Question on tank bouyancy and weights

[black1]

I dive Faber steel HP 120's and the full weight w/valve is -16.2lbs and empty or 500psi is -7.2 lbs. My question is i dive south florida warm water all year round and i wear a 3mm year round and use 6lbs of weight, could i drop all my weight and just use the tank weight as needed, i have a good lift wing so it's no problem getting to the surface, what are your thoughts? i also carry bare minimum (small flashlight. sheers, 2 lift bags and small clips)

 

[elan]

You do not have many options as I see it. Yes definitely you drop the weights. You will be slightly negative when empty and pretty much negative when full. You need to make sure you have backup buoyancy device in case your BCD fails. You cannot compensate 10lb with just lungs and you do not have ditch able weight. You might as well be heavy on the back.

 

[tedwhiteva]

Do a weight check at the end of a dive - hand off your weight belt to buddy with 500-700 psi in tank at 15 feet. If you can stay at 15 feet, you don't need weight belt!

That said, it also means you are always negative on the dive. So in the event of BC failure, you'll have to swim to surface. Good reason to carry a backup buoyancy device aka lift bag.

my $.02 and worth every penny you paid for it

 

[LeadTurn_SD]

I dive Faber steel HP 120's and the full weight w/valve is -16.2lbs and empty or 500psi is -7.2 lbs. My question is i dive south florida warm water all year round and i wear a 3mm year round and use 6lbs of weight, could i drop all my weight and just use the tank weight as needed, i have a good lift wing so it's no problem getting to the surface, what are your thoughts? i also carry bare minimum (small flashlight. sheers, 2 lift bags and small clips)

Are you currently using 6lbs with the 120's, or with a lighter tank (Al80 for example)? I was not sure what you meant.

Another thing to think about if you are diving heavy steel with a thin wetsuit: Getting to the surface may or may not not be a problem.... but I'd worry a little about staying at the surface if you are significantly negative and have a wing failure.

You need to consider just how negative you are at the beginning of the dive, especially if you have no ditchable weight? (Meaning that all your ballast is in the tank, and none can be easily dropped).

You might be just fine with the 120's and a 3mm wetsuit and no ditchable weights, you'll just need to do a bouyancy check to see. The good thing about 3mm wetsuits is that they have less buoyancy to lose at depth compared to thicker wetsuits.... the bad thing is that they do not provide as much buoyancy at the surface. I'd make sure I could keep my head above water at the start of the dive with an empty BC.

Best wishes.

 

[rstofer]

I'd make sure I could keep my head above water at the start of the dive with an empty BC.

Best wishes.

That just doesn't seem possible with the OP's configuration. Over in the BCs forum, there is a sticky spreadsheet that makes the assumption that it requires 10# of flotation to keep the head above water. I have no reason to doubt it.

Given that, the following is a wild 'what if...'.

I don't know how much buoyancy a 3mm wetsuit has but I'm willing to guess about 10# at the surface and at 15' but only 4# at great depth (say 100'). These are all a wild guess and have no basis in reality.

Oddly enough, the spreadsheet shows a requirement for 2.8# of lead to make the 15' stop assuming the wetsuit actually has 10# of lift at 15' at the end of the dive when the tank is only -7.2#. It may not. Perhaps it has lost 3# of lift and the lead isn't required.

More important: there is a need for 19# of lift at the start of the dive to keep the head fully above water. We have -10# head above water, -16.2# tank, -2.8# lead (see above) and +10# for the wetsuit. Total is 19# negative! Break an inflator hose jumping off the boat and the diver sinks like a stone!

Now, I don't know anything about 3mm wetsuits and the buoyancy characteristics will alter these numbers substantially. But not enough to be neutral at the surface with an empty BC and a full tank!

There are two schools of though on initial buoyancy: First, float eyeball level with an empty tank and an empty BC. This might work if the buoyancy characteristic of the wetsuit is slightly higher. The buoyancy for the head has been reduced to, perhaps, half so -5# for the head, -7.2# for the tank, -2.8# for the lead and +10# for the wetsuit leaves the diver 5# negative. Perhaps the wetsuit is a little more buoyant. But then the lead requirement will increase in order to make the stop. This doesn't look encouraging... But it might be swimmable if the diver ditches the weighbelt at the surface when the BC has failed and the tank is empty. I wouldn't want to do it!

The other method is from Deep Sea Supply: eyeball level with a FULL tank and empty BC at the surface. We have -5# for the head (eyeball level), -16.2# for the tank, -2.8# for the lead and +10# for the wetsuit. Total: 15# negative. Even worse! Probably not swimmable even dumping weights. Ditch the gear and watch it sink!

Unless my math, and the spreadsheet, are way off, there is no situation under which such a highly negative tank should be used in warm water with a thin wetsuit.

It would be worth the time to use the spreadsheet with real values for the wetsuit buoyancy.

Richard

 

[black1]

"Are you currently using 6lbs with the 120's"

With my 120's, I will also be using a Diverite Transpac i just purchased with the "Travel EXP" wing (26lbs of lift).

 

[LeadTurn_SD]

Hi Richard, we are often in agreement, and I agree with your post

I dive a 3mm full, using Al80. I'm a big guy (6'5", 260lbs) so I use a 3XL suit. With my, ummm, "bioprene", and neoprene, I could dive a tank that negative and be fine, but it is very possible the OP will be overweighted. He does need to figure out how much "personal" and wetsuit bouyancy he has, then do the math.

Best wishes.

 

[rstofer]

I really don't like the idea of no ditchable weight. As I recall, the very first step when you rescue a diver and get them to the surface is to ditch their weights to establish positive buoyancy.

There are other considerations should the BC happen to fail at the beginning of the dive when the diver is 19# negative with no ditchable weight.

Richard

 

[LeadTurn_SD]

I really don't like the idea of no ditchable weight. As I recall, the very first step when you rescue a diver and get them to the surface is to ditch their weights to establish positive buoyancy.

There are other considerations should the BC happen to fail at the beginning of the dive when the diver is 19# negative with no ditchable weight.

Richard

Agree, regarding ditchable weight. But for very lean divers with thin wetsuits, there just may not be much (or any) ditchable weight even when using Al80's. My opinion is that for safety you must be able to get "positive" at the surface with an empty wing and a full tank.

For very lean divers with thin suits & no ditchable weight, it could even mean ditching their rig at the surface in the event of a wing failure :shocked2: ....

The bottom line is that the OP needs to determine the bouyancy of his 3mm and his personal bouyancy to make an informed, safe choice. He might be able to pull it off, but my guess is that if he is wearing 6lbs currently, he will be significantly overweighted with the 120. Again, I'm not at all clear if 6lbs is what he is currently wearing with the 120 and a thicker suit, or with a lighter tank, or what?

Best wishes.

 

[black1]

This weekend i did a 110ft wreck dive and then a 65ft drift, i dove my steel 120' with a Hollis H100 bc, 8lbs of weight (too much) and 3mm wetsuit with a 3mm tank top with hood neoprene piece and came up both times with 500psi in my tanks and did my safety stop at 15ft and held it with no problems at all. The hollis is heavy wet and i had no problem. I will now be using a transpac (much lighter than the Hollis) and i was just curious about ditching my weight all together, I do have weight pockets attacthed to it so i can test it out and i also have a ditchable weight belt i could try as well, just curious on your guys thoughts about weight or no weight needed with this setup, thanks all.

Maybe i should try my weight belt with around 2 to 4 lbs on it.