Buoyancy

[Graeme Fraser]

Mine has two tank bands, so I'm magically trim!



Here comes my stupidity showing again, but I've never grasped this. A tank weighs what it weighs. Maybe it's heavy steel, but then there's less lead on the weight belt. If two tanks have the same amount of air in them (and I know steel probably has more, but that just makes things even more confusing) then the air in each of the tanks weighs the same, so when it's gone the tanks should each weigh the same amount less. If the divers breath the same amount from each tank. So how does one tank (steel) go from making a diver negatively buoyant to making a diver neutral, and the other (aluminum) goes to making a diver positive, if they're each losing the same weight of air?

In simple terms an aluminium cylinder will transition from slightly negative to slightly positive, whilst a steel cylinder will transition from negative to less negative.

You are right that the difference between start and finish weight is dictated only by the gas consumed.

If laying on the bottom, the tail of an empty (or low) ali cylinder will tend to float as the valve and 1st stage are quite heavy, whilst a steel cylinder is likely to rest flat. This is what can affect the trim of a diver.

Sorry, probably a crap explanation, but it made sense in my sleep deprived head..

 

[The Chairman]

True if the AL tank is mostly empty, not when it is full.

Put a full AL tank with a reg attached in the pool without a BC. They almost always sit with the valve on the bottom and the butt up. It's not much of a lift, but it's there. If it seems to be all flat, lift up on either end. Which takes more effort? No, it's not intuitive, but it's still there. What's funny is people trying to use the tank weight to get their head down and moving the tank so high it's almost over their head. It can be pretty comical. Yes, the difference becomes more pronounced as the tank empties

Put a steel tank with a reg attached sans BC in the pool and it lies absolutely flat on the bottom. Lift up on both ends to see which is heavier. You can't really tell on smaller tanks, like an LP72, but get to the water heaters, and you'll find the butt is actually a bit heavier. Again, put a reg on it to better replicate what's happening on your back.

@pauldw, I think that @JackD342 explained it pretty well. Let me know if I need to try again. You're not stupid for not "getting it" right away, you're just thinking like you're still on land. You have to consider Archimedes principle on a vessel that's not a simple geometric shape nor equally dense along it's length. It's a bit esoteric and not intuitive at all. I have had many disagree with me... until they do the tank in the pool exercise. That's an "aha" moment for many.

So how does one tank (steel) go from making a diver negatively buoyant to making a diver neutral, and the other (aluminum) goes to making a diver positive, if they're each losing the same weight of air?

Weight is usually added to compensate for the buoyancy of your exposure protection. Most people are pretty close to neutral without it... except me, cuz I'm a cork. So if you're weighted correctly, with your head just awash (not eye level) with no kicking and no air in your BC, then it doesn't matter which tank you have on your back. You will need more lead to begin with on an AL tank, but your buoyancy changes equally as you breathe down either tank.

Best way to figure out your starting weight is at the beginning of your dive and not the end so you can enjoy your dive.

• Full tank
• No weights
• Remove all air from BC
• Cross your legs
  • No kicking
• Fold your arms
  • No hand sculling
• Reg in your mouth
  • Breathe normally
• While floating relaxed, add a pound for every inch your head is out of the water
• Recheck
• Stop when your head is just awash
  • Water just barely going over
  • You should sink if you exhale and hold it
• Dive! Have fun!
  
• Empty all your air from your BC at your safety stop, cross legs & fold arms
  • If you sink, take off a pound or two on the next dive
  • If you're floaty, add a pound or two on the next dive
    • Pause your breathing on the exhale rather than the inhale to compensate

Most after dive checks use "eye level" which means the top of your head is four or five inches above the water, or four or five pounds. That happens to be about the weight you use in air during the dive. If you use my weighting technique, you'll find that all you have to do to begin your descent is let out the air in your BC and then to exhale. Taking a breath will stop that descent. Continue with this until you get to the bottom, then add just enough air to compensate for wetsuit compression.

I hope this helps.

