Weighting and being an Air Hog - Advice?

[DeaDLocK]

Hi guys,

I got my OW in April and have logged about 10 dives since.

I weigh about 200 pounds, dive with a full 3mm suit in saltwater and go down with 10 pounds of ballast.

With a full tank of air at the start of the dive, I sink, but very slowly, so about a metre in I flip and swim down. Once I'm past 30 feet I can control my buoyancy well without needing to touch my BCD at all (it stays empty until I surface at the end of the dive). As most of my dives are in the 40-60 feet range, this works well for me, with the only problem being that I have to fight the bouyancy with an empty tank during my safety stop and for sections of the dive that creep above 25 feet or so it's a struggle to stay down.

However, I have noticed recently that I am surfacing with a lot less air than the others. On an average 50 minute dive I usually run into or close to the red zone on the SPG, and on one occasion I had to abort the dive because I was down to my last notch of air. The others surfaced five minutes later, with many of them still having near enough a quarter of air left in their tanks.

Now I attribute my excessive air intake to my size (I am a lot bigger than most other divers in Malaysia, which is where I live), my inexperience and my general lack of fitness. But I can't help thinking that I am getting my basic weighting and buoyancy control wrong.

Is it absolutely normal to use the BCD for bouyancy control underwater? I was under the impression that the ideal method should be lungs only, but I think that by doing this I am filling up my lungs too much with redundant air to compensate for an empty BCD, and hence running out of air way too quickly.

What would be my recommended weighting (a rough idea is good enough)?

I always enjoyed not having to worry about using my BCD while under, but I am beginning to think I should put on more weight and compensate with the jacket.

Any advice appreciated.

 

[friscuba]

50 minute dives for a new diver at your size isn't bad. The more you dive, the longer you'll find your air lasting. Basically, if the others you are diving with are substantially smaller, expect them to have their air last longer because they'll likely have smaller lung volume, which means if you match them breath for breath, you'll still be taking more air out of your tank than they will with each breath.

If anyone gives you a hard time, just say bigger brains require more oxygen and smile.

You might think about another 2 pounds 'til you get your breathing down if you are fighting to stay down in the shallows or at the end of the dive. You probalby won't even notice it except for not having to struggle to stay down, which burns extra air in itself.

If you continue diving a lot, you'll eventually find you are using just your lungs for buoyancy control and probably won't even touch your BCD inflator except at deeper depths unless you are overweighted, but that can take some time. I wouldn't sweat it at this point.

 

[Colliam7]

I weigh about 200 pounds, dive with a full 3mm suit in saltwater and go down with 10 pounds of ballast. ... With a full tank of air at the start of the dive, I sink, but very slowly, so about a metre in I flip and swim down. this works well for me, with the only problem being that I have to fight the bouyancy with an empty tank during my safety stop and for sections of the dive that creep above 25 feet or so it's a struggle to stay down.

As far as the weighting issue goes, perhaps the logical question is, have you checked your weighting at the END of a dve, with that nearly empty tank? On the surface, with the nearly empty tank and no air in your BCD, can you still descend by exhaling? If not, you may need to add a bit of weight, although from your description it sounds like you are close to where you want to be, maybe a little light. If you add weight I imagine you are looking at 1-5 pounds. For example, if you are using the ubiquitous AL80, your tank buoyancy goes from -1.4 to +4.4 as the tank goes from full to empty, a positive shift of 5.8 pounds. Check at the end of a dive, with a supply of 1lb weights handy. As for air consumption, don't worry about it. It will get better with time, and the slight adjustment in weighting may help.

 

[markfm]

I'm a little lighter than you, and my weighting in 3 mm, salt, is 12 - 14 lb. Coupled with your noting having to fight when your tank is low, I'd be thinking adding 2 - 4 lb.

If you can use a pool at your LDS, a weight check is easy. Jump in, get thoroughly wet, no BCD air pockets. With reg in your mouth, get to neutral buoyancy -- add weight (starting from none) until your mask is just breaking the surface and on a full exhale you start to sink, while an inhale gets you back up, mask at the water line.

Now, check your SPG. Add 1 lb for every 500 psi it shows (on an AL80). If you got neutral with 4 lb, but still have 2000 PSI, that means add +4 lb, a total of 8 lb of lead.

That gets you to your fresh water weight. For salt you add 1/40 of your total weight (the weight of you plus all gear, including lead and full tank). Based on your starting weight, your salt delta would be 6 - 7 lb (+5 for your own 200 lb weight, +1 for the AL80, another 1/2 lb for gear, including the weight you need for fresh water).

End of the day, you likely will end up around 12 - 14 lb -- you really are probably a few lb underweighted.

Air consumption generally improves with practice, just plain diving, as your form improves. I get almost twice the bottom time for the same profile as when I was new -- a lot of subtle things, cleaner breathing cycle, less wasted motion, overall better trim in the water.

Using your lungs is good for control within a given range of depth, but I'd absolutely expect to add air to my BCD when at 50'. If your weighting is really spot on (at the end, the safety stop), on an AL80 you would be -6 lb at the start of the dive, plus lose a couple lb of buoyancy due to suit compression. Use the BCD to get yourself clean at that 50', then with lungs you can change +/- 5 or 10'. When I'm at 40' I fully expect to add a little air when I drop to 60', likewise to dump some if I want to go up 20'.

