favorite deco blends

[Divesherpa]

A lot of my associates in the Gainesville area are firm believers in using a 70 foot bottle (50%) and a 20 foot bottle(20%).
I'm more partial to a 110 foot bottle(36%) and a thirty foot bottle(80).
I started doing deco from a wreck-diver and most of these guys are solely cave?
Any opinions on why the difference, and which is better?

 

[Lost Yooper]

Greetings,

I'll take a stab at it as best I can. The 50% mix allows you to off gas the most effectively at a more ideal depth. If you're in decent condition, and you absolutely must ascend due to an emergency from 30'-40', you're more likely to get away with it uninjured if you used 50% at 70'. It's certainly not recommended, but since you have offgassed so much at 70', and fewer atmospheres need be to skip to get to the surface, you should be better off if you had to ascend.
The other part is that you have more volume of gas to use from 70'-20' with 50% than you would if you had 32% from 130'. You're also looking at having less nitrogen in the mix compared to 32% which is a bad thing during deco. With the 32%, you're on gassing nitrogen more than you would be using 50% at 70'.

I've heard a lot of arguments in support of O2 against 80%. One of the big ones is that at 30' the PPO2 of 80% is only 1.52 which isn't much a window (only 1.2 at 20' and 1.04 at 10'). Secondly, 80% isn't going to be as good as O2 as it has nitrogen in the mix (not a good thing for deco). Third, and perhaps most important (IMO) is that if you had gotten to an O2 bottle for while and had to surface for an emergency, you would be much better off than if you had been on 80% at 30'. When you surface from 20' to the surface with O2, you have a better window than if you used 80% to surface.
The reason 80% is taught is a mystery to me. Some supporters say it's safer for those who can't maintain proper buoyancy. Why are they being tought this stuff if they can't maintain buoyancy ($$?)? Some say it's for rough seas, but that doesn't make any sense either. The change in depth/pressure isn't significant in rough seas if you know how to maintain control. Why wouldn't this effect the 30' 80% diver by that reasoning? If the seas are so rough to make it an issue, why did you go diving in the first place?

I've heard more than I have listed here, but that's the guts of it as I know it. I use 50% and O2 for the type of diving I do. The extreme guys use a 190' and 130' 32/30 mix for deco too. For most guys, a 70' and 20' mix is all that's required.

Take care.

Mike

 

[padiscubapro]

There is alot of controversy here.. Idealy the best would be 36% from 110 or 32% from 130 then 50% from 70 then 100 % from 20, but thats alot of bottles to carry (and o2 at depth is an accident waiting to happen).. If I am diving trimix (on open circuit) I use a 32% or 36% as a travel mix/deep deco mix, the switch to 50% at 70 ft, I leave o2 hanging at the 20 ft stop, if all goes well I switch to the o2 but I don't carry that mix with me.. I build tables for full deco with 50% and also with a switch at 20. 100% is useful in seas because you can do your 10 ft stop at 20fsw, using 50% at 20 you lose a bit of efficiency. I also have a low fo2 mix (my bottom gas) to take breaks from the o2 to stop vascular constriction. If I'm not diving a helium blend I usually only use 50% as a deco mix since I'm probably doing some long dives and the o2 will bring my %cns way to high..

on closed ckt I keep my po2 at 1.3 for the entire dive, and flush the bag at 20 ft and run it as an o2 rebreather, wit periodic flushes to get rid of the inert gas thats now in the loop from offgassing .

You also have to decide on what level of cns% you are willing to accept. When I dive a CCR I may have 5 or 6 hours a day in water and will have %cns several times above NOAA recommendations.

http://www.geocities.com/padiscubapro/index.htm
Recreational and Technical Scuba instruction
including Inspiration CCR
ANDI, PADI, TDI, SDI

 

[aue-mike]

The use of 36% or 32% is not efficient as a decompression gas. Even under "ideal" situations, breathing 32% or 36% would be less efficient than staying on backgas that would probably have a lower PN2.

Here is the bottom line -- introducing nitrogen is contrary to what you are trying to accomplish in decompression. If you start your ascent deep enough and ascend slowly enough, the helium is not an issue. That being conducted, nitrogen is the bugger that makes the DCS bubbles that you now want to avoid.

