25 replies to this topic

[PerroneFord]

As I am starting to venture into deco diving, I have a curious question for our divers versed in deco. What do you use for a deco gas, when you are only taking 1. I have run into two schools of thought on this.

The first, says to carry a deeper gas. Something like 50%. The thinking is that you can get on the deco gas earlier and elinate a good deal of the inert gas before getting real shallow.

The second school of thought says that oxygen will help your deco more than anything else, and if the crap hits the fan, you'd REALLY want that oxygen if you could have it. Also, you won't need as much deco gas total if you are using oxygen. So on dives where I'd need to carry an 80 of 50%, I could carry a 40 of oxygen.

I'm curious to see what you guys did or are doing.

Thanks,

-P

[JimG]

As I am starting to venture into deco diving, I have a curious question for our divers versed in deco. What do you use for a deco gas, when you are only taking 1.

I don't think there is any hard and fast "right" answer to this - everything depends on the type and parameters of the dive.

On cave dives for example, I only use O2. An O2 bottle can usually be parked fairly close to the entrance, whereas a bottle of 50/50 might need to be carried for some distance into the system. That may not seem like much of an issue to you now, but when you start using stage bottles, it will be. Besides, most of the caves in central Florida will give you a kind of "natural" intermediate depth deco as you are exiting (i.e. you spend a fair amount of time in the 20-70 foot range on your way out, due to the configuration of the tunnels), so I don't think there is as much benefit to using 50/50 on those profiles, versus using O2 all at 20 ft.

For OW diving, I tend to prefer 50/50 - these dives tend to be deeper max depths but shorter run times, so I think 50/50 makes more sense from a decompression standpoint (gets you off the higher PN2 gas sooner). Another advantage is that if you and your buddy are surfacing in an air-sharing situation, then you can "break" sooner, at 70 feet vs 20 feet. I think the deco part of the ascent is a little easier to manage of the divers are all breathing their own gas (shooting bags, etc).

That's my take on it anyway.

-JimG

[ScubaDadMiami]

I have noticed that I feel better after dives where I used oxygen rather than 50 percent if I only have one deco gas from which to choose. So, on those kinds dives, I opt for oxygen just because I feel like it cleans me out better by the time I hit the surface. (Just because this is the case for me does not mean it will be the case for you.)

Before engaging in elevated oxygen mix decompression of any kind, I would strongly suggest that anyone get proper training. Do not follow what works for me. Take proper training and then learn what is best for yourself.

[PerroneFord]

I took IANTD Adv Nitrox and Deco. Adv. Nitrox ties you to 50%. I don't have issues getting 02 though. I don't do much in the way of ocean diving either so I'm not inadvertently going to be popping to the surface.

This is a more philosophical question. The obvious benefits of having a copious supply of 02 handy, versus getting onto a rich mix deeper in the dive. Both have benefits.

For deco dives where you run into issues, getting 90% of the deco done on rich mixes and having to blow off the last stop might yield better results than having to blow off everything until you get to 20ft or get to the boat. If you have a shark chasing ya, what are you gonna do!

I think most agencies push the deeper mixtures because they are a bit more forgiving. Trying to hold a 20ft stop on oxygen requires skill in some environments. So the recommandation not to carry o2, comes more from liability and so forth, rather than it being the BEST mix.

In looking at my deco profiles, I notice I can get out of the water a bit faster with o2 deco over 50%, and as SDM mentioned, I think the cleanup is a little nicer.

Since I am not doing helium dives at this point, I am not to worried about going from backgas to my deco bottle. And my deco will be short enough to not have to worrying about taking backgas breaks.

Typically, I am looking at basic ocean dives to maybe 120ft for 20-30 minutes. Or in caves, just doing some longer dives.

And JimG, trust me, I carried a stage all the way through cave class. Only time I didn't carry it was in Little River. Believe me, that sucker got clipped off as soon as was convenient. Of course, we were forced to do our first drop in the middle of the Peanut Tunnel! What a PITA, Charlie Foxtrot THAT was...

[pir8]

I like 50% O2 and generally carry one of my lp45s for deco. If you need to carry an 80 to have enough deco gas, you are really doing some extreeme dives.

[PerroneFord]

V-Planner 3.76 by Ross Hemingway,
VPM code by Erik C. Baker.

Decompression model: VPM - B

DIVE PLAN
Surface interval = 5 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 2

Dec to 130ft (2) Air 50ft/min descent.
Level 130ft 27:24 (30) Air 1.01 ppO2, 130ft ead
Asc to 70ft (32) Air -30ft/min ascent.
Stop at 60ft 1:00 (33) Nitrox 50 1.38 ppO2, 26ft ead
Stop at 50ft 2:00 (35) Nitrox 50 1.24 ppO2, 19ft ead
Stop at 40ft 3:00 (38) Nitrox 50 1.09 ppO2, 13ft ead
Stop at 30ft 4:00 (42) Nitrox 50 0.94 ppO2, 7ft ead
Stop at 20ft 18:00 (60) Nitrox 50 0.79 ppO2, 0ft ead
Surface (60) Nitrox 50 -30ft/min ascent.

Off gassing starts at 95.1ft

OTU's this dive: 54
CNS Total: 18.4%

118.3 cu ft Air
36.1 cu ft Nitrox 50
154.4 cu ft TOTAL


DIVE PLAN COMPLETE

130ft for half an hour on air doesn't seem extreme to me... and gets me out of the water in the 1 hour that most dive boats would want. I would be at the absolute limit of a 40cuft bottle on this plan.

[pir8]

Try the same dive with a back gas of 28%

[PerroneFord]

Yes, I am aware of carrying best mix and it's helpfulness...

However, in some places, it is either air or 32% pre-blend.

[pir8]

Then you must do more planning. What size tank were you going to use for your back? you were pushing the limit of a 120. Were you in doubles? I didn't notice your SAC rate. Lots of variables.

[PerroneFord]

I only wear doubles. Generally two 80s or two 108s pushed to about 3400 psi.

SAC rate is high. I'm a big guy.

[pir8]

Is it easier to get 100% than 28%?

Edited by pir8, 09 March 2007 - 06:09 PM.

[PerroneFord]

Depends on where you are. Some shops absolutely will not PP blend in a customer's tank. Can't say I blame them all that much. But asking for a bottle of oxygen is a lot easier.

[pir8]

We have a Membrane system where we bank 40% and then add air to make whatever blend you want. Makes it easy.

[PerroneFord]

We can boost oxygen to 3000psi, so we can make what we want without dumping tanks. We also bank air, 32, 36, 21/35, and 10/70. Not everyone does that kinda thing though..

[pir8]

As long as we follow the computer program there is no need for us to dump a tank.