Buoyancy and Diving

An object’s buoyancy determines whether it floats, sinks, or hovers. As divers, we need to understand the factors that influence our own buoyancy so that we can control it while in the water. This helps us determine when to inflate or deflate the BCD, and how much weight will be required for a dive.

neutral buoyant scuba diver

Archimedes Principle

A Greek philosopher named Archimedes studied the relationship between an object’s buoyancy and the fluid it displaces. From his experiments, he concluded that “an object in a fluid is buoyed up by a force equal to the weight of the fluid it displaces.” In simpler terms, this means that objects that are less dense than the fluid will float, and objects that are denser than the fluid will sink.

Imagine a bucket filled to the top with water. If you place a piece of foam on the water, a small amount will overflow from the bucket. This small amount of water weighs more than the piece of foam, which is why it floats.

Now imagine dropping a brick of lead in the bucket. The lead sinks to the bottom of the bucket and displaces more water than the foam. But this amount of water weighs less than the lead, which is why it sinks.

how archimedes principle affects divers

Both jars in this photo displace the same amount of water. But the jar on the right side is filled with lead, so the water it displaces weighs less than that jar. As a result, that jar sinks, while the jar on the left floats.

Three States of Buoyancy

The term “buoyancy” refers to the upward force placed on an object when placed in fluid. This force isn’t always enough to make an object float, so there are three states of buoyancy.

Positive Buoyancy Objects that are less dense than a fluid will float because they displace an amount of water that weighs more than the object. Negative Buoyancy Objects that are denser than a fluid will sink because the fluid displaced weighs less than the object. Neutral Buoyancy Neutrally buoyant objects neither float nor sink. Instead, they hover in the water. In order to be neutrally buoyant, an object must have the same density as the fluid it’s placed in.

states of buoyancy

Factors Affecting a Diver’s Buoyancy

Your own buoyancy is determined by the difference between the density of the water and the combined density of your body and gear. Since we want to be neutrally buoyant, our weight and volume need to be adjusted to make our density equal to the water’s density. So to be neutrally buoyant in salt water, our density needs to be exactly 64 lbs/ft&#179, and in fresh water, it needs to be 62.4 lbs/ft&#179.

Buoyancy control is one of the most important skills to master. Your weight belt, BCD, exposure suit, cylinder pressure, and lungs are just a few of the factors that affect your buoyancy.

Weight and Buoyancy

In the “Diving Equipment” chapter, you learned that weights are worn to offset the buoyancy of your body and exposure suit. Lead weights are used because their density allows us to add weight without significantly increasing our volume. The following are just a few of the factors that determine the amount of weight you’ll need to achieve neutral buoyancy.

Water Density Salt water is denser than fresh water; so more weight is required for dives in salt water. Body Type and Size If you’re overweight, you’ll require more weight because fat tissues are less dense than muscle tissues. And since most bodies are naturally buoyant, larger people will require more weight than smaller people will. Water Temperature and Exposure Suit Water temperature has an indirect effect on buoyancy because you’ll be wearing a thicker, more buoyant exposure suit in cold water.

weight and buoyancy

Volume and Buoyancy

You’ll be wearing the amount of weight required to achieve neutral buoyancy at the surface. But as you descend, your wetsuit will compress. This decreases your volume, and results in a loss of buoyancy.

To maintain neutral buoyancy, you need to add air to your BCD as you descend. The principle behind this procedure is that as volume is lost in your wetsuit, the same amount of volume should be added to the BCD. As you continue to descend, the air in your BCD also compresses, so you’ll be compensating for volume changes in the BCD as well.

You do the opposite on ascent. As you ascend, your wetsuit in addition to the air in your BCD expands, so you need to release air from your BCD to maintain neutral buoyancy. This prevents an uncontrolled ascent.

bcd inflation and buoyancy

Breathing and Buoyancy

Your lungs also have an effect on your buoyancy. Most people can float comfortably at the surface with a full breath of air, but sink when they exhale. This is because your lungs produce several pounds of lift when full, and loose that lift when empty.

