Big Rollers!

[DEEP SEA]

I test dive a lot and was wondering, when your hanging on a descent line anchored to the bottom at about 30 fsw and big swells keep rolling over your head (say 6 to 8' swells), does the swells add to your depth?

Also, how safe is it being dragged along the bottom on a sled when the bottom keeps varying depths do to reefs and large rocks (good visibility but rapid climbs and falls)
.

 

[Rick Murchison]

Yes. Waves do have an effect on your real depth, and are probably a factor in some DCS and AGE cases. When there are big waves it is extremely important to be even more conservative than normal in your profile, and especially careful not to do any full-lung breath holding near the surface. During the safety stop it is better to ride the swell than to hang rigidly to a rope.
I for one don't like to dive in anything more than about four foot swells.
Rick

 

[Mario S Caner]

Doing Deco stops in large swells can be done safer with the use of a Jon line. Never hold onto the line as it will drag you up and down a matter of a (potentially deadly) few feet in a split second.

 

[large_diver]

I learned the hard way last summer not to swim under an underwater hang bar in big swells (4-8 ft)....DOH!

 

[DiverInAk]

If you are aware of large swells, you should probably try to perform your safety stop closer to the 20ft mark, and if possible extend it for an additional minute or two.

 

[Remora]

Hey Deep Sea!

The Jon line that Mario suggested works great. The point is to be situationally alert while doing your hang, never grip the line but let it ride through the "O" you make with your thumb and forefinger. The deeper stop is also a good recommendation based on the swells. WRT the sled they are quite manourevable but be very careful on the quick ascents. You may have to ensure the boat operator is aware of the underwater topography and reduce his speed. U/W comms gear is excellent for this.

Yet another plug for OTS Mario!

 

[DiverInAk]

This is also another good point for diving with a computer, at least I would think it would help.

 

[DocVikingo]

Hey Mario,

Groan.

DocVikingo

 

[DEEP SEA]

Thanks everyone who replied. I think you answered my question on the surge. I wonder how many divers got hurt and never figured out what happen?

 

[Dr Deco]

Dear readers:

To the responses regarding decompression stops in choppy seas, I would add the following comments. These sea conditions can at times be connected with currents and divers have found themselves with the requirement for expending considerable effort to move underwater. (Something similar to this occurred in Cozumel a year or so ago. It resulted in several cases of DCS in the members of the dive party.)

When physical activity is very high, the tissues can be loaded at depth while the tissue perfusion is high. This is because of increased heart rate, the "muscle pump" and active vasodilator substances released by the tissue itself. During the deco stop, physical activity is then greatly reduced, and off gassing of a tissue might not occur to the extent required to prevent DCS.

While “compartments” are included as a part of the decompression algorithm, it should be remembered that considerable muscular activity “converts” many tissues (eg, muscle, tendons and ligaments) to shorter halftimes. If the activity continues at depth for a period of time, the tissue gas loads can be very large. When this activity is abruptly reduced, along with blood flow, tissues are “converted” to slower halftimes and offgassing is reduced. Large “conversions” of this nature are not expected to occur and, present Haldanian decompression algorithms and theory do not address this in decompression computers.

It is evident that such situations are not built into current decompression computers. Adaptive decompression algorithms do not adapt to this type of situation. It is necessary for the diver to be cognizant of this and, I believe, [red]pause for a longer duration at the stop level. [/red] I would guess that extending the stop duration by a factor of two would not be excessive.

In the future, we may have deco meters that take into account such situations. Until those halcyon days arrive, we must make the corrections ourselves. Knowing this physiological principle is of value for many dive situations.
_________________
Dr Deco


[sp]For those with a special interest in these matters, here is the next scheduled Decompression Physiology course. http://wrigley.usc.edu/hyperbaric/advdeco.htm