The case against ditchable weight

[cool_hardware52]

So you think your 7 mil loses 5 lbs from surface to 15'? Maybe but my 3 mil doesn't.

I don't "think" it does, I *know* it does.

Did you actually read Eric's post? He notes that a diver in less exposure suit might find themselves slightly negative at the surface.

That because thinner suits compress less.

However thin suits require less ballast, so a correctly weighted diver in a thin suit suit need not ever be very negative.

The typical 3mm will be about +4 at the surface, and will probably lose about -1 ~2 lbs at 15 ft. If one assumes an al 80 with about 5 lbs of useable gas that leaves the diver with a 3-4 lbs problem.

It's pretty simple to compensate for about 1/2 (maybe more) of that via breathing. Now the diver has a 1~2 lbs problem. While I would prefer to be neutral to positive at the surface -2 lbs can be tolerated by most divers for quite a period.

My standard advice for divers in 3mm suits and al 80s is to start the dive negative by about -2 lbs. with a full cylinder and empty wing.

Tobin

 

[tursiops]

However thin suits require less ballast, so a correctly weighted diver in a thin suit suit need not ever be very negative.

The typical 3mm will be about +4 at the surface, and will probably lose about -1 ~2 lbs at 15 ft. If one assumes an al 80 with about 5 lbs of useable gas that leaves the diver with a 3-4 lbs problem.

It's pretty simple to compensate for about 1/2 (maybe more) of that via breathing. Now the diver has a 1~2 lbs problem. While I would prefer to be neutral to positive at the surface -2 lbs can be tolerated by most divers for quite a period.

My standard advice for divers in 3mm suits and al 80s is to start the dive negative by about -2 lbs. with a full cylinder and empty wing.

Tobin

Tobin, I'm beginning to see where we differ in our assumptions (not in our physics...I think we agree on that!).
(1) I'm not willing to leave a stressed newish diver at the surface (or underwater) with ANY negative buoyancy that must be compensated by lung usage. When one is entering panic, or even just badly frightened, niceties like keeping one's lungs full (or exhausting them, for that matter) tend to be forgotten. So where you say, "While I would prefer to be neutral to positive at the surface -2 lbs can be tolerated by most divers for quite a period" I've got to go with not asking that diver to tolerate ANY negative at the surface.
(2) I'm also not willing to ask that diver to overcome being positive due to his wetsuit at the surface such that he must actually swim down or pull down to get to his suit-compression depth. Actually pull-down, maybe, not not a real head-first swim down. The main reason I want to avoid this is because for many folks it is significantly harder to equalize while head down, and I don't want to put that on them.

So my more conservative approach to weighting a not-that-experienced diver in a 3mm suit is to get them more like 4-5 pounds neg with a full tank and empty BCD, at the surface. A thicker suit, or a more spherical person with a lot more 3mm neoprene on them, will take a bit more.

We are only differing by about 2 pounds in our starting weight for that normal 3mm diver, so it is not the end of the earth...although to read this thread it sounds like it! My prototypical newish, verging on stressed, diver has enough to worry about; I don't want to add to it.

 

[cool_hardware52]

Tobin, I'm beginning to see where we differ in our assumptions (not in our physics...I think we agree on that!).
(1) I'm not willing to leave a stressed newish diver at the surface (or underwater) with ANY negative buoyancy that must be compensated by lung usage.

I fear you may not yet understand what I recommend WRT weighting.

I've never advocated using only your lungs to offset *negative* buoyancy. I have suggested it is both possible and reasonable for divers to use their lungs to counter modest amounts of unwanted *positive* buoyancy. Failure to achieve this either sends the diver to the surface (slowly) or leaves them on the surface. Neither is much of a risk for single tank rec diving.

When one is entering panic, or even just badly frightened, niceties like keeping one's lungs full (or exhausting them, for that matter) tend to be forgotten. So where you say, "While I would prefer to be neutral to positive at the surface -2 lbs can be tolerated by most divers for quite a period" I've got to go with not asking that diver to tolerate ANY negative at the surface.

Again, with little to no exposure suit and a fit diver it may not be possible to avoid having the diver neutral or positive with using their BC, as their reg, cylinder and simple BC will leave them negative. I routinely council people to consider using at least a 3mm suit, even it they leave 1/2 zipped to mitigate this risk.