This affects your buoyancy as you dive. If you’re neutrally buoyant, you’ll rise as you inhale, and sink as you exhale. For this reason, it’s important to maintain a consistent breathing pattern to balance out these shifts in buoyancy.

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You can use your lungs to make temporary adjustments in buoyancy. For example, when swimming over rocks or reefs, you can take shallower breaths to compensate for wetsuit and BCD expansion until you return to your original depth.

Fins! Negative bouyancy.

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Ellabee83

I have had a quick look but can’t see that it’s been answered specifically.

So. I’m a big lass (more positive buoyancy!) and diving in the UK, so wearing a 7mm wetsuit (even more positive!). I’m having to compromise on weights (12kg) as if I go in fully weighted it’s prohibitive in terms of wandering around with it on (a lot of long walks to get to shore/ pontoons) and also once I’m under 6m+, they become too heavy and I end up wallowing in the murk and needing loads of air to get me up at all.

We’ve found that lowering my tank helps, but I am rolling A LOT and then once we get that issue sorted, my legs float and I’m literally vertical.

Boat diving, I have to actively swim down, and once I’m down, I’m great and trim is really good. Shore diving, especially in our waves, I require an extra 5-6kg to get me weighted enough to stay stable. If I lose any of that, I am again, literally vertical and upside down.

I’m looking at some ankle weights to assist as my boots are 6mm Seac Pro hard sole boots- again, long walks- which is obviously going to affect trim. My current fins are tusa solla which are pretty light, I think 0.8kg each. So there’s quite a lot of positive buoyancy at my feet vs a lot of weight around my core.

I’m also looking at getting some tech fins as they’re denser and have negative buoyancy.

So far, I’m considering

Apeks rk3 hd
Scubapro supernova
4th element tech
Oms tribe

Does anyone have any particular comments on how these are for helping with negative buoyancy, and if so, any help on sizing?

As I mentioned, my boots are seac pro hard soles in a small (roughly 10cm x 25cm), I’m a 6-7 UK (39/40 EU) and according to the dimensions of the rk3hd info sheet, the size of my boots puts me into a super, which seems ridiculous.

Any input would be really appreciated.

tbone1004

Technical Instructor

Messages 19,998 Reaction score 13,476 Location Greenville, South Carolina, United States # of dives I’m a Fish!

I would get Scubapro Jet fins vs. any of those listed, maybe Hollis F1 but with any of those types of fins I hope your calves are in good shape because they are a bit of a butt kick.

Now, to address the real issue which is likely inappropriate gear configuration and real weighting issues.
Bioprene floats, but it does not need to be compensated for by the BC because it is considered static buoyancy, i.e. if it takes 10kg to sink you in a bathing suit at the surface you will still need that to stay down at 30m because adipose tissue is considered non-compressible at reasonable diving depths.

A 7mm suit should need no more than 10kg to get it to sink and that is not an unreasonable amount of buoyancy to compensate for with the BC and even with a large size tank a 30lb wing should be more than adequate. I was trying to figure out what you think you need in terms of lead and it sounds like 12+6kg=18kg total? That’s ~40lbs which is a lot, but not unreasonable. What type of BC are you using? You would greatly benefit from a stainless steel backplate as it would not only help you shed some of the lead you are carrying on a belt *depending on the BC you are coming from you could easily shed 4kg for a negligible total rig weight increase*, and the one-piece harness will help with stability and security on your body far more than any standard BC would because it is custom fit to your curves and with the crotch strap it will actually stay where you put it. The single tank adapters will also help the tank be much more secure. That change alone will be a huge change of experience for you and likely be a game changer for your diving since you’ll be able to actually dive adequately weighted and balanced.

Check out the recent blog post that @oya made about weight positioning which is a good read as well. Roger tends to be a bit passionate about the topics he blogs about so understand that going in, but the passion is there for a reason.

