buoyancy and diving ascents

[TSandM]

It's pretty standard to teach students to do their ascents by getting more or less vertical (or quite head up, anyway) and negative, and swim up with their fins. This actually works fairly well for the type of diving that most instruction envisions: warm water diving, with thin exposure protection and aluminum tanks. It doesn't work well in colder water, or at all, really, with dry suits. And it has one problem which is present in all environments, which is that, if you are dependent on swimming to move upward, anything that distracts you from kicking will cause you to sink. It is also relatively difficult to hold a specific depth this way, since you have to match the degree to which you are negative quite precisely with your finning effort, or you will move up and down in the water column. It is taught because it is physically easy to do, and because it minimizes the risk of becoming overly buoyant and ascending too fast.

The other approach to ascending is to use buoyancy to move up in the water column. If you think about it, if you are actually neutral, then if you take a deep breath and begin breathing with your lungs quite full (don't hold your breath on ascent!) you will get floatier, right? And you will move up in the water column. If you then exhale all the way, you will slow or stop; if you don't, you can vent a bit of air from your BC (or suit, if you are diving dry) and then you WILL stop. Using this technique results in a very smooth and controlled ascent, and one where it is very easy to stop at any point, simply with breath control (following by a bit of BC adjustment, perhaps). It is also a much preferable approach to ascent in a dry suit. It does take practice, however.

In any ascent, you DO want to get a bit head up as you get close to the surface, to check for hazards. No one wants to surface into the bottom of the boat, or worse, the props!

 

[Flguy76]

Thanks for all the info. I have began to use a surface check to get myself neutral with an eye level float check during a breath hold with a full 3500 psi tank and then add about 5 pounds to compensate for a near empty tank ( i dive an AL80) i was surprised though at the amount it weight it took to go from fresh to salt as on my first dive after weighting myself in the pool I was unable to descend. i estimated about 4 pounds which i found to be insufficient for me and that was without any exposure suit at all. achieving neutral buoyancy isn't always so difficult once at depth but being over weighted is not fun either and can ruin the dive. The amount of air and when to vent during an ascent was always a question with me. my instructor demonstrated a proper ascent rate and just said to vent periodically. (the padi maual says to vent as needed which i took to mean if you start to get too positive) but i have had a tendancy to vent to much in at times. When surfacing without a line or some sort of visual reference it can be hit or miss.I have always tried my best to start my ascent by watching my timer and ascending 10 feet and always taking at least 10 seconds if not more to do so and then try to maintain that rate. I recently purchased a computer that i have not dove yet but i never like to depend completely on technology which can fail.

 

[TSandM]

I got the tip from jonnythan here on Scubaboard, years ago, that if you are ascending and exhale fully and you do not stop, you should vent until you do. That was what allowed me to begin bringing neutral ascents under really good control. An instructor, several years later, told me that there is a window of depth that you can control with your breath -- within that window, you can stop your descent by inhaling, or stop an ascent by exhaling. Once you get outside of that window (which may be 3 to 5 feet, depending on your depth and your lung size), you will HAVE to use your BC or dry suit to correct your movement.

On descent, I use my ears -- any time I need to clear, I put some air in my BC or suit. On ascent, it's more difficult, but I try to remember that three foot window, and vent a little at least every three feet or so. Eventually, you will get to the point where you'll feel your gear lift off your back, or feel your dry suit get too loose, and you'll know it's time to let air out.

 

[PeterNBiddle]

10%? Wow. I just dove on 8 pounds, worked a treat. I maybe could have cut a pound or two more.

Peak Perf Buoyancy was GREAT for me. Strongly recommend it and then practice what you learn.

Get in the water and dive.

 

[supergaijin]

Vertical ascents are also taught in order to be able to release expanding air from a 'typical' BCD. Ascending horizontally usually means the air is in one of the chambers running the length of the BCD backplate and it may be difficult if not impossible to vent sufficiently.

I know my own BCD requires me to make quite a pronounced effort to remove the last half litre or so to compensate fo a half kilo (1lb). IMO it is more 'natural' to make a vertical ascent from depth following a U-shape profile ie. wreck at depth.

On multi-level reef dives horizontal ascents are more common and this is where the inexperienced get caught out, as they do not feel the BCD lifting under their armpits due to information overload aka enjoying the dive too much.

The amount of air and when to vent during an ascent was always a question with me. My instructor demonstrated a proper ascent rate and just said to vent periodically. (the padi maual says to vent as needed which i took to mean if you start to get too positive) but i have had a tendancy to vent too much in at times.

Try to feel the BCD lift you next time you're ascending- this works particularly well if you're wearing a weightbelt as your body is sinking and the BCD is lifting. Depending on your other factors of buoyancy (suit, tank, weight, body volume etc.) you can control a lot with your lungs. I dive pretty light (4lb) and can control all of that weight quite easily with my lungs... but choose to use the BCD as it is a whole lot easier. Learning how much air you're adding and releasing requires practice- much like bunnyhopping or stalling a stickshift car, it takes some time and practice to feel the gear working properly.

I do not teach OW students to dive with the assumption that they will begin diving dry- that is 'classic' PADI modular training.

 

[TSandM]

supergaijin, this is one of the places where things are different depending on where you learn and where you dive. We teach students with the assumption that, even if they are not in dry suits NOW, they will be shortly, at least if they dive locally at all. What we teach works everywhere; what you are teaching does NOT work where we are.

 

[supergaijin]

I teach with the assumption that divers are able to dive independently in conditions similar or better in those in which they were trained.

When I dived in NZ with 7mm Farmer John's, there was plenty of air in the BCD (compensating for dramatic suit compression) to be able to vent in a horizontal position. Even at the end of the dive during a safety stop, there is enough air to migrate towards the shoulder dump or deflator.