(2) I'm also not willing to ask that diver to overcome being positive due to his wetsuit at the surface such that he must actually swim down or pull down to get to his suit-compression depth. Actually pull-down, maybe, not not a real head-first swim down. The main reason I want to avoid this is because for many folks it is significantly harder to equalize while head down, and I don't want to put that on them.

My basic ballast recommendation is "Eye level at the surface with no gas in your BC and a full cylinder" This may need to be modified a bit with thinner suits, but most divers weighted this way can easily initiate a decent by kick up and exhaling, no need for a free diver like descent.

The reality is most divers will err on the side of slightly too much weight anyway.

We are only differing by about 2 pounds in our starting weight for that normal 3mm diver, so it is not the end of the earth...although to read this thread it sounds like it! My prototypical newish, verging on stressed, diver has enough to worry about; I don't want to add to it.

I consider any diver I can help get within 1-2 lbs of ideal ballast a win.

Tobin

 

[Eric Sedletzky]

The end goal is to be able to hold a stop with a near empty tank with no air in the BC and control the stop with breathing alone.
It just so happens that with this weighting I can float on the surface with no air in my wing with a full tank. I use everything from steel 72 up to a steel E7 120. The adjustment for each tank is different weight belts, and I have several for all my tank/suit/plate combinations. All it is is a balancing act and simple math.
Most of my diving is in a 7mm beaver tail suit, nothing fancy. I also have a 3 mil and a 5 mil. I use those in fresh water up at Lake Tahoe in the summer. Again, no air in wing and can float fine. Many times I don't even use a wing and can snorkel out to my drop spot and swim down, swim around, and swim back up.
I don't know how to explain it, but I can float no problem with no air in my wing and surface swim out in the ocean too with a big tank. At the end of the dive I can hold a stop with no air in my wing and then surface swim back in with no air in my wing. Sometimes I don't use a wing in the ocean either, and I can still surface swim out do my dive and swim back in. I'm not a fat guy either, well maybe a little roll for a 53 year old guy, but I try to keep in shape.
For a while I liked to practice doff and dons in a pool with nothing but a steel 72 held onto my back with straps. I would wear board shorts, no wetsuit of any kind, and I could stay on the surface when I jumped in. I would exhale and begin to sink slowly, then inhale when I was down to stop the descent, then swim around in any position at any depth perfectly neutral. If I can do this in a pool of fresh water then I could do it in a warm ocean and be even lighter in the water.
I just don't understand why the concept of being able to float on the surface without the aid of a BC or wing is so difficult for some people to grasp.
People used to dive with no BC's all the time and there was nothing to save them on the surface so they had to know how to float. They dove just fine. So now we have BC's, which you'd think would make the world of diving a much better place, but instead we have people coming up in distress then sinking back down and drowning!?!? I don't get it?

Just one question to the naysayers and push button divers about being positive or at least neutral on the surface with no air in the BC:
Do you seriously feel that it's OK for a diver to rely solely on the suspension of a bag of air to stay on the surface with integrated weights (or any weights) that are difficult to remove when seconds count? Or to be overweighted to the point that a diver must rely on the BC to stay on the surface anytime during the course of a dive? I could re-read the thread, but I don't remember seeing anything about redundant buoyancy either for integrated jacket divers.
I'm assuming this whole conversation is about recreational single tank diving since the OP asked about losing integrated weight pockets out of a jacket BC, Right?
If it gets into tech diving then there's a whole different set of rules, but I don't think that's what we're talking about.

 

[Storker]

Many times I don't even use a wing and can snorkel out to my drop spot and swim down, swim around, and swim back up.
I don't know how to explain it, but I can float no problem with no air in my wing and surface swim out in the ocean too with a big tank. At the end of the dive I can hold a stop with no air in my wing and then surface swim back in with no air in my wing. Sometimes I don't use a wing in the ocean either, and I can still surface swim out do my dive and swim back in.

You have to explain the physics behind this, because I can't see any way that this is physically possible.

The buoyancy swing of an Al80 (3000 -> 500 psi) is 2.3 kg (~5lbs), an HP100 (3440 -> 500 psi) is 2.9 kg (~6.5 lbs). If you're neutral on the surface with a full tank, how can you not be buoyant at the end of the dive when you're two to three kilos lighter? And if you're neutral on the surface at the end of the dive, how can you be neutral with a full tank when you're two to three kilos heavier?

And then there are posters who obsess about overweighting with a couple of pounds...

