Hold that line boys! How precise should you hold depth?

[Rhone Man]

Normally in the ocean, I plan on my last stop being at 20'. It lessens the impact of the ups and downs caused by swells, and it keeps you well beneath the ladder, props, rudders, etc. Once you finish you can do a nice slow gradual ascent to the ladder.

Reminds me of a great story I read about a dive on the Andrea Doria. Bunch of divers down on the wreck, and then a storm starts to blow up and it gets worse and worse. Soon the boat is groaning against the grapnel line, but the skipper cannot do anything because he has divers down decompressing. He stations a man on the deck to cut the line as soon as all the divers are safely recovered. The storm gets worse and worse. Eventually the troughs between the waves become so deep that when the boat is in the bottom of the trough, the divers clinging onto to the line for dear life doing their last decompression stop are actually above the crew on deck and wearing an extremely distressed look on their faces.

The story ends happily with all divers recovered, and the boat returning to port. But I doubt those divers kept the last deco stop at +/- 1 ft.

 

[Rhone Man]

HOLY CARP! Are you kidding?

Amusing typo of the day:

 

[RJP]

Amusing typo of the day:

Wasn't a typo. Did it on purpose. Figured that any BBS software that censors ****, *****, ****, ******, or ****** would also censor the word "crap."



EDIT: looks like I was wrong!

 

[Gilldiver]

Reminds me of a great story I read about a dive on the Andrea Doria. Bunch of divers down on the wreck, and then a storm starts to blow up and it gets worse and worse. Soon the boat is groaning against the grapnel line, but the skipper cannot do anything because he has divers down decompressing. He stations a man on the deck to cut the line as soon as all the divers are safely recovered. The storm gets worse and worse. Eventually the troughs between the waves become so deep that when the boat is in the bottom of the trough, the divers clinging onto to the line for dear life doing their last decompression stop are actually above the crew on deck and wearing an extremely distressed look on their faces.

The story ends happily with all divers recovered, and the boat returning to port. But I doubt those divers kept the last deco stop at +/- 1 ft.

Its happened more then once and can happen anywhere that the current can change directions with the tide, or just because. Add in winds against that current and the waves will start to build. The great Lakes can see this as fresh water will build higher with a lesser period due to the lower density.

 

[Rick Murchison]

...what's the dealeo with holding to within 1'?... What is the acceptable variance here? ...

As I've mentioned before, deco algorithms are akin to "measuring with a micrometer, marking with chalk and cutting with an ax."
The "dealeo" is that +/- 1' is the standard for acceptable deviation from stop depth/buoyancy control for decompression dive training.
The goal is +/- ZERO!
In the real world it is never possible to reach the goal, but usually possible to meet the standard... but we also know that sometimes (often?) - due to weather, current, brain farts, etc - it is not possible to meet the standard either.
Your real question - and real concern - is "at what deviation does the decompression algorithm fail, and I get DCS?" or, in a more practical view, "how much slop can I get away with?"
And the answer to that question is a moving target! People have gotten bent even with very tight control; people have escaped getting bent with horrible deviations from the computed stop depth.
I like to look at it like target practice... if there is a 5 mil ballistic dispersion inherent in the projectile, then the best I can expect on a 1000’ shot is to come within 5’ of the aim point, and there’s a chance of hitting the aim point so long as my actual aim is within 5’ of it. But my best chance of actually hitting the aim point is with my actual aim precisely on it.
In measuring performance on such a shot, my goal is to hit the aim point every time. My standard might be 10’… so to meet the standard I must aim no more than 5’ off center.
If my projectile has a real world 25’ effective blast radius then I can get away with being off as much as 20’ and still get the target every time, and be off as much as 30’ before a guaranteed miss.
If the target’s shooting back (DCS), it’s worth my while to practice precise aim every time I engage
Bottom line is that you're wise to be as precise as you can possibly be, but don't panic if you're off a bit; if you're off a lot, pray harder.

Rick

 

[Meng_Tze]

Uh.... it is a ppO2 issue, amongst other issues.

The deco algorithms are based on an ascent profile and deco stops. If you don't follow the algorithm then you might as well make up your own schedule. Deco is Voodoo Magic anyway, I'm not going to make it Double Voodoo Magic by introducing my own untried and untested and unproven schedule into the scenario. If the algorithm says stop at 20 feet for 7 minutes, then that's what I'm doing. Not 25 feet for 7 minutes, not 20 feet for 6 minutes............No, I'm doing 20 feet for 7 minutes.

