Single bottle dives and choice of deco gas

[gsk3]

When planning a dive with a single deco bottle, when do you decide to plan on 50% vs. O2, and why? Assume standard gases and depths in the 100-130ft (30-40m) range.

So far I've heard two reasons for the choice:
- Change from bottom gas: If you're using 32% or 30/30, it doesn't make much sense to use 50% for deco.
- Fast vs. slow tissues: If you are doing shallower dives and going into deco, you are doing longer dives. Therefore you'll wind up with more slow tissue saturation and more time on O2 at 20ft is better for that.

My current thinking for each depth is something like:

Depth.........Bottom gas...Deco gas
100 ft...............32%.......100%
110 ft....30/30 or 25/25.......100%
120 ft....25/25 or 21/35........50%
130 ft....25/25 or 21/35........50%


If the bottom gas is the reason, where would you draw the line? E.g. for a 110 ft dive on 30/30 would you use 100%, but the same dive on 25/25 use 50%? Or 120ft dive on 25/25 use 100% but same dive on 21/35 use 50%?

 

[limeyx]

For me, I would use O2 for most stuff 80-120 as a first choice, partly because the 50% is a bit less of a change from the BG, but also because I "like" O2

Once the rock bottom starts to get "too big" for the dive, then I would generally switch to, or add a 50% bottle.

Anything deeper than 120, then it would be 50% first choice.

 

[JS1scuba]

For dives in the 100-130 fsw range that REQUIRE decompression stops, using a standard 50/50 nitrox or 100% oxygen would be best. If the total decompression time reaches or exceeds 30 min then you would want to use BOTH EAN50 and 100% oxygen as it will make for a more efficient decompression and let you open the famous oxygen window a bit more.

cheers
JDS

 

[fdog]

I'll echo Nick, and add that potentially rough seas will push me towards 50%.




All the best, James

 

[fdog]

I'm pretty sure what you meant, Joel, but just to clarify, you're refering to 50% (not 50/50), right? Lest someone get confused...


All the best, James

 

[gsk3]

So that adds a third consideration (rock bottom) and confirms that either at 120 or 130 plans should change to 50%. Thanks.

Joel - I'm not doing dives past 30 mins of deco currently, but I'm aware that they would add a second bottle. Do you have thoughts on when to plan on 50 vs. 100% for the one bottle dives?

 

[Rainer]

My current thinking for each depth is something like:

Depth.........Bottom gas...Deco gas
100 ft...............32%.......100%
110 ft....30/30 or 25/25.......100%
120 ft....25/25 or 21/35........50%
130 ft....25/25 or 21/35........50%


If the bottom gas is the reason, where would you draw the line? E.g. for a 110 ft dive on 30/30 would you use 100%, but the same dive on 25/25 use 50%? Or 120ft dive on 25/25 use 100% but same dive on 21/35 use 50%?

My personal and local team feelings are that we'll dive 32% to about 110' with O2 for deco, and 21/35 and 50% from 130' (to about 160'). The awkward range is really 110-130' dives. I've done several things in that range (25/25 with O2, 21/35 with 50%, 30/30 with O2). I can't say I have a favorite or that we've really settled on a standard. If I know there will be portions down to 130' (especially with any overheads), we'll take 21/35 and 50%. If I know I'm going to max out at 120' and will be there for awhile, 25/25 (or 30/30) and O2 is the call. If we're unsure of exact depths, 21/35 and 50% offers the greatest flexibility. You do have to realize you need to make "corrections" to the RD rules for 100-120' averages (1:1 from 150' will underestimate the required deco).

In general, O2 deco just "feels" better post-dive than 50% deco, and I find it better for slow tissues on longer bottom times (usually the shallower dives). Also no reason to use a deco gas that has as much if not more N2 than your backgas. That said, O2 benefits need to be balanced against min gas requirements. Fewer mix options is also of course nicer for standardization and team planning...

For those I dive with, 32%, 25/25, or 30/30 is always O2 (or if the deco is really short enough, backgas). 21/35 is always (at least) 50%.

 

[gsk3]

Ugh. I just wrote out a whole reply about RD in this range, but browser reset and erased it all. Summary was using either 1:1 on time past the MDL or 1:1 - 5 min per 10 feet above 150, then taking the more conservative of the two, seems to work.

Quick example: 110ft 30 minutes O2 deco.
First rule gives 10 minutes deco. Second rule gives 10 minutes deco. DecoPlanner VPM+2 gives 7 min.

110ft 50 minutes O2 deco:
First rule gives 30 minutes deco. Second rule gives 30 minutes deco. DecoPlanner VPM+2 gives 16 min.

Looks like a 1:2 ratio in this range might be more appropriate than 1:1 with a correction. Either way, I'm using RD as a backup rather than primary deco planning so not a huge issue for me currently.

 

[Rainer]

Sorry, you misunderstand me. I'm talking about trying to apply RD 1:1 rules for 21/35 and 50% in the 100-120 range. The usual RD algorithm can underestimate the required deco (relative to VPM+2). I take (and was taught) 1:1 applies without modification (apart from the +/- 5min for 10' increments) in the 130-170 range for 50% as the sole deco gas.

For O2 deco, those I dive with use a completely different algorithm (basically time over = deco required, shaped *very* differently than how you shape 50% deco). I won't comment on the specifics of deco profiles on open fora, but feel free to PM for details.

I use RD as my primary deco dive planning in the ranges/depths/BTs I'm comfortable with. Having to pull out a laptop for each dive is for the birds.

Ugh. I just wrote out a whole reply about RD in this range, but browser reset and erased it all. Summary was using either 1:1 on time past the MDL or 1:1 - 5 min per 10 feet above 150, then taking the more conservative of the two, seems to work.

Quick example: 110ft 30 minutes O2 deco.
First rule gives 10 minutes deco. Second rule gives 10 minutes deco. DecoPlanner VPM+2 gives 7 min.

110ft 50 minutes O2 deco:
First rule gives 30 minutes deco. Second rule gives 30 minutes deco. DecoPlanner VPM+2 gives 16 min.

Looks like a 1:2 ratio in this range might be more appropriate than 1:1 with a correction. Either way, I'm using RD as a backup rather than primary deco planning so not a huge issue for me currently.

 

[gsk3]

Sorry, you misunderstand me. I'm talking about trying to apply RD 1:1 rules for 21/35 and 50% in the 100-120 range. The usual RD algorithm can underestimate the required deco (relative to VPM+2). I take (and was taught) 1:1 applies without modification (apart from the +/- 5min for 10' increments) in the 130-170 range for 50% as the sole deco gas.

Ah, I did misunderstand. E-mailed you.

Having to pull out a laptop for each dive is for the birds.

I just got an iPad. The $45 for Vplanner mobile is looking mighty tempting.