressures. It does become an issue as you pass 240-250 bar, though, and a 300 bar tank holds about 10% less air than nominally. So a 10L 300 bar tank holds only about 2700-2800 surface liters.

 

[Bob DBF]

But, OTOH, I find disappointing that threads that start out very well like " Why ‘everyone is responsible for their own risk-based decisions’ isn’t the right approach to take. " unfortunately seem, more often then not, to degenerate into endless philosophical and political debates.

The good news is you can just move on to another thread. Oh, by the way, you may not want to go to the Pub.


Bob

 

[wgw04024]

How did your mask just 'fill with water' exactly?

 

[Eric Sedletzky]

Here is something that has not been mentioned so far.

One of the most common reasons that new divers end up rocketing to the surface (and it happens more than it should) is contrary to what you might think--new divers are frequently overweighted, often by quite a bit.

So why would being overweighted make you rocket to the surface? You would think it would be just the opposite. The reason is that in order to become neutrally buoyant when overweighted, you have to add a lot of air to the BCD. For every kilogram you don't need, you have to add about a liter of air to the BCD. As you get shallower while doing something like clearing a mask, all that extra air will expand, and the next thing you know you are on an out-of-control rocket to the surface.

The less weight you wear, the less air you need in the BCD. The less air you have in your BCD, the easier it is for you to control your buoyancy, usually simply through the way you breathe.

Very good explanation.
This is also why I scream my head off about getting the extra weight off down to where a diver can hold a 15’ safety stop with an empty BC. 15’ is the depth where the most radical pressure changes occur, right in the middle of the first ATM.
So any air in a BC will expand and contract the most with the slightest increase or decrease in depth.
I’ve seen divers constantly have to fill and dump air trying to hold a 15’ stop when ocean swells are going overhead. Very frustrating.
Get rid of the weight and get rid of the air.

 

[Bubblesong]

... where a diver can hold a 15’ safety stop with an empty BC. 15’ is the depth where the most radical pressure changes occur, right in the middle of the first ATM.
So any air in a BC will expand and contract the most with the slightest increase or decrease in depth.
I’ve seen divers constantly have to fill and dump air trying to hold a 15’ stop when ocean swells are going overhead.
Get rid of the weight and get rid of the air.

Is this with a full tank of air, or a “sucked down to 500psi tank of air” safety stop? Because we all do our neutral buoyancy at the beginning of dive with full tank, but after losing the weight from the air in the tank, I always feel too buoyant at end of dive. How many lbs of weight is lost from an aluminum 80 or other tanks for that matter?
Not be indelicate, but peeing underwater reduces your overall weight too.

 

[MichaelMc]

Is this with a full tank of air, or a “sucked down to 500psi tank of air” safety stop? Because we all do our neutral buoyancy at the beginning of dive with full tank, but after losing the weight from the air in the tank, I always feel too buoyant at end of dive. How many lbs of weight is lost from an aluminum 80 or other tanks for that matter?
Not be indelicate, but peeing underwater reduces your overall weight too.

This is with 500psi. Air is 0.08 lb. per cubic foot. For AL80 each 100 psi is 0.2 lb. = (100/3000) * 77 * 0.08.
For pee, you’re changing the shape of the body and its weight, and pee is water, so zero net impact, on buoyancy. Human pee has specific gravity around 1.02, sea water is 1.025, so negligible difference.

 

[boulderjohn]

Is this with a full tank of air, or a “sucked down to 500psi tank of air” safety stop? Because we all do our neutral buoyancy at the beginning of dive with full tank, but after losing the weight from the air in the tank, I always feel too buoyant at end of dive. How many lbs of weight is lost from an aluminum 80 or other tanks for that matter?
Not be indelicate, but peeing underwater reduces your overall weight too.

MichaelMc gave you the correct numbers above, which means that an AL80 has about 6 pounds of air in it when full. If you are at the end of the dive and have not sucked the tank completely dry, you have lost 5 pounds of air. Consequently, many people advise that when you do a buoyancy check prior to a dive, you should add 5 pounds to the perfect weighting determined by that process. Others disagree, arguing that when you do a weight check at the beginning of the dive, you have enough air trapped in your gear (which you will lose during the dive) to make up that difference. I believe that is partially true, but I also believe few people trust the results of a true weight check and are actually at least a few pounds heavy when they claim to be perfectly weighted.

