Too heavy after a certain depth ?

[gcarter]

You made some claims. I asked you to substantiate them. It's not a game. It's part of the discussion and exchange of ideas that is supposed to happen on a forum like this. You are obviously unable to substantiate even the most basic facts relevant to the claims you are making. You may be able to bluff a few ignorant people. The rest of us recognise you simply don't know what you are talking about. Thanks for sparing us another irritating and futile discussion.

OK.

Go with that.

 

[scubafanatic]

Might I suggest equipping your BCD wing inflators with Scubapro BALANCED power inflators. While they ain't cheap @ $ 100/each, they act like a scuba regulator with a balanced 2nd stage, meaning the inflator increases the air flow into the wing with depth, so the inflation rate remains constant regardless of depth, you don't experience the issue like you do with the cheapo inflators that can't keep up the air flow rate with increasing depth.

 

[NAM001]

the variation of density BECAUSE OF DEPTH is next to nill as far as a diver is concerned. Temperature has a significant effect on the density. Any submarine sailors can testify to that. a temp change from 80f to 50f could result in a 20-40 thousand lb bouyancy change in a ship. It is not much considering that change is felt over a 300-400 foot vessel 20 ft in diameter. So for a diver it is non exstant to speek of. In my opinion the suit compression is logrithmic in that say every atm you loose say half of the remaining bouyancy, there is just not much left after 4 or 5 ....1/2 cuts. If i remember right you loose about 90 % of the lift at 100 ft. This is the lift that is being refered to in the OP. Yes the more lift you loose the faster your decent rate becomes. Could you get to the point that you would be unable to inflate the bc and go up? Yes you could but you would have to work to do it. you would have to be so heavy that you were using both the suit and the bc to comp with at a shallow depth. And then at septh the loss of teh suit lift would have to be greater than the remailing reserve in the bc or wing.

For the context we're talking about, up to say 100 m depth, what would be the variation in water density?

 

[socc]

So is their a way to calculate how much weights you need depending on your gear ?
The only way that I learnt in my OW course was the one that you go inside the water and after exhaling your eyes should be in the water level, which means your weights are ok !
Is that a reliable method ? Do you have to use a different method for when using wet suit and when using dry suit ?

 

[NAM001]

If you do that you cant get into the situation you are asking about providing you have the correct sized bcd or wing. Ideally you have no air int he wing or bcd and at say 20 ft and nearly an empty tank you adjust you weight to become neutral. The method you reference will show gross weighting errrors adn will get you close.

At this point of the discussion forget about numbers and think about a see saw on a play ground. weight on one side and weight on the other representing lift. all your tanks and gear is -X for bouyancy ( heavy, sinking) to counter this is your suit which is +y ( lift ) x does not = Y at this point. Now you add or remove weight on the lift side so that Y +/- weight = X . now the math tanks and rig = -10 your suit +14 so you add - z of lead on the belt to make the negs = the pos's. -10 -4 counters + 14 ,,,,,-10 + -4 + 14= 0 or neutral. So you are now neutral with a tank with say 400 psi in it. Now fill the tank to full 3000 psi that is about 65 cu ft. air is 8# per 100 cu ft. so you just added about 6# of air weight. You are now 6# heavy. This is what your bcd is for (variable bouyancy device to counter the bouyancy changes. so to start your dive you will need 6# of lift in your bcd to counter the full tank of air. now you go down to 30 ft you suit now only lifts 7#. you are now 7# heavy so you put more air in the bcd. 13# lift now. you go to 100 ft and you suit only has 2# lift. so its more air in the bcd. Your bcd now has 16# or so of lift in it to counter the full tank and the loss of lift from the compressed suit. You now breath 1/2 of your air and your tank getl lighter and you are floating up so you vent the bcd to dump some of the lift. your air is now 3/4 gone adn you head to safety stop depth you have 500 psi,,, so your tank is 6# lighter , your suit expands and is giving you 14 lift again and you need NO air in the bcd. THIS IS THE GOAL , to hold safety stop with neutral bouyancy and no bcd lift.