Ellabee83

I would get Scubapro Jet fins vs. any of those listed, maybe Hollis F1 but with any of those types of fins I hope your calves are in good shape because they are a bit of a butt kick.

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Now, to address the real issue which is likely inappropriate gear configuration and real weighting issues.
Bioprene floats, but it does not need to be compensated for by the BC because it is considered static buoyancy, i.e. if it takes 10kg to sink you in a bathing suit at the surface you will still need that to stay down at 30m because adipose tissue is considered non-compressible at reasonable diving depths.

A 7mm suit should need no more than 10kg to get it to sink and that is not an unreasonable amount of buoyancy to compensate for with the BC and even with a large size tank a 30lb wing should be more than adequate. I was trying to figure out what you think you need in terms of lead and it sounds like 12+6kg=18kg total? That’s ~40lbs which is a lot, but not unreasonable. What type of BC are you using? You would greatly benefit from a stainless steel backplate as it would not only help you shed some of the lead you are carrying on a belt *depending on the BC you are coming from you could easily shed 4kg for a negligible total rig weight increase*, and the one-piece harness will help with stability and security on your body far more than any standard BC would because it is custom fit to your curves and with the crotch strap it will actually stay where you put it. The single tank adapters will also help the tank be much more secure. That change alone will be a huge change of experience for you and likely be a game changer for your diving since you’ll be able to actually dive adequately weighted and balanced.

Check out the recent blog post that @oya made about weight positioning which is a good read as well. Roger tends to be a bit passionate about the topics he blogs about so understand that going in, but the passion is there for a reason.

That’s really helpful thanks.

So I use an AP Valves Buddy Profile BCD and I’ve got a 15l steel tank.

Weight wise, yes, about that. I think it’s about 28lb on a boat dive, and I need the extra when I’m shore diving as the minute I am deep enough to actually swim, I start rolling around. With 28lb on a boat dive I’m able to control at the bottom really well. As soon as I get to about 6m on a shore dive, it’s the same, but the swim out can be a bit of a pain, especially in any currents.

I’ll check out the post, thank you. I’m not sure if I can swap the back plate in my BCD but I can have a look into it.

Fins! Negative bouyancy.

Welcome to ScubaBoard, the world’s largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

Ellabee83

I have had a quick look but can’t see that it’s been answered specifically.

So. I’m a big lass (more positive buoyancy!) and diving in the UK, so wearing a 7mm wetsuit (even more positive!). I’m having to compromise on weights (12kg) as if I go in fully weighted it’s prohibitive in terms of wandering around with it on (a lot of long walks to get to shore/ pontoons) and also once I’m under 6m+, they become too heavy and I end up wallowing in the murk and needing loads of air to get me up at all.

We’ve found that lowering my tank helps, but I am rolling A LOT and then once we get that issue sorted, my legs float and I’m literally vertical.

Boat diving, I have to actively swim down, and once I’m down, I’m great and trim is really good. Shore diving, especially in our waves, I require an extra 5-6kg to get me weighted enough to stay stable. If I lose any of that, I am again, literally vertical and upside down.

I’m looking at some ankle weights to assist as my boots are 6mm Seac Pro hard sole boots- again, long walks- which is obviously going to affect trim. My current fins are tusa solla which are pretty light, I think 0.8kg each. So there’s quite a lot of positive buoyancy at my feet vs a lot of weight around my core.

I’m also looking at getting some tech fins as they’re denser and have negative buoyancy.

So far, I’m considering

Apeks rk3 hd
Scubapro supernova
4th element tech
Oms tribe

Does anyone have any particular comments on how these are for helping with negative buoyancy, and if so, any help on sizing?

As I mentioned, my boots are seac pro hard soles in a small (roughly 10cm x 25cm), I’m a 6-7 UK (39/40 EU) and according to the dimensions of the rk3hd info sheet, the size of my boots puts me into a super, which seems ridiculous.

Any input would be really appreciated.

tbone1004

Technical Instructor

Messages 19,998 Reaction score 13,476 Location Greenville, South Carolina, United States # of dives I’m a Fish!