I once had a student for OW dives in cool, fresh water who had had another instructor from our shop for the pool sessions. Knowing he would do his OW dives in a 7mm suit, he had insisted on using a 7mm suit for the pool sessions so he would start the OW dives with his weighting needs dialed in. He told me that because of that, he knew 22 pounds was exactly what he needed. I looked at his slight build and said, "No way." By the time we finished his OW dives, we was down to 10 pounds, and he was much, much happier with his control of buoyancy and trim. I have no idea how his instructor-supervised weight check had gone, but it resulted in him thinking he needed more than twice the weight he actually needed.

BTW, in my own personal recreational diving, I dive a few pounds overweighted. I like the fact that that slightly easier bubble is easier to manage when I need to dump air. When I dive a few pounds overweighted, though, I am probably diving with more than a few pounds less than a typical recreational diver my size would use.

 

[boulderjohn]

Here is something else to consider when checking weight. The most common method for checking weight at the beginning of a dive is to dump all air from the BCD while holding a normal breath. With perfect weighting, you will float at eye level, and when you exhale, you will sink slowly.

Over the years, we have had more than a few threads on ScubaBoard in which divers have talked about the importance of dumping weights while on the surface after an OOA emergency. Otherwise, they argue, you will not have the strength to remain on the surface while kicking and without the ability to add air to the BCD. Yes, there have been real cases in which divers have gone OOA, reached the surface, and then been unable to stay there through swimming efforts and sunk back down beneath the surface and drowned.

Ignoring the facts that you should have been neutrally buoyant prior to ascending and the expansion of air in the BCD should make you positively buoyant on the surface, ignoring the fact that you can inflate your BCD orally, the simple fact is that floating on the surface with an empty tank should be effortless for a diver who was properly weighted for the dive. In fact, such a diver should have trouble descending intentionally. If you are unable to stay on the surface with an empty tank, you are grossly overweighted.

 

[Eric Sedletzky]

I still maintain that the only true way to get weighting perfect (as a baseline) in any certain gear combination is to do the 15’ stop, empty BC, 300-500 psi method.
There’s a lot to take into consideration. Suit compression and cooling, working air out of padding/hard to vent areas, etc. which affects the final outcome by a few lbs.
Trying to do weight checks at the beginning of a dive, you might get close but it’s still not precise. At the end of the dive it’s precise, and based on that info a diver can choose to do whatever they want in regards to weighting. At least they’ll know where the hard baseline is.
Change tank size, get a new BC, or get a new suit and of course it’s all out the window, start over.

 

[Soloist]

I still maintain that the only true way to get weighting perfect (as a baseline) in any certain gear combination is to do the 15’ stop, empty BC, 300-500 psi method.
There’s a lot to take into consideration. Suit compression and cooling, working air out of padding/hard to vent areas, etc. which affects the final outcome by a few lbs.
Trying to do weight checks at the beginning of a dive, you might get close but it’s still not precise. At the end of the dive it’s precise, and based on that info a diver can choose to do whatever they want in regards to weighting. At least they’ll know where the hard baseline is.
Change tank size, get a new BC, or get a new suit and of course it’s all out the window, start over.

This is EXACTLY how I weight myself. I emphatically believe if a single tank recreational diver used this method of weighting they should be able to swim themselves up to the surface at the beginning or end of a dive without air in their BC or dropping weights. The diver should be able to maintain their head above water without having to ditch weight in the event of a BC failure. If they are unable to do so, baring any physical limitations, they are carrying too much lead in my opinion. Am I missing something here?

I have been on the surface with a failed BP/W in Cozumel. What’s interesting is I had no idea the wing wasn’t holding any air. I was rolling up my SMB on the surface and noticed I was having to do a little finning while doing so. I hit the inflator button a couple of times, but by the time the SMB was stowed the boat arrived. Due to being distracted by the SMB I didn’t discover the corrugated inflator hose was compromised and sheared in half until the next day. In other words, it was a nonevent. With a full tank and a negative entry there would obviously be a more finning involved, but not unreasonable. When carrying a pony I have no ditchable weight, so being able to ditch weight is rarely an option for me.