So now lets talk about the bcd size needed.... At most you were -18 at 100 ( -6 from full tank and -12 from the suit compression) . so you need at min a bcd that will lift 18#. Since you are a buddy you will need some spare lift to assist your buddy if he has a lift failure. so you now need a min of say 30. Why 30 well you are -18 and he is say -18. that is -36. if you fill you bcd you are now only -6 and you can fin kick your self up till your suits expands.

Keep in mind this only deals with bouyancy and not TRIM. Trim involves where you put the weight that makes you neutral.
Also remember if you go from a 7mm to a 3mm suit the whole process begins from scratch at ssafety stop depth 400# and adjusting weight to get neutral again.

There are a lot of things in OW that is done for the instructors benifit that does not necessarily work in non training diving. trim and bouyancy is the biggy.

Lastly dry suits are a different animal. with a shell DS you have no compression to counter. a crushed NEO suit has little if any. a non crushed neo is the same as a wet suit. I will say this ,,, one thing at a time.

Hopes this helps some.

So is their a way to calculate how much weights you need depending on your gear ?
The only way that I learnt in my OW course was the one that you go inside the water and after exhaling your eyes should be in the water level, which means your weights are ok !
Is that a reliable method ? Do you have to use a different method for when using wet suit and when using dry suit ?

 

[socc]

If you do that you cant get into the situation you are asking about providing you have the correct sized bcd or wing. Ideally you have no air int he wing or bcd and at say 20 ft and nearly an empty tank you adjust you weight to become neutral. The method you reference will show gross weighting errrors adn will get you close.

At this point of the discussion forget about numbers and think about a see saw on a play ground. weight on one side and weight on the other representing lift. all your tanks and gear is -X for bouyancy ( heavy, sinking) to counter this is your suit which is +y ( lift ) x does not = Y at this point. Now you add or remove weight on the lift side so that Y +/- weight = X . now the math tanks and rig = -10 your suit +14 so you add - z of lead on the belt to make the negs = the pos's. -10 -4 counters + 14 ,,,,,-10 + -4 + 14= 0 or neutral. So you are now neutral with a tank with say 400 psi in it. Now fill the tank to full 3000 psi that is about 65 cu ft. air is 8# per 100 cu ft. so you just added about 6# of air weight. You are now 6# heavy. This is what your bcd is for (variable bouyancy device to counter the bouyancy changes. so to start your dive you will need 6# of lift in your bcd to counter the full tank of air. now you go down to 30 ft you suit now only lifts 7#. you are now 7# heavy so you put more air in the bcd. 13# lift now. you go to 100 ft and you suit only has 2# lift. so its more air in the bcd. Your bcd now has 16# or so of lift in it to counter the full tank and the loss of lift from the compressed suit. You now breath 1/2 of your air and your tank getl lighter and you are floating up so you vent the bcd to dump some of the lift. your air is now 3/4 gone adn you head to safety stop depth you have 500 psi,,, so your tank is 6# lighter , your suit expands and is giving you 14 lift again and you need NO air in the bcd. THIS IS THE GOAL , to hold safety stop with neutral bouyancy and no bcd lift.

So now lets talk about the bcd size needed.... At most you were -18 at 100 ( -6 from full tank and -12 from the suit compression) . so you need at min a bcd that will lift 18#. Since you are a buddy you will need some spare lift to assist your buddy if he has a lift failure. so you now need a min of say 30. Why 30 well you are -18 and he is say -18. that is -36. if you fill you bcd you are now only -6 and you can fin kick your self up till your suits expands.

Keep in mind this only deals with bouyancy and not TRIM. Trim involves where you put the weight that makes you neutral.
Also remember if you go from a 7mm to a 3mm suit the whole process begins from scratch at ssafety stop depth 400# and adjusting weight to get neutral again.

There are a lot of things in OW that is done for the instructors benifit that does not necessarily work in non training diving. trim and bouyancy is the biggy.

Lastly dry suits are a different animal. with a shell DS you have no compression to counter. a crushed NEO suit has little if any. a non crushed neo is the same as a wet suit. I will say this ,,, one thing at a time.

Hopes this helps some.

Hello KWS,

First of all many thanks for your very detailed and explanatory reply !

