Changes in Pressure from Swells

[CIBDiving]

Sorry - sometimes they actualy expect me to DO something at work!

Rick, You make some good points. And the depth change with the tides is, of course, true. But tides are Not swells and the physics that generate the tides are completely different from wind generated swells. Tides are the result of gravity while wind waves are not. I suppose the question comes down to - exactly what are swells and how are they formed? I'm not sure science has ever satisfactorly answered that question!

Do you have any Proof of the statement "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" ??

 

[CIBDiving]

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?

That is exactly what I think happens the crest adds pressure to all the surronding water and the trough subtract this - you end up with a 'mean' depth and No change in pressure - unless you are in the swell itself above the level of the trough.

 

[RIOceanographer]

Do you have any Proof of the statement "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" ??

People actually do this. One of the ways physical oceanographers measure wave height is to place a pressure sensor at a fixed depth and then use the pressure changes caused by the waves passing over head to deduce wave height. So yes, it does change pressure.

Here is an example of such an instrument (scroll to the bottom for the explanation of wave height measurement):

http://marine.rutgers.edu/leo/leo15_prof_b.html

Rick Murchison's explanation was correct.

 

[cudachaser]

I've noted up to a 2ft delta on my computer at 30ft with 3 to 5ft swells

 

[The Kraken]

But aren't the peak and trough of the wave equidistant from the mean line of mean sea level?

IE: If there is a six foot swell, isn't the peak 3' above msl and the trough 3' below msl?

If so you have an equal increase and decrease in pressure which, given the short span of time the pressures are being applied, results in no appreciative increase of relative depth at all.

the K

 

[gangrel441]

Of course, since the question is regarding an embolism, which unlike DCS, can take an instant to happen, the short time span is negated. The extreme example of this question would be an example where a diver takes in a deep breath terminating at the exact point in time when the swell crosses over head, then holds his breath beyond the exact time that the trough passes directly overhead.

I still don't think under any circumstances in which divers would actually consider being in the water, that the swells/troughs would result in a change in pressure great enough to actually cause an embolism, unless done deliberately and under controllded circumstances.

But still, don't hold your breath.

 

[alpaj]

Hmm interesting question. Having taken a few university physics courses and I decided to think about the problem. Best mention right of the start that fluid mechanics is tricky stuff. First off some one mentioned compression of water. That's not possible. As well some one mentioned above that scientists use pressure sensors to measure wave amplitude (height). In this case depth of the sensor is of importance since pressure differences between troughs and crests become insignificant at increasing depth bellow the wave. This depth in part depends on the total depth of the water column. The water collumn bellow the wave does not follow the same pattern as the wave on top. This can be seen in the following animation.

http://orange.math.buffalo.edu/444/deep_water_wave_plus.gif

Somebody asked if a diver will move in the water column (veritcal) bellow the wave. The answer can be seen in the link above. One would have to be very close to the surface in order to move. The force of these waves are "averaged out" with increasing depth. Thus the abillity to detect pressure differences due to wave action decreases exponentially (very quickly) with decreasing movement in the water column below the wave. As demonstrated above decreasing movement happens not far bellow the wave itself. Notice the dots in the bottom few rows (horizontal) hardly move. And the various forces of the wave is what makes them move. The fellow that claimed to have been measuring pressure differences (of TWO feet) due to waves at 30 feet depth needs to check his gauges. For this to measurable at 30 feet he must have been diving in VERY deep water (deep water aids water column movent due to waves because the ground does not impede movement) AND in order to move a water column 30 feet down he must have had around 18 foot waves (amplitude). Hmm I find that unlikely. Sorry not to insult, but I imagine he's got a computer. I think that the computer may indicate changing numbers like that when it is on the verge of indicating one depth versus another. (Hmm I bet the computer has two foot increments?). So to sum it all up it is insanely unlikely that a fellow would get an embulism due to wave action. Unless he was diving in Hawaii and had a wave break over his head . Even for Wave action to affect the diver at 10 feet one would have to have around 7 foot high waves. Hmm even with waves like that I think the bigger concern would making it onto the rescue helicopter.

 

[donacheson]

People actually do this. One of the ways physical oceanographers measure wave height is to place a pressure sensor at a fixed depth and then use the pressure changes caused by the waves passing over head to deduce wave height. So yes, it does change pressure.

Here is an example of such an instrument (scroll to the bottom for the explanation of wave height measurement):

http://marine.rutgers.edu/leo/leo15_prof_b.html

Rick Murchison's explanation was correct.

From that link: "Surface waves at LEO-15 are measured with a bottom-mounted pressure sensor, not a surface buoy. By the time they reach the seafloor, shorter period waves have been attenuated more than longer period waves. Waves with periods below about 4.5 seconds are rarely observable with the LEO-15 bottom-mounted pressure sensor. Surface buoy observations include waves with periods below 4.5 seconds, so they report higher significant wave heights than the LEO-15 pressure sensor."

The pressure change beneath a wave decays exponentially with depth. I no longer have my grad school texts, but I'd expect that pressure (in atmospheres) varies as [waveheight/33]*[exp(-constant*depth/wavelength)] for depths below the wave trough and probably over bottoms much deeper that 1 wavelength. For long waves in shallow water, numerical simulation would probably be required.

 

[fdog]

This may be an offshoot of "common oldtimer knowledge" here on the Northern California coast:

>Surf report is 6'-8'.
>Diver X, slightly addled, decides that today is a fine day for diving.
>Also is making beach entry. (not as bad as it sounds)
>Makes entry in textbook fashon, regulator in mouth.
>Diver X makes it past the frothy stuff.
>Still in "shallow water", about waist deep, sees wave approach.
>As wave begins to pass over, Diver X ducks under wave.
>Instinctively inhales big breath.
>Reg supplies gas at pressure of water column (8' or roughly 3.5 psi over 1 ATM)
>Diver pops out backside of wave to surface.
>Chance of overpressure injury.

"Street wisdom" is (Big surf) + (Beach entry) = (use snorkel past the frothy stuff)

All the best, James

 

[gangrel441]

James

Has this phenomenon ever been documented, or is it just common knowledge passed on from diver to diver? If you know if a specific case of this happening, it may put this thread to rest.