Best way to approach being overweighted?

[mdoreen]

Give me tank pressure when full, manufacturer and size. I'll tell you on paper what the difference is

Thanks so much for the tips above - hand on chest for venting drysuit will be particularly useful I think. There's so many of these things that seem like they should be common sense but I haven't thought of them yet, grateful to have people tell me rather than trial and error!

The steel tanks I rented were 80 cu ft, 3300 psi in the water immediately before descending. Not sure on the manufacturer, but I'll check this weekend and let you know, I'm interested in the math and physics of the whole thing.

 

[mdoreen]

Oh, good. I didn't quite get that from your post, but I could have misread.

Air weighs about 1.2 grams per liter, and if you consume 90% of an aluminum "80" (which is further than I would typically feel comfortable), you'll use 2000L of gas, so that's 2.4kg or 5.4 lbs. But that calculation doesn't change between aluminum and steel, it's just the weight of the air. A high-pressure steel 100 is going to have about 20% more gas, so the swing would be another .5kg or 1 lb. Probably not a big deal unless you are significantly underweight.

Makes sense. So the difference in weight of the two tanks is a difference in the weight of the actual metal? ie aluminum is positively buoyant, so as you breathe more you're working against the positive buoyancy of the tank? Sorry if that's not right, just trying to understand the mechanics.

 

[Marie13]

Makes sense. So the difference in weight of the two tanks is a difference in the weight of the actual metal? ie aluminum is positively buoyant, so as you breathe more you're working against the positive buoyancy of the tank? Sorry if that's not right, just trying to understand the mechanics.

Here’s an easier way - aluminum tanks get floaty as you breathe them down so you have to carry more lead to counteract the floatiness. Steel tanks stay negative when you breathe them down so you don’t need the extra lead you do with aluminum tanks.

 

[Wathdoc]

That's good. For cold water (7mm+ or Drysuit), AL80 are the main culprits for smaller volume divers or divers with lots of exposure. They just aren't very "sinky" at the start of a dive on the surface and you have to add extra weight to compensate for them being positive at the end of your dive. LP Steels are good. HP Steels are great.

You will be practicing weight checking a lot as you dial in your exposure protection. Makes notes on your weight, if you were under/over weighted at the end of the dive, and the different in your configuration (especially your undergarment changes).

DUMP Drysuit
You should have drysuit squeeze when you trying to descend from the surface. If not, try to dump more from your suit. So make sure you put your shoulder dump all the way up and try to get all the air out as you are in a vertical position. Your hand should be on your chest to not build up air in the elbow to wrist section.

DUMP BCD
You are correct, if you are in trim position... you need to twist to get the dump valve to the highest position. You want the dump basically facing the sky and pushing your butt up towards the sky helps get the last bit of air out.

Lastly, if you must. SWIM DOWN. Lots of small volume divers do in cold water if they don't have very negative tanks.

The continuing exhale... The whole idea is to get your body volume as small as possible. Basically take your lungs to as low as you can maintain... say 20% volume with all the air out of your belly. You can inhale when you need to, but it should be small and quick. Exhaling is a lot longer. It may take up to a minute to get down, be patient.

Sounds like you are on your way and will figure it all out in time. Happy diving.

This reply is spot on if I might say so. With steel tanks, drysuit and bcd apart from incorrect weighting the major problem is either trapped air in one of those compartments or too much inflation of the lungs. Having a degree of suit squeeze early on is very important in the first few meters aids decent. As others have said, don't stress and deflate your lungs' lot of people don't. You should the slowly drift down even if you need a couple of gentle kicks to aid it.

 

[inquis]

I'm interested in the math and physics of the whole thing.

I find it simplest to consider that there are two aspects of cylinders, with no difference due to tank material (steel vs. aluminum): the force gravity imparts (i.e., dry weight) and the buoyant force.

The buoyant force is simply due to the volume (shape). If you fill a bathtub to the rim, then put your tank in the tub and push it down until it's just completely submerged, a) you'll make a mess on the floor, but b) the weight of the water (# of pounds) that was displaced (onto the floor) equals the buoyant force.

When diving, the difference between dry weight (W) and buoyant force (B) is what matters. If B is more than W, the object floats. It's not just limited to the tank, though. The exact same principle applies for you, your torch, your drysuit, etc. The bottom line is the TOTAL volume vs. the TOTAL weight (dry). For an AL80 tank, it turns out that the buoyant force is larger than its weight. However, it's not worth getting caught up on that any more than you worry about whether your body displaces more water than it is heavy.

Unsurprisingly, most people use the word "buoyancy" to be that bottom-line, net difference between total weight and total buoyant force.

If the net buoyancy is positive (buoyant force larger than weight), then you need to change that to be neutral. Adding lead increases both weight and buoyant force: weight by a lot and buoyant force by a little. The net result is the balance shifts toward the "weight" side, reducing buoyancy.

Switching out an aluminum tank for a steel tank (of comparable gas capacity): weight goes up and buoyant force goes down. Both serve to reduce buoyancy. A high-pressure steel more so than a low-pressure steel. If you were too buoyant before, you'll be better now. If you were already neutral (or even negative) before, you're going to be worse now. In the latter case, you'd need to add buoyancy back in -- as you might expect, reducing the amount of lead is an easy way to do that.