 

[cool_hardware52]

And then there are posters who obsess about overweighting with a couple of pounds...

At the risk of repetition…...

If a diver in a thick suit and modest sized single tank (no 190's) starts the dive weighted so they are eye level at the surface with no gas in their wing and a full cylinder they can easily hold a 15 ft stop when after they have exhausted most of their gas simply because their suit will be far less buoyant at 1.5 ata than it was at 1 ata.

This same diver will be buoyant at the surface at the end of the dive because they are lighter by the weight of the gas they consumed.

None of this requires violating any of the laws of physics.

There simply is no need to be negative. Slightly positive floats, slightly negative doesn't Pretty simple.


Tobin

 

[Eric Sedletzky]

You have to explain the physics behind this, because I can't see any way that this is physically possible.

The buoyancy swing of an Al80 (3000 -> 500 psi) is 2.3 kg (~5lbs), an HP100 (3440 -> 500 psi) is 2.9 kg (~6.5 lbs). If you're neutral on the surface with a full tank, how can you not be buoyant at the end of the dive when you're two to three kilos lighter? And if you're neutral on the surface at the end of the dive, how can you be neutral with a full tank when you're two to three kilos heavier?

And then there are posters who obsess about overweighting with a couple of pounds...

I never said neutral at the end of the dive on the surface, I meant positive. It doesn't matter if it's slightly positive or really positive, the point is it's positive meaning that I won't sink unless I work at it.
That also means slightly positive at the surface at the beginning of the dive with a full tank (no air in wing). My theory is that my black or dark colored wetsuit has been out in the sun and the gas bubbles are pumped. That possibly supports the heaviness of the air I'm carrying, however even after a considerable surface swim sometimes where you'd think the suit would begin to cool off and the gas bubbles would get smaller I can still float, so who knows?
Then as I dive the suit obviously compresses and losses buoyancy at depth, this is where the need for a wing and a few huffs of air are needed. Then also at depth the ocean water begins to cool the suit so not only is it compressed but the gas bubbles are cooling and reducing in size also thus reducing buoyancy. So by the time the air is used up (5-6 lbs) I'm on my way up with a much less bouyant suit because suits don't just spring back to full size as you come up, it takes a while, and it's also cooled down. So the combination of the two has offset the loss of air weight somewhat to a trade off. This is how we are able to set our weighting so we can dive with no BC's. There's a lot more to it than just losing 5 lbs of gas.
The fine tuning of weight comes in play with every different suit I own and it's particular characteristics.
There's also the pure physics of the 15 foot stop, which is the most buoyant shift critical area in the water column as far as weight swing. This is why it's a stop BTW. And because of the wild weight swings at the 15' stop, I don't like to have any air in my wing because it sucks to have to constantly feather the trigger on the inflator and dump, so if I have to do that it obviously tells me I have too much weight on that doesn't need to be there. But I figured that one out on my own 15 years ago, with no help from any instructors BTW.

With my wetsuits: The cheap stuff smashes down something terrible, and my Rubatex suits spring right back, plus they are nitrogen filled so it's less prone to crushing. I took me a lot of diving and experimentation to figure all this out and I don't expect other people to take it to quite this degree, but it's what I do.

 

[tursiops]

There's also the pure physics of the 15 foot stop, which is the most buoyant shift critical area in the water column as far as weight swing. This is why it's a stop BTW.

No, not really. that is a coincidence. 15 ft is the stop because it is between 20 ft (too deep to get much off-gassing) and 10 ft (too much off-gassing).

As an aside, I wish people wouldn't talk about how much their buoyancy changes because they are shallow. What they mean is that one foot of vertical motion when shallow has more effect on buoyancy than one foot of vertical motion when deep....it is a proportional thing, not an absolute thing. For you calculus buffs, it is d/dz of Boyle's law.

 

[dumpsterDiver]

I think people should be weighted so they can breathe normally and comfortably and still maintain neutral buoyancy at any depth. The statement that divers don't need BCs unless they wear wetsuits to be dangerous advice and inconsistent with the directives of all training agencies AND inappropriate for the basic scuba forum.

 

[cool_hardware52]

I think people should be weighted so they can breathe normally and comfortably

Ya, me too. Breathe, as in inhale and exhale.

Virtually every BOW student realizes on their first dive that breathing impacts buoyancy.

I do have to wonder why you persist in intentionally distorting what others advise.

Tobin