Buoyancy and Bobbing

[krukster86]

Really basic question here about buoyancy. Every time I have gone underwater in my short experience with diving, it has been in a somewhat different environment with different gear thickness required. With weighting, we were never really given a chance to check buoyancy, just a matter of estimating how much you need on the boat with trial and error or "rule of thumb". Every time I have gone diving, I tend to bob up and down when breathing in and out (respectively) while cruising underwater. is this a symptom of overweighting myself?

*EDIT: This is particularly bad the first 1/2 of the dive, and it goes away the second half. I find myself exhaling slowly, then coming within inches of coming in contact with the "floor", so I either have to inhale prematurely just to come back up or I put some air in my BCD.

 

[00wabbit]

I"m a newbie too. Moving up and down while breathing in and out is normal. As you take in air your chest expands and you displace more water so you go up. When you exhale the water pressure compresses your chest and you go back down. At first it makes you feel like you are moving up and down too much. After a while you get used to it and you can even use it to your benefit. If you are swimming along and need to pass over something you can take a deeper breath and fly over it then exhale and come back down.

I am on dive 14 and I am just getting used to the feeling. Once you get used to it and stop fighting it you can use less air because you aren't constantly adjusting the BC or stressing about going up.

 

[krukster86]

I"m a newbie too. Moving up and down while breathing in and out is normal. As you take in air your chest expands and you displace more water so you go up. When you exhale the water pressure compresses your chest and you go back down. At first it makes you feel like you are moving up and down too much. After a while you get used to it and you can even use it to your benefit. If you are swimming along and need to pass over something you can take a deeper breath and fly over it then exhale and come back down.

I am on dive 14 and I am just getting used to the feeling. Once you get used to it and stop fighting it you can use less air because you aren't constantly adjusting the BC or stressing about going up.

Thanks! I am still getting used to this feeling. On my last dive, the guy I was buddies with is VERY experienced, and it seemed like he was not moving up and down AT ALL when breathing. I thought I was doing something wrong.

 

[electrix]

Since it goes away after your tank gets lighter ( al 80 ? ) you are probably about right otherwise you will bob to the surface toward the end of your dive. If you can still submerge at the end of your dive it sounds good.

 

[krukster86]

Since it goes away after your tank gets lighter ( al 80 ? ) you are probably about right otherwise you will bob to the surface toward the end of your dive. If you can still submerge at the end of your dive it sounds good.

Yeah I do know that the tank gets lighter at the end of the dive. I guess I just have to get used to the bobbing. I know it comes natural due to air in and out of the lungs, but I just seem to notice that I am bobbing a lot more than other people I dive with. I guess this is one of those things that takes practice.

 

[chrpai]

Breath control / visualization is an important aspect of buoyancy control. It's normal to go up and down but it can be amplified if you are overweighted and/or wearing a thick wetsuit. These two things require you to put more air in your BCD then you would otherwise need. The more air that's in your BCD, the more expansion and compression that will occur as your depth changes.

Do your NB checks to make sure your weighting is where it needs and just dive a lot. You'll get the hang of it in time.

 

[BabyDuck]

yeah, i second that you may be overweighted. it's normal to have some 'bob' with breathing, but it becomes exaggerated if you are carrying a lot of weight so have a lot of air in your bc so it expands as you go up several feet...

do a real weight check. if you weren't really taught, there are some excellent threads on it. and remember that the weight you need is the weight you need until it isn't anymore. your weight needs change as you become more comfortable, as you change gear, for lots of reasons. do your weight check!

if you aren't overweighted, check your breathing. are you exhaling completely? are you inhaling in too long a time frame which gives you more chance to start floating? people all do this different, but work on smooth full breath in with a pause (not a hard hold, just a break from moving air) then a complete exhale. most divers end up with a rate of maybe 10-12 breaths per minute, some lots lower, but it will likely be lower than your rate on land. once you start getting your groove, you can breathe 'around' different amounts of air you are keeping in your lungs. for example, you can have full lungs and breathe out about half then totally back in then out about half. or, totally full then totally empty in smooth waves to avoid lots of depth change. don't worry about this, just know that you might start doing it later & it is just fine.

