Water in BCD

[dfx]

Maybe I spoke too soon when I said I think logically. But here goes. If 1 cu ft of air at 3000 psi weighs 16 lbs, then why does an empty tank only gain about 5 lbs when you add 80 cu ft at 3000 psi? Or am I in a different context?

Yes, those 80 cu ft are at atmospheric pressure, aka 14.7 psi. The internal volume of an AL80 tank is around 0.38 cu ft or so. Take 80 cu ft of air, squeeze it into the small volume tank, and you're at 3000 psi.

 

[Panama Jones]

Thanks eponym...was scared to death, I don't think I could have ever gotten a 1300lb tank on my back (probably if I put in on empty then filled it).

---------- Post added May 10th, 2013 at 08:01 PM ----------

Yes, those 80 cu ft are at atmospheric pressure, aka 14.7 psi. The internal volume of an AL80 tank is around 0.38 cu ft or so. Take 80 cu ft of air, squeeze it into the small volume tank, and you're at 3000 psi.

Ahh...I see your math now. But why would we need to know what one cu ft volume of air compressed to 3000 psi weighs at compression? are there tanks that size?

 

[eponym]

Thanks for the chuckles, Jones.

And if you can, take the time to fill out your profile. Eager minds want to know where you are, what you've seen, etc.

 

[dfx]

Ahh...I see your math now. But why would we need to know what one cu ft volume of air compressed to 3000 psi weighs at compression? are there tanks that size?

Only to compare it with the weight of water.

 

[Panama Jones]

Thanks for the chuckles, Jones.

And if you can, take the time to fill out your profile. Eager minds want to know where you are, what you've seen, etc.

Good point, had a few questions and wanted to sign up quick. Tonight for sure.

New diver (50 - 100 dives). But all over the place... Chile, Mediterranean, San Diego, Mexico, Cuba and the cold lakes in Alberta. Live in Alberta Canada. So diving so few dives altogether but in fresh and salt, cold and warm, all different levels of exposure suits (except dry) and different tanks hasn't given me the stability to figure out a lot of things. So I make some jokes on this site but all in all I really am learning from it. Hopefully style isn't too blunt.

 

[Frosty]

Got blown out in Key largo, and wanted to dive so went over to Jules and buddied up with an instructor who doing some practice with a new OW student of his so that Jules would let me in. The women had buoyancy issues. She got down with no problem but then would soon float up. He kept adding more and more weight.

Totally bogus. If you can get down, then you do not need more weight. Adding more weight was just doing the obvious. Making her add more air to the BCD and becoming even less stable.

Perfect example why any toning down of the basic physics is not a good idea in MHO.

have i missed something here steve?
OW student decends down the line then floats back up.If theres no air in the bc then why not add weight?
or are you saying there WAS air in the BC and he added weight instead of dumping air?

---------- Post added May 11th, 2013 at 12:24 PM ----------

Good point, had a few questions and wanted to sign up quick. Tonight for sure.
New diver (50 - 100 dives). But all over the place... Chile, Mediterranean, San Diego, Mexico, Cuba and the cold lakes in Alberta. Live in Alberta Canada. So diving so few dives altogether but in fresh and salt, cold and warm, all different levels of exposure suits (except dry) and different tanks hasn't given me the stability to figure out a lot of things. So I make some jokes on this site but all in all I really am learning from it. Hopefully style isn't too blunt.

Panama Im not being a smart alec here but that is EXACTLY why you use a log book.Note your weight,water type,exposure suit etc.That gives you a baseline for the next dive in the same conditions.
You have probably worked out that basic dive training has to be taken down to the lowest common denominator.That being a 10 year old child.
The other thing is that because there are just so many variables you can introduce into any situation the manual would be about three feet deep if it covered everything off.
I recently did my rescue diver course and yes theres a lot of it that there is THE way to do things.Theres also a lot of it where its so dependant on the individual situation you can't have a fixed rule.Your best bet is to have training to cover the options and do what you feel is the best.

 

[TMHeimer]

Side note-- If you are diving with a tank that has a little water in it you are heavier because unlike a BCD, a tank is not a flexible container. Correct?

Forgetting the specifics of weight of air, amount of PSI, etc..... It's the total weight of the tank, no? ei., water weighs a lot more than air, so a tank completely full of water would weigh WAY more than one full of air and would sink you like a rock, no? Same as a tank full of air is heavier than one that's almost empty. So it's the total weight of the non-flexible container that counts....whatever is inside the tank becomes part of the tank's total weight....With the BCD, the water inside it is pretty much in contact with the water outside it--not separated inside a non-flexible container, thus no effect on the diver's weight. Correct?

 

[TSandM]

I think the point with the weight-adding post is that, if the student can sink, she is not terribly underweighted. She may not be carrying enough weight to sink with an empty tank, but she's certainly carrying enough to sink with a full one. If she immediately floats back up, it's probably either a technique problem with BCD usage, or badly controlled breathing. Adding more weight doesn't help with either, because although it can make up for poor breathing, the ensuing buoyancy instability due to the large gas volume in the BCD is worse than the original problem.

 

[TMHeimer]

Nope, not kdding, I actuallly challenged him on it and he told me how wrong I was. Funny though. Hope he isn't teaching his students to add water when they can't stay down. Ok, Maybe not so funny.

Well hey, maybe he is right. I find that after a dive if I have water in the BC when holding it up to drain it, I find it is heavier........
That COULD be his reasoning.......

 

[dfx]

I once had an instructor tell me that water in your BCD at the end of a dive is a symptom of being underweighted. Reason being that you are opening your BCD dump valve trying to sink but all the air is gone, thus letting in water. I added a few pounds and hardly any water in my BCD now.

There's some truth in that. If you're underweight and you try to sink more, you'll try to vent your BCD even though there's no air in it. That's when you get water in. Of course there's other ways to get water into your BCD as well.