Steel tank Wetsuit question

[jonnythan]

That makes no sense.

If you have 10 spare pounds of gas to vent, as well as 10 pounds of lead on your belt, ditching either one will have the *exact same effect*.

If you ditch the lead, THEN breathe down another 10 pounds of gas, maybe you'll be in trouble.

Of course, if you vent 10 pounds of gas, then you breathe another 10 pounds (if you have that much), you'd be in the same trouble.

The only conceivable differences are:

1) Each affects your trim a little differently

2) Venting 10 lbs of gas prevents you from breathing down enough to make you uncontrollably buoyant since you planned to breathe that much over your dive anyway.

I can't imagine you'd use 10 lbs of gas on the ascent, and if you did, you better not be venting it before you start.


Genesis: which weighs more, 10 lbs of feathers or 10 lbs of bricks?

 

[jonnythan]

Genesis once bubbled...
If you ditch STATIC mass, you permanently alter that relationship.

Once the gas is gone, you've lost "static mass," whatever that means.

 

[scubatexastony]

If nothing went wrong with this divers b/c and he used his air (the 10 lbs or more in question) he must be under weighed in the first place if he can't make a safety/deco stop...... or am I missing the boat here? again I ask.....where's his lift bag?

tony

 

[Genesis]

If you have 10 spare pounds of gas to vent, as well as 10 pounds of lead on your belt, ditching either one will have the *exact same effect*.

If you ditch the lead, THEN breathe down another 10 pounds of gas, maybe you'll be in trouble.

Actually, the inflection point comes earlier than that.

Do the math - you start balanced with nearly no gas, zero air in the wing, and at the surface. You have 10lbs of lead on your belt.

You fill the tanks with 18lbs of gas. You are now -18, AT THE SURFACE.

At the BOTTOM, you are -18 PLUS whatever your exposure protection lost on the descent. Call it another -7, for a total of -25.

You can swim up a maximum of -15.

If you ditch the 10lbs of lead, you can swim up, right? You're now -15.

Ok, so you do. But the 7lbs your suit lost comes back as you ascend. You're now -8. If you breathe EIGHT lbs of gas, you're hosed, not 10.

Of course, if you vent 10 pounds of gas, then you breathe another 10 pounds (if you have that much), you'd be in the same trouble.

The only conceivable differences are:

1) Each affects your trim a little differently

2) Venting 10 lbs of gas prevents you from breathing down enough to make you uncontrollably buoyant since you planned to breathe that much over your dive anyway.

I can't imagine you'd use 10 lbs of gas on the ascent, and if you did, you better not be venting it before you start.

What possible purpose do you have for the gas if you will not be using it for the original dive plan?

This is a correllary to the "get back on the boat with 500 psi!" admonition. Its stupid to blow a safety stop because you would invade that 500psi reserve. What use is the 500psi of gas to you once you're back on the boat? None.

Ditto here - hauling up an extra 10lbs of negative buoyancy in the form of gas that you have no conceivable need for in your now-aborted dive doesn't make a lot of sense, especially when you trade off either something expensive (e.g. a cannister light) or something that damages your ability to hold a safety stop (e.g. your lead), or both in the process.

There are situations where ditching gas makes no sense; if you're in water where your reg may freeze, or if you need to stop descending NOW (no floor!), then you've got a different set of problems.

But in those cases you have no business being in the water without some form of redundant buoyancy.

Ditching static mass at depth should, IMHO, be considered unacceptable. Options to make that something you never even CONSIDER doing should be part of your dive planning, again, IMHO.

 

[scubatexastony]

Now maybe it'll sink in.....

and where's his friggin BAG?

 

[Genesis]

Which is why he's in this pickle in the first place

 

[Se7en]

I was going to be sarcastic and say that I have this new fangled device called a SPG to tell me how much air I have in my tanks... but I won't.

Lets instead, take a look at the scenarios.

Option 1 (near start of dive): I have 15 lbs of air in my tanks, and a 10 lbs belt.

Option 2 (near end of dive): I have 5 lbs of air in my tank, and a 10 lbs belt.

In option 1 we're 15 lbs negative. Now we can easily swim up 5lbs, so we need to dump 10 lbs.
You dump air, I dump belt.
Both of us are 5 lbs negative now. We both swim up. Yah us. Lets have a beer to recover.

If during that swim up I use more than 5lbs of air, I become positive, and shoot to the surface. Ouch! these skin bends hurt, get me to a chamber quick. Bad diver, no beer.
If during that swim up YOU use more than 5 lbs, you drown. Get your body to the morgue.

Option 2, I dump and do a polaris. Chamber time
You don't have enough air to dump. You become one of the 90% of fatalities that are found with their weightbelt in place.

Genesis - I think it's an interesting idea, and one that we can file away as an option if we don't have anything else to ditch, but I don't think it's the correct solution if we do have other stuff to ditch.

FWIW, I also put my weight belt on under my harness, and in 12 years of diving have never dumped it (except during training, at the surface). Like you, I don't want it to ever come off - except if the only other option is to drown.
(Actually, I often don't wear a weightbelt at all, my gear is heavy enough. But that's diving dry, or single tank in the tropics - either way I can swim the gear up comfortably)

Se7en

 

[Se7en]

Genesis once bubbled...

you have no business being in the water without some form of redundant buoyancy.

Ditching static mass at depth should, IMHO, be considered unacceptable. Options to make that something you never even CONSIDER doing should be part of your dive planning, again, IMHO.

I agree wholeheartedly!

 

[jonnythan]

Genesis once bubbled...


Actually, the inflection point comes earlier than that.

Do the math - you start balanced with nearly no gas, zero air in the wing, and at the surface. You have 10lbs of lead on your belt.

You fill the tanks with 18lbs of gas. You are now -18, AT THE SURFACE.

At the BOTTOM, you are -18 PLUS whatever your exposure protection lost on the descent. Call it another -7, for a total of -25.

You can swim up a maximum of -15.

If you ditch the 10lbs of lead, you can swim up, right? You're now -15.

Ok, so you do. But the 7lbs your suit lost comes back as you ascend. You're now -8. If you breathe EIGHT lbs of gas, you're hosed, not 10.

At the bottom, you're, say, -25.

Vent 10 lbs of gas, you're at -15, so you do.

Then the 7 lbs your suit lost comes back as you ascend. You're now -8. If you breathe EIGHT lbs of gas, you're hosed [because you're OOA AND shooting to the surface.].

Notice the similarity?

Ditching static mass at depth should, IMHO, be considered unacceptable.

Agreed. So should dumping gas to become less negative. I can't possibly imagine how that would be a viable part of any contingency in any situation whatsoever.

 

[jonnythan]

Genesis once bubbled...
Ditto here - hauling up an extra 10lbs of negative buoyancy in the form of gas that you have no conceivable need for in your now-aborted dive doesn't make a lot of sense, especially when you trade off either something expensive (e.g. a cannister light) or something that damages your ability to hold a safety stop (e.g. your lead), or both in the process.

There's always a need for that gas.

What if your higher breathing rate (and mad dumping) cause your first stage to freeze and cause a freeflow?

What if your buddy's reg fails?

What if you get entangled in something?

Do you really wan to be at the bottom of the ocean with *just enough* gas to get to the surface when you could have dumped the same mass (in lead) and had tons of spare air?

There is no imaginable advantage to dumping live giving air instead of dead lead.