Changes in Pressure from Swells

[CIBDiving]

Are you holding on to somthing like a mooring line that keeps you in place, or are you completely neutral & floating a fixed distance from the surface? maybe even a lift bag?

It makes a difference.

Well holding on to something that is bouyant on the surface surely makes a difference - it WILL move, but if you are referenced to the bottom ( a subsurface mooring ball comes to mind) with no current so the line runs verticly. what then? does a diver move up and down? I can't remember! the last time I was in that situation was a number of years ago now - but I have no recollection of vertical movement.

 

[CIBDiving]

Pressure is measured by the amount of water, ie. weight, above us.

the post should have said : "the amount of water DIRECTLY over your head..."

No it's not. if that was true then we would only feel pressure on the tops of our heads, think about it.

 

[gangrel441]

Ahhh...but do we physically go up and down with the troughs? Think of when you are under a boat with a hang bar. Does the hang bar remain at a constant depth, or does it rise and fall in relation to the diver?

I am not really sure what the effect is on pressure on the diver, But I would think the swells would have to be pretty big for this to be a problem...probably big enough that in most cases, the dive would have been called.

 

[Rick Murchison]

Sometimes it is useful to use extreme exhibits to understand what happens in more "normal" circumstances.
The passage of waves does change pressure - and therefore "depth." Let's use the world's longest period wave as a starting point... the tides. I think you can all agree that as the tidal (not tsunami but daily tidal shift) wave passes overhead, the depth changes; indeed the measured depth under that wave changes by the entire height of the wave from peak to trough.
On the other hand, a two foot wave with a 4 second period passing over an 80 foot bottom will be all but undetectable at the bottom with a depth gauge. This is because the pressure from the passing wave is distributed into the water not just straight down, but in all directions. Right at the surface, however - if the pressure instrument were placed at precisely the level of the trough, it would measure a depth change from zero at the trough to nearly the entire height of the wave as the crest passes overhead.
So... the depth change experienced by a diver at a specific depth above the bottom depends on two things:
(1) the height of the waves passiing overhead, and
(2) the period of the waves.
The higher the wave and the longer the period, the deeper and the more pronounced the depth change.
As a practical matter for scuba divers, if you are in the surge zone then there is a measurable depth change occuring as the waves pass over (the changing depth - water rushing from higher pressure to lower pressure - is what's causing the surge ). If you can't feel the surge, the depth change is insignificant.
Rick

 

[Diver Dennis]

Not really, because the pressure is distributed throughout at depth. The water surrounds our whole body and therefore our whole body is under pressure.

 

[gangrel441]

The pressure is still created by the amount of water above you pushing down, plus the amount of atmosphere above the water, pushing down on that. I wonder, though...does the weight of the swells on either side of the trough negate any difference in pressure created by the trough?

It's a good question, but I would guess that since we don't hear about divers geting embolized in this way on a regular basis, that this danger is not a high risk.

Best I can come up with is, don't hold your breath.

 

[Diver Dennis]

The pressure is still created by the amount of water above you pushing down, plus the amount of atmosphere above the water, pushing down on that. I wonder, though...does the weight of the swells on either side of the trough negate any difference in pressure created by the trough?

It's a good question, but I would guess that since we don't hear about divers geting embolized in this way on a regular basis, that this danger is not a high risk.

Best I can come up with is, don't hold your breath.

Good point. I would image, as Rick said, that the further down you are the less effect the change in surface height would be. This would apply to what you say gangrel because the farther from the surface you are the less the pressure change would be. That is an interesting point about whether the swells would put enough pressure on the trough to negate the depth change. Water is pretty dense so I don't know how much you could compress it.

 

[gangrel441]

I don't know if compression is the issue. I am thinking along the lines that the swells are pushing in more directions than just straight down, that the downward force being exerted by that water is being spread down and out. You are correct that water is a liquid, and by definition does not compress.

Also, the change in depth isn't quite as important as the percentage change in pressure, IIRC. This is why such pressure changes are more important in shallow water than at depth. Change in pressure from 90 to 60 fsw is from 4 ATA to 3 ATA, or 25%. Change in pressure from 30 fsw to the surface is from 2 ATA to 1 ATA, or 50%.

Still, I am thinking those toughs would have to be pretty big to get one worrying about an embolism.

 

[Diver Dennis]

Also, the change in depth isn't quite as important as the percentage change in pressure, IIRC. This is why such pressure changes are more important in shallow water than at depth. Change in pressure from 90 to 60 fsw is from 4 ATA to 3 ATA, or 25%. Change in pressure from 30 fsw to the surface is from 2 ATA to 1 ATA, or 50%.

Still, I am thinking those toughs would have to be pretty big to get one worrying about an embolism.[/QUOTE]


I agree.

 

[gangrel441]

I would say a more immediate concern is that, during a safety stop, the change in pressure and wave action from the trough could bring a diver bobbing to the surface prematurely. As I recall from OW training, the recommendation for such conditions is to do the safety stop just a bit deeper to help maintain depth and proper bouyancy.