 

[Skeeter1097]

First of all it takes 15 to 30 feet to smash the air out of your wet suit and BCD. That is way after you hit 20 feet you don't have to fight to get down. When you are at a safety stop with 300 lbs in your tank. You should not have to fight to stay down. If you are you need more wight, 2 or even 4 lbs. Remember this it is easer to get reed of wight if needed then to stop your assent because of lack of wight.
My 2 cents 2 lbs extra is better then 2 lbs short.

 

[Puffer Fish]

Some excellent advice above... to your question regarding using air in a vest... if you can use only your lungs at 50 feet and a 3mm suit, at the start of a dive, you are under weighted.. That 5 lbs of air the tank will loose from the start to the end of a dive is far more than your lungs will compensate for and you have a compressed suit (in addition).

You might take some happiness in the fact that using a little air in your BC helps with trim and stability.

As air consumption goes.. you are not doing bad... as you are also using energy to keep down at the end. You will find that there are some people.. mostly women, that don't seem to be using air. We hate them for that (just do it quietly, so they don't notice).

With regards to your breathing effecting weighting... there is a lot of bull out there.. Fact is, a new suit will compress with time and need less weight..you might breath a bit deeper with time, and me more comfortable exhaling.. but it would never be more than a pound.

Good question by the way...

 

[dave4868]

You're using your air pretty well considering your size, the depths you described and the extra exertion required to swim down against the positive buoyancy caused by your underweighting.

You may have been misled by the false concepts that some people espouse, that less weight is always better and that breath-control of buoyancy can take the place of the bcd. (Damn those skinny little guys with the worn-out thin wetsuits!)

As people here have said, it makes more sense to determine the true weight you need so you can be neutral with an empty tank at the depth of a safety stop or at the surface.

It also makes sense to use the bcd to make the major buoyancy adjustments and to breathe normally most of the time, but occasionally briefly fine-tune your buoyancy with a shallow or deep breath, then go back to normal breathing.

If you do those things, your air consumption will improve compared to finning against positive buoyancy and messing around constantly with breathing patterns.

Dave C

 

[TSandM]

One of the main purposes of a BC is to compensate for the weight of the air in your tanks, and the loss of buoyancy from your wetsuit at depth. Since, when you start the dive, you are carrying about six pounds of gas on your back, of which you will use about five, you would like to be weighted so that you are five pounds NEGATIVE at the surface at the beginning of the dive (or some people use 15 feet, the safety stop depth. But I don't want to be unable to control my ascent from 15 feet, so I use the surface). When you add that five pounds of negative buoyancy to the buoyancy you lose as your wetsuit compresses with depth, you should be somewhere around 6 or 7 pounds negative at 100 feet at the beginning of a dive. You might have enough volume in your lungs to compensate for that (I don't) but it would mean very awkward breathing to do it.

Before there were buoyancy compensators, people swam themselves down and controlled their buoyancy at depth with their breathing and their fins, and you can definitely dive that way. But BCs were invented so you don't have to struggle.

You should begin the dive with air in the BC, and gradually vent it as you use the gas on your back, and as you work your way shallower.

Any effort or struggle you have to make underwater will increase your gas consumption rate. If you are having to swim yourself down, that costs you gas. Similarly, if you are struggling to hold your stop, that's costing you gas, too.

 

[Pearldiver07]

With a full tank of air at the start of the dive, I sink, but very slowly, so about a metre in I flip and swim down. Once I'm past 30 feet I can control my buoyancy well without needing to touch my BCD at all ..... creep above 25 feet or so it's a struggle to stay down. .

A couple of things I noticed in your description. Since you're having problems in shallow depth, both beginning and end of dive I'd suggest, like others, to re-check your weight. It sounds like you're not allowing for expansion and contraction of the wet suit's neoprene cells.

As for air, I see it as a very related issue here. Any effort you have to expend handling buoyancy issues (i.e., staying up or staying down) is done in addition to the normal finning to move about. That's all added work.

When you're really neutral you needn't expend any energy to stay in place. I always have my students cross their legs as part of a buoyancy check. It's amazing how many people are finning downward or upward without even noticing it. When swimming, they think all of that effort is going toward horizontal movement. This exercise will help you discover how true that is.

Hope this helps,

Steve
PADI Course Director

 

[Kingpatzer]

A lot of people brag about how little weight they need.

Frankly, it's a load of bs . . . SOME weight can be lost due to skill. But the physics are the physics -- if you have a hard time being slightly negatively bouyant with a nearly empty tank, then you don't have enough weight.

Don't focus on how many pounds you need or don't need in comparison to anyone else. If you are having any problem with your safety stop, add more weight.

Outside of that, air consumption will go down with experience. Just go dive nad have fun and dont' worry about comparing your self to anyone else as it relates to things like how much weight you use or how fast you use your air.

Do worry about comparing yourself to others when it comes to bouyancy control, trim,, etc.