Why is buoyancy more sensitive in shallow waters?

[butwhytho]

When I was newer to diving, I got the advice "breathe shallow when shallow, breathe deep when deep", and I noticed on my own dives that I change depth more easily with breathing in shallow waters vs deep. Why is that? The best answer I could find was something regarding pressure gradients, but I still don't know why pressure gradients in deep and shallow water causes the difference.

 

[lowwall]

When I was newer to diving, I got the advice "breathe shallow when shallow, breathe deep when deep", and I noticed on my own dives that I change depth more easily with breathing in shallow waters vs deep. Why is that? The best answer I could find was something regarding pressure gradients, but I still don't know why pressure gradients in deep and shallow water causes the difference.

For a given change in feet/meters, the percentage change in pressure is greater when you are shallow. The percentage change is how much the gas bubble in your wing and lungs change.

Simple example. Let's use metric to make this really easy. If you go from the surface to 2m deep, the pressure increases .2 atmospheres (from 1atm to 1.2atm absolute) which is 20% of your starting pressure and thus the volume of the gas in your wing/lungs decreases 20%.

If you start at 20m deep and drop to 22, the pressure goes from 3.0atm to 3.2atm absolute. It's the same increase in absolute pressure of .2atm, but the percentage increase is only 6.67% so the gas bubble in your wing/lungs only changes 1/3 as much as it did dropping those same 2m from the surface.

 

[Rilelen]

Yep the relative pressure change is greater when shallow, which means gas expands and compresses more rapidly. It’s the relative difference (not the absolute difference) in change in pressure that makes buoyancy “feel hard” when shallow. You’re more stable at depth, even though there’s more pressure.

It’s like being in a car quickly braking and accelerating between 0-10mph, versus being in a car cruising at 70mph.

 

[Nemrod]

Just breathe normally at all depths, no need to pant like a (hot) dog at shallow depths.

One cf at 99 feet (4 atm) becomes two cf at 66 feet (3 atm) becomes four cf at 33 feet (2 atm) becomes eight cf at the surface (1 atm). The volume change from 99 feet to 33 feet is the same as the volume change from 33 feet to the surface.

 

[Edward3c]

When I was newer to diving, I got the advice "breathe shallow when shallow, breathe deep when deep", and I noticed on my own dives that I change depth more easily with breathing in shallow waters vs deep. Why is that? The best answer I could find was something regarding pressure gradients, but I still don't know why pressure gradients in deep and shallow water causes the difference.

It’s all to do with boyle's law which should have been covered during your basic training.

 

[lowwall]

It’s all to do with boyle's law which should have been covered during your basic training.

Boyle's law states that the Pressure and Volume of a gas are inversely correlated (if you hold the temperature and mass of the gas constant}.

Wikipedia has a nice gif to illustrate it:

But knowing this doesn't automatically mean you will know the answer to the OP's question.

 

[Wobbegong_th]

First, in shallow water, pressure change is larger in relation to depth change.
At 0m, ambient pressure is 1 ATM (atmosphere)
At 10m, ambient pressure is 2 ATM - double the previous pressure
at 20m, ambient pressure is 3 ATM - increased by 1/2 previous pressure
at 30m, ambient pressure is 4 ATM - increase by 1/3 previous pressure

and as pressure related reversely with volume (higher pressure - less volume)
if you're starting with 1 ltr at surface in flexible container (a balloon, for example),
at 10m, the balloon volume will be reduced to 0.5 ltr - half of the previous level/depth, reduced by 0.5
at 20m, the balloon volume will be reduced to 0.33 ltr - 0.66 of the previous level/depth, reduced by 0.34
at 30m, the balloon volume will be reduced to 0.25 ltr - 0.75 of the previous level/depth, reduced by 0.25
Can you see the 'gap' of the change in volume at each step change in depth??

And Volume related directly to buoyancy. (larger volume - higher buoyancy)

The above applied mostly to the air in your bcd bladder.
When consider your breathing, it's the "density" of the gas that come into play to make you need to breath harder at depth.

 

[Duke Dive Medicine]

When I was newer to diving, I got the advice "breathe shallow when shallow, breathe deep when deep", and I noticed on my own dives that I change depth more easily with breathing in shallow waters vs deep. Why is that? The best answer I could find was something regarding pressure gradients, but I still don't know why pressure gradients in deep and shallow water causes the difference.

It's not the breathing per se. Your buoyancy changes as a function of the weight of the water that your body displaces (Archimedes' Principle). For a given lung volume, the weight of the water that your lungs displace at 30 feet will be essentially the same as the weight of the water they displace at 120 feet. The perceived difference in buoyancy is more likely due to wetsuit compression.

Best regards,
DDM

 

[tursiops]

Boyle's law states that the Pressure and Volume of a gas are inversely correlated (if you hold the temperature and mass of the gas constant}.

The perceived difference in buoyancy is more likely due to wetsuit compression.

 

[Duke Dive Medicine]

Popcorn is too esoteric for me. Kindly hit me with a ball bat if my physics is wrong