No Technical Training for Me.

[Rick Murchison]

Rick, you're talking a change in depth as the "bottom" of the wave goes over, then the "top" and so on?

total change in depth would be what? the "height" of the wave?

The very first thing we must define here is that depth, for the purposes of decompression discussions, is simply a measure of pressure, not the physical distance to the surface of the water directly above you. With this definition in mind, the change in depth experienced by an object suspended in the water is less than the vertical distance from crest to trough because water pressure is exerted in all directions, not just directly downward. So, even if you are located directly below the trough of a wave, you're still getting some pressure exerted from the crests on either side - but the total is still less than when a crest is directly overhead. For this reason, the effect of passing waves on ambient pressure for an object suspended in the water column is less the deeper the object is; the closer to the surface the object is the greater the effect. The period of the wave has a lot to do with it too.
Imagine the pressure measured halfway down a watermellon in the ocean, and six foot waves.
If the waves are very close together, as they pass, the watermellon will experience very little up and down movement, but it will experience a substantial pressure (depth) change as it is submerged with each passing wave. But if there is a very long period between waves, then the watermellon will be able to rise and fall with the swell with very little change in depth. The effect on a diver at a shallow safety stop follows the same rules, and rough, short period seas can make a deeper deco stop desirable to minimize cyclic pressure/depth changes during the stop. (Note that for an object on the bottom this principle is reversed - the very long period wave will have a greater effect on the pressure on the object than the short period wave - it all depends on (1) the actual change in ambient pressure and (2) whether the object moves vertically enough to compensate for the changing pressure).
Rick

 

[Rick Murchison]

It will be unchanged (well … much less that the amplitude)

Well, which is it? Can't be both.
Rick

 

[H2Andy]

if i am doing deco at 20 feet in 6-8 foot waves

how much pressure change are we talking about? and how often?

 

[Thalassamania]

Well, which is it? Can't be both.
Rick

It’s like the gas laws, there’s a difference between ideal behavior and observed behavior. It's going to depend on several factors including the mass of the object and the vertical drag component of the object’s shape. Mass will resist movement with respect to the gravity vector, drag on the shape will provide the acceleration of the object as it overcomes its mass to change position with respect to the gravity vector. Then, due to its potential energy, it will overshoot its “perfect target” by some small amount. It will be different for different masses and different shapes as well as vary directly with wave amplitude and indirectly with depth. A foot of change is easy to feel on your ears and three feet is enough to really hurt, so I’d guess, for a diver, it’d be in the range of a foot or so.

 

[Thalassamania]

if i am doing deco at 20 feet in 6-8 foot waves

how much pressure change are we talking about? and how often?

Eight foot seas I guess a foot or less. Time scale is half the period.

 

[Rick Murchison]

if i am doing deco at 20 feet in 6-8 foot waves

how much pressure change are we talking about? and how often?

My impression is that in the Gulf, probably ~1-2 feet. In the Atlantic, probably ~1 - 1.5 ft (longer period in the Atlantic, usually). At ten feet with those seas I'd guess ~ 2 ft+/- If you wanted to find out for sure you'd need a good real-time pressure gauge that you could read in less than 1ft increments - or you could make one out of a small water bottle and a 2ft length of good stiff acrylic tubing.
I've never actually measured it - I've always subscribed to the theory that if I can feel the depth (pressure) change in my ears then it's enough to be "less than optimum" - and to consider being deeper for a good deco stop.
But now that it's come up, I'll make a gauge and test it out the next time I can get offshore.
Rick

 

[Soggy]

But now that it's come up, I'll make a gauge and test it out the next time I can get offshore.

In order to get an accurate measurement, you'll need a device that has a similar mass as you and a similar drag coefficient in the same direction. This gauge will need to be separated from you so that your own fluctuations in depth due to breathing do not have an effect on the measurement.

The real answer is that it just doesn't matter in real diving situations, regardless of the subtle differentials that may be created.

 

[Rick Murchison]

In order to get an accurate measurement, you'll need a device that has a similar mass as you and a similar drag coefficient in the same direction. This gauge will need to be separated from you so that your own fluctuations in depth due to breathing do not have an effect on the measurement.

Actually the data I want is the data that I can use, so all I need is something I can hold at eye level and mark while I do some stops at various depths in rough seas. So I'll have my exact mass and drift coefficient, and include any inherent errors in my own technique. Then I'll send it to you so you can play with it and make up your own mind.

The real answer is that it just doesn't matter in real diving situations, regardless of the subtle differentials that may be created.

I think it does. That's why I'll conduct the experiment - and I'll either confirm what I think or debunk it. At least we'll have some real numbers to decide around.
Rick

 

[Thalassamania]

I’d be real interested in the results, please be sure to get good data on the size of the swells and their period.

 

[Soggy]

Actually the data I want is the data that I can use, so all I need is something I can hold at eye level and mark while I do some stops at various depths in rough seas. So I'll have my exact mass and drift coefficient, and include any inherent errors in my own technique. Then I'll send it to you so you can play with it and make up your own mind.

I'm not sure how that data will be useful because your inherent errors are not predictable and thus it will be difficult if not impossible to decipher what is a technique issue and what is a wave-related change in pressure. Maybe you have thought of a way to correct for that, but I can't think of any. I'm no engineer, though.

I drag a flag float around in shallow dives (5-20' deep) with swells going over head quite frequently. I do not experience drysuit squeeze and release, as I would if your theory were correct. I do end up in a washing machine getting tossed back and forth (surge), but there is no indication of pressure changes.