 

[The Chairman]

About your breathing... there are two volumes associated with your lungs: Tidal volume and vital capacity. The average buoyancy change in a male's tidal volume (normal breath) is about a pound (500ml) but the change in their vital capacity (all the air in your lungs) is a whopping 10 pounds (4600ml). The deep breathing recommended by some agencies causes a wild buoyancy swing, so breathe normally. However, your vital capacity means your lungs have a lot of power over your position in the water column. If you come up to a coral head, rather than changing your attitude or adding air to your BC, simply take in a super breath and breathe yourself over the obstacle. Do a super exhale to get yourself back down. Remember: You can pause your breathing at any time, but never, ever occlude your glottis (hold your breath). That would be dangerous as you ascend and could easily injure you. Use your chest muscles to accomplish this, not your glottis. The glottis is the muscle you use when you cough. You close it to build up pressure by tightening your chest, and it makes an explosive sound when you release it. It is far, far stronger than your lungs. I always teach that Boyles Law really stands for "Breathe Or Your Lungs Explode, Stupid!" Keep that glottis open.

 

[The Chairman]

On your breathing part deux... Relaxed divers need less air in their lungs than anxious divers. Again, the goal is to be calm and breathe normally. If you've done all this and are having a problem getting down, zen out. Your anxiousness translates to being far more buoyant than you want or need. Do what you need to do to relax and you'll see yourself magically disappear below the waves. But, if you have to add an extra pound or two due to your anxiety, there's no harm or foul in that. I often add an extra pound to a student who seems to be tense. Nothing makes you even more tense than not being able to descend.

Diving should be fun and will be if you get in control of it. Trim is a HUGE part of that equation and sadly, many, many instructors don't understand that relationship either. It's all about thrust, and if your legs are significantly below your head, then you're introducing thrust induced buoyancy when you kick and will lose it when you stop to look at something, making you look like a yo-yo. Get flat, which is the point of this thread anyway, and you'll find it doesn't take a hundred dives to get your buoyancy dialed in. In fact, you'll find that once you've put in the small amount of air to be comfortably neutral near the bottom, that the rest of the adjustments you'll need during the dive can be done with only your breathing. Get trim and you'll have snobbish amounts of neutral buoyancy in no time and you'll be considered a diving God by your friends.

 

[scrane]

On one of my last dives one of the guys I've been with on the boat says to me:"ya know, that was the first time I seen you flat in the water". Normally I stay vertical. That way I can easily pivot 360 degrees and see everything there is to see around me. If I want to get closer to the reef I blow out some air and pull up my legs. If I want to get really close I'll flip up my legs into the SB approved horizontal position.
While swimming in proper trim my vision is limited to straight down or straight ahead , although I find bending my neck annoying. If I need to get somewhere, obviously I will reluctantly go horizontal and chug along.
So, Generally I prefer to dive vertical when I can. It works as well for me today as when I learned to dive 45 years ago.
Buoyancy control is another matter. One reason why I can dive in my non standard position is that I can effortlessly levitate above the coral by my breathing and judicious use of the BC. Buoyancy control is paramount.
So, what I'm saying is: horizontal trim is a useful tool, but not the only way to go.

 

[MichaelMc]

So, Generally I prefer to dive vertical when I can. It works as well for me today as when I learned to dive 45 years ago.

So do you mostly drift dive guided only by the current with your head say at least 7' above the reef? Or just hang vertical under the boat? How often do you "reluctantly go horizontal and chug along"? Your first option for looking closer at the reef is to exhale and pull your legs up?

If I want to get closer to the reef I blow out some air and pull up my legs. If I want to get really close I'll flip up my legs into the SB approved horizontal position.

 

[rongoodman]

Buoyancy control is another matter. One reason why I can dive in my non standard position is that I can effortlessly levitate above the coral by my breathing and judicious use of the BC. Buoyancy control is paramount.

So what you're saying is that you've given up the quickest and most useful tool to position yourself in the water column, your fins. Any movement of your feet is only going to send you in one direction, up.