I think I get it. Something that many beginners forget (at least thats what I think), is that air actually has weight ! Also, in my OW course no one really told me about all this kind of stuff and I think that is probably for most people. They just tell you, that you put weights to compensate the positive buoyancy from the suit which looses lift at depth, etc. but nothing in detail to understand what is really going one and how important might this be. Probably this is because it the first certificate you get and it might not be that important, but I actually find it important and I am trying to improve my knowledge in diving using this forum. Some people that do not do that though, they might have troubles (hopefully not, but...) !!!

So the method I mentioned above is quite reliable but not perfect, right ?

Also, from what I understood, with almost empty tank you should have neutral buoyancy with zero lift from the BCD (i.e. empty BCD) at safety stop depth. In addition, the BCD should be able to compensate the weight of a full tank at safety stop depth to start with.
Then, as the suit looses buoyancy by going deeper, the BCD should be able to compensate the full tank and lost lift from the suit.
Finally, as you breath air the tank becomes lighter so after some point you get quite a lot of lift back and your BCD needs lift to compensate. To end with, as you start to ascend, air in the BCD expands giving you more lift and the suit starts giving you more lift as well.
Did I understood right ?
So at the end of the day you have to make sure that your BCD can compensate mainly the weight of a full tank and the lost lift from a neoprene suits depending on the planned max depth ?

Another question I have is: Does the BCD loose lift "power" (sorry I did not know how to state it otherwise) as you go deeper ? If so why ? Or its lift "power" is the same whatever the depth ?

Many thanks,

Socc

 

[Foxfish]

So is their a way to calculate how much weights you need depending on your gear ?
The only way that I learnt in my OW course was the one that you go inside the water and after exhaling your eyes should be in the water level, which means your weights are ok !
Is that a reliable method ? Do you have to use a different method for when using wet suit and when using dry suit ?

That was what I was taught also. It is common for new divers to have trouble with descents because they tend to find it hard to relax and exhale so they hold their breath or they keep finning. To compensate they add a lot of weigh to their BCD. There are some other good threads on the forum that discuss this in detail.

If you follow the method you describe with your weighting then you shouldn't have the problems you mentioned in the OP. I would certainly not try to calculate the weight I need. If you find you are starting to surface at the end of your dive after emptying all the air out of your BCD then be sure to add an additional weight.

By the way. I've watched the video of the Russian diver Yuri Lipski who went straight to the bottom of the Dahab blue hole. My take of this was that it was intentional. As others have pointed out, diving to that depth with a single tank of air is very risky. If is a relatively simple and safe dive if you the required training and equipment.

 

[teknitroxdiver]

Might I suggest equipping your BCD wing inflators with Scubapro BALANCED power inflators. While they ain't cheap @ $ 100/each, they act like a scuba regulator with a balanced 2nd stage, meaning the inflator increases the air flow into the wing with depth, so the inflation rate remains constant regardless of depth, you don't experience the issue like you do with the cheapo inflators that can't keep up the air flow rate with increasing depth.

I don't believe that's an actual feature of those.

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[CT Sean]

So at the end of the day you have to make sure that your BCD can compensate mainly the weight of a full tank and the lost lift from a neoprene suits depending on the planned max depth ?

That's part of it. A BC also needs to be able to float itself without the diver in it should it need to be removed for any reason (tangled up in something or kayak diving as examples) It would be a shame to remove it and watch it sink to the bottom if you let go of it.

Another question I have is: Does the BCD loose lift "power" (sorry I did not know how to state it otherwise) as you go deeper ? If so why ? Or its lift "power" is the same whatever the depth ?

Many thanks,

Socc

No, a BC does not lose "lift power". A BC that displaces 30 lbs of water on the surface when full will displace 30 lbs of water at 100 feet when full as well. It just takes 4 times the amount air to fill it.

 

[socc]

That's part of it. A BC also needs to be able to float itself without the diver in it should it need to be removed for any reason (tangled up in something or kayak diving as examples) It would be a shame to remove it and watch it sink to the bottom if you let go of it.





No, a BC does not lose "lift power". A BC that displaces 30 lbs of water on the surface when full will displace 30 lbs of water at 100 feet when full as well. It just takes 4 times the amount air to fill it.

That makes perfect sense !
Thanks !

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