That is not the case everywhere... with 3mm shorties or no wetsuit, a properly weighted diver will have little to no air in the BCD and able to maintain neutral buoyancy in the shallows. Unless you make a physical effort to get the shoulder up (and the easiest way is to adobt an upright postition), you will not be able to release the little remaining air in the chambers of a 'typical' BCD.

During training I have students practice controlled ascents switching from horizontal trim to a more vertical position (and back) while ascending. This also works in temperate waters.

However buoyancy control via the drysuit is different from only using the BCD- thus the need for drysuit orientation where the conditions differ from those in which the student trained. You don't need to reinvent the wheel to train drysuits.

 

[bj139]

Horizontal body position creates more drag when moving vertically, as in descending and ascending, so you have more time to react to undesired vertical movement. Even a 45 degree body position creates a lot more drag so use that drag to your advantage. Venting air from your BC can be done by rolling to the right while lifting your BC hose up with your left hand which vents your drysuit at the same time since the exhaust valve is on your left arm.

 

[mtngoat2674]

Vertical ascents are also taught in order to be able to release expanding air from a 'typical' BCD. Ascending horizontally usually means the air is in one of the chambers running the length of the BCD backplate and it may be difficult if not impossible to vent sufficiently.

I know my own BCD requires me to make quite a pronounced effort to remove the last half litre or so to compensate fo a half kilo (1lb). IMO it is more 'natural' to make a vertical ascent from depth following a U-shape profile ie. wreck at depth.

On multi-level reef dives horizontal ascents are more common and this is where the inexperienced get caught out, as they do not feel the BCD lifting under their armpits due to information overload aka enjoying the dive too much.

Try to feel the BCD lift you next time you're ascending- this works particularly well if you're wearing a weightbelt as your body is sinking and the BCD is lifting. Depending on your other factors of buoyancy (suit, tank, weight, body volume etc.) you can control a lot with your lungs. I dive pretty light (4lb) and can control all of that weight quite easily with my lungs... but choose to use the BCD as it is a whole lot easier. Learning how much air you're adding and releasing requires practice- much like bunnyhopping or stalling a stickshift car, it takes some time and practice to feel the gear working properly.

I do not teach OW students to dive with the assumption that they will begin diving dry- that is 'classic' PADI modular training.

I would never dive this way. First, I wear a BP/W, so it doesn't lift under my arms in a way you would notice, but if my BC requires that much lift to get me to move, I know I'm overweighted. I also know my BC doesn't fit properly if it moves around significantly on my body.

There is no reason, with a modern BC with multiple dumps, that you can't effectively vent from a horizontal position. Whether I lift my posterior slightly above the plane of my body and use a rear dump, or roll slightly to either side and use a shoulder dump or inflator, removing air is very simple with knowledge and an awareness of how you are positioned in the water an where the air is located.

I learned in the tropics, and moved to the northwest. My first cold water dive was dry, without any pool training in the suit. I did fine, and improved dramatically within a few dives.

Now I can maintain completely horizontal trim on controlled blue water ascents or in low vis. There's no good reason to ascend in a vertical position. The only time I ever end up that way is when I'm trying to assist another diver who is inexperienced or has poor trim to keep them from ascending too fast.

It requires practice, it requires an understanding of how pressure effects your body and equipment, and it requires relaxing and trusting your gear.

---------- Post added October 6th, 2013 at 09:31 AM ----------

When I dived in NZ with 7mm Farmer John's, there was plenty of air in the BCD (compensating for dramatic suit compression) to be able to vent in a horizontal position. Even at the end of the dive during a safety stop, there is enough air to migrate towards the shoulder dump or deflator.

That is not the case everywhere... with 3mm shorties or no wetsuit, a properly weighted diver will have little to no air in the BCD and able to maintain neutral buoyancy in the shallows. Unless you make a physical effort to get the shoulder up (and the easiest way is to adobt an upright postition), you will not be able to release the little remaining air in the chambers of a 'typical' BCD.

However buoyancy control via the drysuit is different from only using the BCD- thus the need for drysuit orientation where the conditions differ from those in which the student trained. You don't need to reinvent the wheel to train drysuits.

A BCD does not need to have a substantial amount of air in order for a diver to vent the air effectively. That is a completely inaccurate statement, regardless what suit the diver is wearing. At higher pressures, larger volumes vent more QUICKLY, which just means that a good diver always is aware of their position in the water and begins dumping air when they start rising too quickly. If your ascent "gets away from you", then you Waited too long.

If a diver cannot maintain neutral buoyancy in the shallows with little to no air in their BCD, they are not weighted correctly, or they are not using their BCD properly by altering only small amounts of air. At shallow depth, even small changes of air in the BC make a huge difference.

True, diving a drysuit is pretty much the same. They, too, are designed to be used optimally with proper trim and buoyancy techniques, having the diver horizontal.

 

[Steve_C]

I have a classical type BCD. I spend most of the dive horizontal. I can easily dump a bit when needed by a tug on the inflator hose. My not so great rotator cuff combined with wanting to stay compact leads me to avoid lifting the inflator hose. I was diving a rental BCD in Greece and noticed my control was ok but not as good until I realized that the BCD was requiring me to tilt up about 20 degrees in order to dump some air the same way. So there is a lot of variance in BCDs.

As for 10% of body weight, forget the fact the same person could be naked or have on a 10/14 wetsuit. Mass is not mass. Muscle is close to neutral while fat is buoyant and bones sink. A tall bony with little body fat diver will need less weight than a diver with the same body mass but with 50 pounds of fat. I did the computation once, cannot remember if it was fresh or salt but as I recall 10 pounds of average fat took a pound of lead if I remember right.