 

[The Chairman]

So, what I'm saying is: horizontal trim is a useful tool, but not the only way to go.

And we completely disagree.

Most of the cool stuff is close to the bottom, and often under outcroppings right at the bottom. You can't see it unless you're really flat. As for looking ahead, tilt your head 30 degrees to the right or left before you try to look straight ahead.

Rototillers was the name given by our dear old @Uncle Pug for those who were determined to dive with their feet down. Why? When they got close to the bottom, the thrust from their fins stirred the bottom and utterly destroyed the visibility behind them. Oh, they had great vis, but not anyone who was behind them. More often than not, they had no clue of the devastation in their wake. There is a discussion way back in time, where a guy claimed that he just doesn't silt others out when diving with his head up, legs dangling and then his buddy chimed in. It was truly a candid discussion. Denial: not just another river in Egypt.

So, some get the important relationship between trim and buoyancy and others just don't. Master trim and buoyancy and watch your control, comfort and fun increase significantly. That, or let the vagaries of Boyels law and vector forces master you.

One reason why I can dive in my non standard position

I beg to differ on this, but that position is pretty standard the world over. It shouldn't be, but it is.

 

[The Chairman]

Caveat: I do go vertical on every dive, once I'm on my deco stop, whether it's mandatory or precautionary. I rotate and look for approaching boats as I ascend from that stop.

 

[wetb4igetinthewater]

Put a full AL tank with a reg attached in the pool without a BC. They almost always sit with the valve on the bottom and the butt up. It's not much of a lift, but it's there. If it seems to be all flat, lift up on either end. Which takes more effort? No, it's not intuitive, but it's still there. What's funny is people trying to use the tank weight to get their head down and moving the tank so high it's almost over their head. It can be pretty comical. Yes, the difference becomes more pronounced as the tank empties

Put a steel tank with a reg attached sans BC in the pool and it lies absolutely flat on the bottom. Lift up on both ends to see which is heavier. You can't really tell on smaller tanks, like an LP72, but get to the water heaters, and you'll find the butt is actually a bit heavier. Again, put a reg on it to better replicate what's happening on your back.

@pauldw, I think that @JackD342 explained it pretty well. Let me know if I need to try again. You're not stupid for not "getting it" right away, you're just thinking like you're still on land. You have to consider Archimedes principle on a vessel that's not a simple geometric shape nor equally dense along it's length. It's a bit esoteric and not intuitive at all. I have had many disagree with me... until they do the tank in the pool exercise. That's an "aha" moment for many.


Weight is usually added to compensate for the buoyancy of your exposure protection. Most people are pretty close to neutral without it... except me, cuz I'm a cork. So if you're weighted correctly, with your head just awash (not eye level) with no kicking and no air in your BC, then it doesn't matter which tank you have on your back. You will need more lead to begin with on an AL tank, but your buoyancy changes equally as you breathe down either tank.

Best way to figure out your starting weight is at the beginning of your dive and not the end so you can enjoy your dive.

• Full tank
• No weights
• Remove all air from BC
• Cross your legs
  • No kicking
• Fold your arms
  • No hand sculling
• Reg in your mouth
  • Breathe normally
• While floating relaxed, add a pound for every inch your head is out of the water
• Recheck
• Stop when your head is just awash
  • Water just barely going over
  • You should sink if you exhale and hold it
• Dive! Have fun!
  
• Empty all your air from your BC at your safety stop, cross legs & fold arms
  • If you sink, take off a pound or two on the next dive
  • If you're floaty, add a pound or two on the next dive
    • Pause your breathing on the exhale rather than the inhale to compensate

Most after dive checks use "eye level" which means the top of your head is four or five inches above the water, or four or five pounds. That happens to be about the weight you use in air during the dive. If you use my weighting technique, you'll find that all you have to do to begin your descent is let out the air in your BC and then to exhale. Taking a breath will stop that descent. Continue with this until you get to the bottom, then add just enough air to compensate for wetsuit compression.

I hope this helps.

Pete,

Can I quote you on a FB note, with a link to your comment? This is useful stuff.