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I would get Scubapro Jet fins vs. any of those listed, maybe Hollis F1 but with any of those types of fins I hope your calves are in good shape because they are a bit of a butt kick.

Now, to address the real issue which is likely inappropriate gear configuration and real weighting issues.
Bioprene floats, but it does not need to be compensated for by the BC because it is considered static buoyancy, i.e. if it takes 10kg to sink you in a bathing suit at the surface you will still need that to stay down at 30m because adipose tissue is considered non-compressible at reasonable diving depths.

A 7mm suit should need no more than 10kg to get it to sink and that is not an unreasonable amount of buoyancy to compensate for with the BC and even with a large size tank a 30lb wing should be more than adequate. I was trying to figure out what you think you need in terms of lead and it sounds like 12+6kg=18kg total? That’s ~40lbs which is a lot, but not unreasonable. What type of BC are you using? You would greatly benefit from a stainless steel backplate as it would not only help you shed some of the lead you are carrying on a belt *depending on the BC you are coming from you could easily shed 4kg for a negligible total rig weight increase*, and the one-piece harness will help with stability and security on your body far more than any standard BC would because it is custom fit to your curves and with the crotch strap it will actually stay where you put it. The single tank adapters will also help the tank be much more secure. That change alone will be a huge change of experience for you and likely be a game changer for your diving since you’ll be able to actually dive adequately weighted and balanced.

Check out the recent blog post that @oya made about weight positioning which is a good read as well. Roger tends to be a bit passionate about the topics he blogs about so understand that going in, but the passion is there for a reason.

Ellabee83

I would get Scubapro Jet fins vs. any of those listed, maybe Hollis F1 but with any of those types of fins I hope your calves are in good shape because they are a bit of a butt kick.

Now, to address the real issue which is likely inappropriate gear configuration and real weighting issues.
Bioprene floats, but it does not need to be compensated for by the BC because it is considered static buoyancy, i.e. if it takes 10kg to sink you in a bathing suit at the surface you will still need that to stay down at 30m because adipose tissue is considered non-compressible at reasonable diving depths.

A 7mm suit should need no more than 10kg to get it to sink and that is not an unreasonable amount of buoyancy to compensate for with the BC and even with a large size tank a 30lb wing should be more than adequate. I was trying to figure out what you think you need in terms of lead and it sounds like 12+6kg=18kg total? That’s ~40lbs which is a lot, but not unreasonable. What type of BC are you using? You would greatly benefit from a stainless steel backplate as it would not only help you shed some of the lead you are carrying on a belt *depending on the BC you are coming from you could easily shed 4kg for a negligible total rig weight increase*, and the one-piece harness will help with stability and security on your body far more than any standard BC would because it is custom fit to your curves and with the crotch strap it will actually stay where you put it. The single tank adapters will also help the tank be much more secure. That change alone will be a huge change of experience for you and likely be a game changer for your diving since you’ll be able to actually dive adequately weighted and balanced.

Check out the recent blog post that @oya made about weight positioning which is a good read as well. Roger tends to be a bit passionate about the topics he blogs about so understand that going in, but the passion is there for a reason.

That’s really helpful thanks.

So I use an AP Valves Buddy Profile BCD and I’ve got a 15l steel tank.

Weight wise, yes, about that. I think it’s about 28lb on a boat dive, and I need the extra when I’m shore diving as the minute I am deep enough to actually swim, I start rolling around. With 28lb on a boat dive I’m able to control at the bottom really well. As soon as I get to about 6m on a shore dive, it’s the same, but the swim out can be a bit of a pain, especially in any currents.

I’ll check out the post, thank you. I’m not sure if I can swap the back plate in my BCD but I can have a look into it.

Source http://scuba-tutor.com/dive-physics/buoyancy/

Source https://scubaboard.com/community/threads/fins-negative-bouyancy.625972/

Source https://scubaboard.com/community/threads/fins-negative-bouyancy.625972/

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