 

[g1138]

Let's not jump ahead and say the OP's overweighted. Depending on his tank and his exposure protection he just may be spot on.
Only a self buoyancy check at 500psi will tell.

To krukster86:
Perhaps at the beginning of your dive you just need a bit more air in your BC. If at the end of your dive you feel spot on, then I think that may be the only problem. Add one more quick tap of air into your BC at the start of your dive so you don't have to breath in sooner than you want to.

You may also not want to exhale fully (if you are doing so), but perform a normal slow exhale as you would on land. There's no need to fully empty your lungs unless you want to descend; keep the residual lung volume as it is.

 

[antattack]

Unless you are underweighted, bobbing has little to do with your ballast. It has to do with how your are breathing. Also, new divers often mistake the natural motion of the ocean with their buoyancy control. Agree with previous post- only a check when your tank is at 500# will tell for sure.

I'm training to be a dive master. One thing I've noticed is that new divers tend to take deep breaths- deeper than when they are on the surface. Also, when they are THINKING about breathing, they tend to breath deep and slow. Deep and slow is a recipe for bobbing in the the water.

Once divers get under way, and start looking around, their breathing becomes more natural and they bob less. Until they start thinking about breathing, then they start breathing deep...

This is one of the great things about training as a Dive Master. I have to spend long hours in the pool watching students, and that means I get to hover, and hover, and hover in ten feet of water. I'd recommend that you and your buddies spend two minutes practicing your hover at the beginning of every dive. Just descend to 15-20 feet and hover. Maybe practice buddy breathing while you are at it. Get used to how the ocean moves you around and remember, your training isn't over just because you got your C card Practice a bit, then go have some fun!

 

[vixtor]

I will also agree with the posters saying that going up&down too much may be a symptom of overweighting. The idea is that it is normal to go up when you inhale (increase your volume) and down when exhaling (decreasing your volume), because of the Archimedes law.

Also, because of Boyle's law (P*V=constant), when you go up, all the gases that you are carrying are expanding. And when you go down, they compress. Now, let's assume you have 1 liter of air in your BC, at a depth of 10 meters (2 bar). If you go up half a meter, pressure decreases from 2 bar to 1.95 bar. So we have 1*2=x*1.95, x=1.025 liters (0.025 liters increase in volume). If you have 4 liters of air in your BC, then it expands to 4.1025 liters (0.1 liters increase). So the more air you have in your BC, the more it will expand when your lung expansion sends you up, which will accelerate your ascent even more. The same happens when you go down. This is a chain reaction, so if you breathe too slow it may send you up and down a lot (the more you go up, the more it expands, and the more you go down, the more it compresses). The good news is that it would be hard for the expanding parts of the equipment (exposure suit and BC) to expand enough so you could not reverse the direction by exhaling (a 3-5 liters expansion would be enormous and not expected during a 0.5-1m ascent/descent), so you don't need to be worried about this bobbing - you will be always able to reverse it by inhaling/exhaling (as needed).

In the first part of your dive, depending on your tank, you are 2-3kg heavier than at the end of the dive, so you need to have more air in your BC to compensate. It makes sense to notice a decrease in bobbing in the second part of the dive, for the reasons explained above, because you are carrying less air in the second part. If you are properly weighted, your BC is almost empty, so the air required to float the tank is a significant percent of the total air in your BC.

Please note that there are other expanding parts of your equipment - the exposure suit (wet or dry). About it you cannot do much, because it depends on water temperature. But you should expect more bobbing when diving with a thick suit than when diving tropical in a 1-2mm shorty.

Also, remember that bobbing will be wider at shallow depths, and will decrease in deep water. In our example, 2/1.95 is much more than 4/3.95 (so the same bobbing will be less accelerated by BC/suit at 30 meters than it is at 10 meters).

As a conclusion - exaggerate bobbing (compared to what you know it is normal) means overweighted. But bobbing is to be expected no matter how good your weighting is. Do a proper check if you can, because proper weighting will reduce bobbing for sure - probably enough to enjoy the improvement.