PO2 plays a definite role, but also the gradient you are trying to achieve and total exposure (effectively RT). Doing deeper stops than required has more negative outcome than doing the stops shallower. Doing your 20ft stop at 25ft is worse than doing it at 15ft.

Especially with longer hang times, achieving the correct depth (but at a minimum shallower if you cannot) is crucial.

 

[TSandM]

People are actually doing the experiment every day, because divers are doing the same bottom profiles using different decompression programs, and generating different tables, and doing different decompression using different gases. The answer is that there are a number of things that work, and that people using things that usually work sometimes get hurt anyway, and we don't always know why.

I am entirely certain that changing a schedule from 5 minutes at 60 feet and 6 minutes at 50 to 6 minutes at 60 feet and 5 minutes at 50 is not going to hurt me. How much I have to change that schedule to raise the risk significantly is something I don't know, so if I've generated a schedule, I'm going to try to follow it fairly closely, but I'm not going to panic if I overstay a stop by 60 seconds, especially if that stop is relatively shallow (overstaying becomes a bigger issue deeper, and cutting them short is a bigger issue shallow).

I still think the basic issue is one of control -- poor control puts you and potentially your team at risk. Excellent control allows conscious decisions to deviate WITHIN parameters you decide, and then the risk is your own to decide.

 

[Rick Murchison]

PO2 plays a definite role, but also the gradient you are trying to achieve and total exposure (effectively RT). Doing deeper stops than required has more negative outcome than doing the stops shallower. Doing your 20ft stop at 25ft is worse than doing it at 15ft.

Especially with longer hang times, achieving the correct depth (but at a minimum shallower if you cannot) is crucial.

In my opinion, shallower is bad advice... very bad indeed, especially at shallower stops. While doing the 20' stop at 25' on pure oxygen is bad from an oxtox standpoint, doing it at 15' is bad from a bubble standpoint, and gains nothing on the nitrogen side of the problem, so neither is a good idea.
However... when dealing with pure oxygen, so long as you remain within the safe oxtox limits, doing the 10' stop at 20' has the same nitrogen gradient as doing it at 10', with the additional benefit of greater safety from a bubble standpoint.
As bubble growth is a function of the change in pressure over the total pressure, your suggested five foot shallower stop in the 20' range will result in a 10% growth in any existing bubbles, which could very well be the difference in no DCS and DCS.
As moving from 20' to 10' causes any existing bubbles to grow 23%, doing that 10' stop at 20' on oxygen looks like a mighty good trade-off for an inconsequential increase in oxygen exposure and zero decrease in nitrogen gradient.
--
If I must deviate from stop depth, I will, but I'll be adding lots of time to the shallower stops if I have the gas for it.
I think I'll start another thread on strategies for deviations from planned schedules...
Rick

 

[ianr33]

As moving from 20' to 10' causes any existing bubbles to grow 23%, doing that 10' stop at 20' on oxygen looks like a mighty good trade-off for an inconsequential increase in oxygen exposure and zero decrease in nitrogen gradient.

I checked the math on that and agree that the pressure decrease from 20 feet to 10 is 23%,so the Volume of any bubbles will increase by 23% . Radius will increase as the cube root of the volume so will only go up by a factor of 1.07. i.e.a 7% increase.Not a good thing but not as bad as 23%.

Which of those figures is the most relevant to getting bent I dont know.

I tend to do the first couple of minutes of an O2 stop at 21 or 22 feet then ease up to 20. When the 20 foot stop clears I will slowly ascend to around 15 feet and do the remainder of deco at that depth.
longest I have been on O2 is around 30 minutes.Will do a 5 minute backgas break in the middle of that but even so I tend to get a dry cough the next day which I suspect is O2 related.

One other (kind of ) relevant point is that I set my computer to 95% O2 rather than 100. If its set to 100 it will get upset about ppO2 at 21 feet and upset about a missed stop at 19 feet. I can of course hold a stop to +/- six inches but I prefer to give myself a little leeway just in case.

 

[rjack321]

I think I'll start another thread on strategies for deviations from planned schedules...

As long as you're not on deep air and have a buddy on the same page, just recalculate on the fly. The fantasticoness of ratio deco is exposed

Certain individuals who shall not be named are better off with computers recalculating for them tho.