So choice of tank vs. lead act the same with regard to total buoyancy (whether you sink or rise). One major reason people use steel tanks, though, is that it distributes the weight higher on the body, affecting TRIM (whether you rotate). (Same thing with stainless steel backplates.) People that use aluminum tanks very often need to put some of their lead on a tank band or trim pockets. They don't carry MORE weight (as that would impact buoyancy), they just change where it's positioned.

Hope it helps!

 

[Mark Newland]

Here’s an easier way - aluminum tanks get floaty as you breathe them down so you have to carry more lead to counteract the floatiness. Steel tanks stay negative when you breathe them down so you don’t need the extra lead you do with aluminum tanks.

Looking at your experience vs mine and I ask myself, do I want to argue this with you? HELL NO, I would lose. However, I do question it.

I already know that aluminum tanks go from "negative buoyancy" to "positive buoyancy" as the tank become empty. I also know that steel tanks REMAIN negatively buoyant throughout the dive. As I understand it steel tanks go from HEAVY to less heavy yet still negative buoyant as they become empty so you still have to take into consideration the "floatiness" of them at the end of the dive.

For a proper discussion, let's compare aluminum 80's vs steel 80's (same air volume). Is the amount of "delta" of going from a full aluminum 80 to an empty aluminum 80 the same as the "delta" going from a full steel 80 to an empty steel 80. Since the weight of air is the same, the delta should be the same and hence the "floatiness" over time should be the same. However, I do agree that the overall buoyancy of steel tanks is "heavier" and therefore less weight is needed on your body (or vest, etc).

 

[Marie13]

Looking at your experience vs mine and I ask myself, do I want to argue this with you? HELL NO, I would lose. However, I do question it.

I already know that aluminum tanks go from "negative buoyancy" to "positive buoyancy" as the tank become empty. I also know that steel tanks REMAIN negatively buoyant throughout the dive. As I understand it steel tanks go from HEAVY to less heavy yet still negative buoyant as they become empty so you still have to take into consideration the "floatiness" of them at the end of the dive.

For a proper discussion, let's compare aluminum 80's vs steel 80's (same air volume). Is the amount of "delta" of going from a full aluminum 80 to an empty aluminum 80 the same as the "delta" going from a full steel 80 to an empty steel 80. Since the weight of air is the same, the delta should be the same and hence the "floatiness" over time should be the same. However, I do agree that the overall buoyancy of steel tanks is "heavier" and therefore less weight is needed on your body (or vest, etc).

The explanation I gave is simple for new or non-divers. My LP85s and LP50s don’t get floaty - or at least don’t get breathed down enough to get floaty.

 

[TMHeimer]

Looking at your experience vs mine and I ask myself, do I want to argue this with you? HELL NO, I would lose. However, I do question it.

I already know that aluminum tanks go from "negative buoyancy" to "positive buoyancy" as the tank become empty. I also know that steel tanks REMAIN negatively buoyant throughout the dive. As I understand it steel tanks go from HEAVY to less heavy yet still negative buoyant as they become empty so you still have to take into consideration the "floatiness" of them at the end of the dive.

For a proper discussion, let's compare aluminum 80's vs steel 80's (same air volume). Is the amount of "delta" of going from a full aluminum 80 to an empty aluminum 80 the same as the "delta" going from a full steel 80 to an empty steel 80. Since the weight of air is the same, the delta should be the same and hence the "floatiness" over time should be the same. However, I do agree that the overall buoyancy of steel tanks is "heavier" and therefore less weight is needed on your body (or vest, etc).

I think you and Marie say the same thing with different words. Since the AL tank is positive at the end of the dive it is working to float you. Since the steel tank remains negative (though only slightly), it is part of the package that is weighing you down. Neither has anything to do with how much lead you need. The weight check is all you need.
If the AL tank were even just a tiny bit negative when near empty I figure it would not be "floaty" and so different a feel than the steel tank.

 

[mdoreen]

I find it simplest to consider that there are two aspects of cylinders, with no difference due to tank material (steel vs. aluminum): the force gravity imparts (i.e., dry weight) and the buoyant force.

Ah, that does make sense! As TMHeimer said, I know it's not too relevant to finding out the actual amount of weight I need, but I do find it interesting to think about how it all works.

 

[inquis]

it's not too relevant to finding out the actual amount of weight I need

I think it's quite relevant. With steel tanks you need less lead because they are less buoyant and somewhat serve the same purpose as the lead.

As an example, all of these configurations are neutral for me with my gear/exposure suit in salt water plus:

• 10 lbs lead + AL backplate + AL80 tank
• 7 lb lead + SS backplate + AL80 tank
• 0 lb lead + AL backplate + HP120 tank

(Actually I'm slightly overweighted with the latter, but it's the closest to neutral I can get with that much air.)

My point being, look at the total picture. Neutral can be achieved in various ways. In the first configuration, I have to have 7 of the 10 lb on my tank band for optimal trim. The second, I only need 4 of 7 on the tank band. The third doesn't trim the greatest (requiring more extension on the legs and less of the arms), but 50% more air is beneficial in some circumstances.

Bottom line: total lead required to be neutral depends on your configuration, including choice of cylinder. There are advantages and disadvantages to every choice of configuration. Personal call as to which of those are more important.