How Much Deco Can you Blow Off?

[lamont]

People are talking about how much time to blow-off, but I tend to think about it more in terms of what % of the deco to blow off.

My question would be which would you rather be:
1) 90 minute deco obligation, blow off 20 minutes
2) 30 minute deco obligation, blow off 20 minutes
Or, do you think that they are equally dangerous?

Other way of thinking about this is:

1) blow off 60 minutes of 90 minute deco (2/3rds blown off)
2) blow off 20 minutes of 30 minute deco (2/3rds blown off)
3) blow off 5h20m of an 8 hour deco (2/3rds blown off)

Since I know divers that have inadvertently done #2, while I don't think you get away with #1 without at least a chamber ride, and #3 would kill you, I don't think strict percentages are right. It is probably closer to blowing off 20 minutes of deco being equivalent, but if you're doing an 8 hour deco you can probably get away with cutting it 20 minutes short more than you can on a 30 minute deco, so that breaks down at some point too.

 

[liuk3]

Agreed. I wasn't trying to make hard and fast rules as you tend to get yourself in trouble that way with all this voodoo. I was just trying to also relate deco obilgations closer to my own experience level. I am currently not doing dives incurring 8 hours of decompression. It is interesting hearing people's perspectives on this. Obviously there's no right answer, just different considerations if you find yourself in a position where you need to do so.

 

[liuk3]

Lamont, what is the dividing line between T1 and T2 ?

I assume he is using GUE's definition of T1 and T2, but I could be wrong.

 

[ianr33]

I assume he is using GUE's definition of T1 and T2, but I could be wrong.

Which is? (I dont do the GUE thing )

 

[lamont]

I assume he is using GUE's definition of T1 and T2, but I could be wrong.

yeah, 1 deco bottle + normoxic = T1 vs. 2+ deco bottles + hypoxic = T2

 

[liuk3]

Which is? (I dont do the GUE thing )

Sorry.

T1 is generally 1 deco bottle, 30 minutes or less of deco, and 170 max depth.
T2 is generally multiple deco/stage mixes, longer deco, and deeper depths.

You can find the exact standards here:
Standards and Procedures | Global Underwater Explorers

 

[rjack321]

In an emergent situation, the options you have are to get back in the water and do some IWR, or hang at the surface for a couple of hours breathing O2 and hydrating. I've heard of divers blowing off deco time at 20' stops for a variety of reasons that did just that and didn't require a chamber ride.

If your time on the surface is short that's probably more like omitted deco. I'm not adverse to omitted deco nor IWR. In many places either are a hell of alot LESS risky than waiting X hours for the chamber.

 

[JS1scuba]

It all depends on a few factors ?

1. If I have to bring someone up to save a life I will blow off all the stops to get them to the surface, hand them off, and drop back down and have another diver bring me gas to complete my omitted deco schedule Then while they work on the guy on the surface they should prepare to evac me when I come up. (always carry emergency gas on board for this)

To the reality of the question.

Decompression schedules vary so much as to what is minimum decompression based on perfect profiles, gas selection, conditions etc and an acceptable level of risk. For example is one uses the USN as a rock bottom minimum decompression and you are using say a Hamilton DCAP as your schedule then you have a wide amount of deco you can "blow off" if you need to.

As a rule of thumb that I use if I need to get up is to blow off all deep stops deeper than 3 atmospheres (this saves gas) then get up and shallow and assess the situation. (compare your schedule of your typical to a classic neo-haldanean model - you will see what I mean)

For Example a 200 fsw 25 min dive with 18/35 50 EAN and 100% has a first stop using Buhlman of 76 fsw with a total runtime of 80 min. Whereas a bubble model has that same bottom time have a first stop at 150 feet with a runtime of 84 minutes. But the USN has the first stop at 50 fsw and a total run time of 69 minutes .......... which one is right ?

In my decision tree the USN of 69 minutes is my minimum deco and from there I make the decision as to how much risk I will take in blowing off stops.

If i am flying a computer (which I do) and the computer gives me less deco than all of it I will fly the computer, shave off as much as 25% of the total deco time and then on surface do 1 hour of 100% oxygen.

NOTE: THE ABOVE IS MY PERSONAL CHOICE -- I accept decompression risk and fully understand the S/S of decompression illness. I've managed deco incidents on boats and ran a diver HBO facility, so I am not pulling this out of my butt.

Blowing off decompression for the sake of doing it is stupid and reckless. If you have a REALLLY good reason to do so have a plan in place before you need to. Make sure your boat and crew know what your protocol is. Make sure you have extra gas on board with regs on them so it can be dropped to you. (I always have extra with me for me and clients)

**** happens. Be prepared if you slip in it.

Cheers
JDS

 

[Kevrumbo]

Came up in another thread.

I know this is one of those unanswerable questions,but, just for discussion:

How much deco can you miss and still have a chance of no major ill affects? To put some boundaries on it, lets suppose the dive was planned using V Planner +2 , 30/85 gradient factors or similar and the missed deco was on O2. Also lets only consider "normal" dives. Skipping the last 10 minutes of deco when you've already done 10 hours of it in Wakulla probably doesn't matter too much !

My personal, WAG is that if its my lucky day then I might be able to miss 10 minutes of O2 deco and be none the worse for it. Miss 30 minutes though and I would expect my diving career to be over.

Thoughts?

My Buddy and I were roughly 25 to 30% short of total optimal deco time when I got a DCS type I hit in Truk Lagoon. There was no margin left for any error, looking back now in hindsight --and my buddy had a perfect slow ascent profile on the flawed deco schedule with no problems post-dive. Otoh, I strained a left shoulder at depth manipulating stage/deco bottles; popped-up like a ballistic missile the last 3m/10'; and further injured my shoulder hauling my tanks onto the boat upon surfacing in a rain squall. Used and miscalculated while at depth, an on-the-fly extension of a Ratio Deco 1:1 schedule at 48m/160' (based on GF 30/85 and V-planner VPM-B +2).

http://www.scubaboard.com/forums/near-misses-lessons-learned/264517-type-i-bends-hit-chuuk.html

 

[karstdvr]

I blew off 7 minutes deco because an 8ft gator was 3 feet away in the small cavern. No ill effect,even after exertion of beaching myself versus proper tank removal.