DIR- GUE DIR 90-minute limit on ocean dives

[jborg]

I was reading through Fundamentals of Better Diving in advance of Fundies, and came across an interesting and (to me) possibly dubious passage. This is the old first edition, not the more modern second edition -- I'm not sure why, but that's what we get sent in PDF form. I have the new edition in paper form and don't recall seeing this passage there, but I don't have it at hand for comparison right now. Anyhow, what I reacted to was the following in the equipment chapter under the heading "Stage/Decompression Bottles" (emphasis mine):

A stage bottle is a bottle used to extend bottom time, whereas a decompression bottle is a bottle used during the ascent portion of the dive to promote efficient decompression by reducing inert gases (e.g., N2 and He) while elevating oxygen percentages. Stage and decompression bottles are almost exclusively used in technical diving, where longer bottom times and/or multiple gas mixes are the standard. These bottles usually rely on the same or similar equipment and are filled with appropriate gases for a given dive.​

While ocean diving, stage and decompression bottles should be made of aluminum so as to not overweight the diver. Because conditions in the ocean are unpredictable, diving in the ocean should be limited to 90 minutes of total immersion and less should conditions warrant. For this reason, cylinders as small as 40cf are usually perfectly adequate for most ocean decompressions, with 80cf cylinders being the permissible upper limit.​

Is this is a current part of the GUE thinking, or something that's been superseded/deprecated?

Note: I'm specifically looking for input from GUE divers here to understand the GUE point of view. Yes, I could, and maybe will, ask my instructor, but that's just one opinion and you guys are out there right now...

 

[jadairiii]

Cant tell you "current thinking" but the logic behind this is that a lot can go wrong in open water diving the longer you are in the water. Consider doing a drift dive (in S Florida) with a 3 kt current, you can be quite far down range from where you started. Add to that, if you are holding steady on a down line in that current for a lengthy period of time. In that second scenario, if one diver blows off the line and everyone else stays, and you have a 3 hour deco, where does the boat go?

Not to mention, just supporting divers on long decos in rough conditions with a "small boat" (in bad weather a 40' dive boat gets real small).

logistics of doing long deco dives in open water, safely, are complicated at best.

 

[rjack321]

I've done longer than 90mins in quasi controlled conditions (eg shore diving in a lake) and everything was fine. I've also gone slightly over in the Great Lakes (100mins) and surfaced to a thunderstorm & lightening 25+km from safe harbor and scary waves.

Nothing about weather or currents or any other 'issues' has changed since that book was written and 90mins is still a good number to shoot for.

 

[Imla]

I can't remember that being a theme during my T1/T2/CCR2 classes with the exeption of the point jadairii is making above.
Limiting exposure to what is needed to achieve the objective that you want to achieve, keeping in mind local temperatures, weather volatility, currents, traffic and so on.
The previous city I lived in was well know for having the possibility for "4seasonsinaday".
Also keep in mind that fundamentals gives you a very small, very square box to play within. As you "ascend in the ranks" you get bigger boxes, softer walls, and more room to play within the boundaries of knowledge and experience.
Also... if I recall correctly... T2 is limited to 60 minutes of unadjusted decompression in deco planner. Basically, with D12s, a s80 bottom stage and a 7L/s80 50% and an s40/7L O2 stage, you should be pretty spot on for just about any T2 dive within that time restraint, and that keeps your total dive time under 90 minutes...

 

[nadwidny]

I've done longer than 90mins in quasi controlled conditions (eg shore diving in a lake) and everything was fine. I've also gone slightly over in the Great Lakes (100mins) and surfaced to a thunderstorm & lightening 25+km from safe harbor and scary waves.

If I recall, I was with you on both those dives....

 

[jborg]

Thanks for good points, all!

 

[Nick_Radov]

GUE divers will sometimes do longer ocean dives but those are "expedition" type dives with complex logistics. There might be a chase boat, safety divers, floating deco station, etc. For routine ocean diving with your buddies it's a lot safer to stay roughly within the 90 minute limit (plus or minus).

The other factor is hypothermia. If you have a bad drysuit flood at the end of your bottom time you can probably tough it out through 60 minutes of deco but more than that and your body can start to shut down.

If you need more bottom time then just do two dives with a decent surface interval.

 

[huwporter]

Just about every T2 certified diver here has done longer than 90 minute run times in the ocean - BUT it's essential to be aware that conditions can change rapidly, to be knowledgeable about how your your local conditions work, to know the weather forecast, and understand that the longer dives get, the more important and more involved support needs to become.