Here is the problem with using 32% or 36% (aside from the other big problem of introducing N2) -- watch what happens to the PPO2 from your first deco stop to your next gas switch. For example, from 110' to 30' or 20'. You have a huge gap there (80-90') where the O2 ceases to be useful and the N2 takes control. You are basically pissing in the wind half of the time. However, if you use 50/50, the differential is only 50 feet from 70 to 20 feet and you don't lose as much benefit in the ascent. Furthermore, the PPN2 is less than with a 32% or 36% so you are doing what you can to supress N2 introduction. Even better would be to switch to 50/50 heliox and remove the N2 altogether, but I won't get into that today.

The use of 50/50 starts at a shallower depth and allows the use of smaller cylinders. This allows better streamlining for swimming/scootering at depth. Also, less weight gearing up topside and less cumbersome.

Now, using 80% is plain counter-productive to decompression. These are your final stops where you really want to clean your system out before removing all pressure and surfacing. 100% O2 is much, much, much more efficient at cleaning your system out. Ask yourself this -- if you are trying to remove N2, why introduce any more???

Please try and produce one rational reason to use 80%. Anyone.

Don't even try the story about rough seas. If it is too rough to compromise decompression at 20' -- there is no reason you should even be in the water and you should be more worried about how the hell you are getting back on the boat.

You are resting during deco. Should a swell pass by and you "drop" to 25' for a second, that is not a real big deal. I have been is some seriously crappy seas, and deco has never been compromised. This is real diving, not theoretical or cyber diving. Forget what you have been taught and just think it out. Logic and common sense will show you the right answer.

Cheers,
Mike

 

[Lost Yooper]

Yeah, I don't carry the O2 with me either, as a matter of convenience more than anything. I'm not afraid of switching to it prematurely (that's a matter of discipline and technique), but I just don't want to carry it. I carry a complete set of tables for various circumstances.

Mike

PS. Nice post Mike.

 

[Green_Manelishi]

Hi AUE-Mike, nice to see you on this board.

Over-all I agree with your statement about 100% being
"better" than 80% but I have a good friend who has found
that, for whatever reason, he cannot deco on 100%. He
experiences too much lung discomfort so he therefore uses
50% and 80% (starts at 30 feet).

Also, what is your opinion of "5 minutes at 70 on 50%" if your
max depth was only 130 or so for 20 minutes or so? If that's
been my profile I carry one bottle with 80% or 100% (if I can
get it) and start my high-O2 at the appropriate depth.

No flames please, merely asking your opinion/experience.

Thanks

 

[Lost Yooper]

Hey GM,

How long does it take before your friend starts feeling discomfort on O2? Appropriate backgas breaks should take care of that, and will actually allow you to offgass faster than staying on O2 without breaks. Long exposures (20mins+) to O2 are actually counter productive to offgassing.

For rather simple dives, I would just carry a 50% bottle and deco out at 70' for 5mins and ascend with it to the mandetory stops. You have to stay at 70' for a sufficient amount of time for the window to do some good. This way, you are cleaned out earlier in the dive in case you have to surface prematurely. Taking both, of course, doesn't significantly effect deco time. The deco times on just 100% is practically the same as it is on just 50%, the difference is that deco is occuring deeper with 50% .

Mike

 

[aue-mike]

I always carry all my gasses with me. However, the dives I do are open water in the ocean, many times drifting, so rigging the bottle at depth is not an option. I still would be hesitant (aside from caves, etc.). If your bottles are properly marked and you are not complacent about gas switches, it is not dangerous to carry it to depth.

Hi GM-
Using EANx 50 following a moderate depth dive is fine. It just comes down to whether or not you want to bring the extra bottle with you. Maybe just a little overkill though...

If nothing else, it is good training for deeper dives. I usually only take down 100% for shallow stuff, but if you want to be super safe use the EANx 50 also. I am not sure I would just do an arbitrary 5 minute stop though. Just do the switch, pause for a minute or so to let the body catch up, and continue with your slow ascent until your mandatory stops.

Cheers,
Mike

 

[Green_Manelishi]

My recollection, but I'd have to ask him to confirm, is that
my buddy's discomfort is relatively quick and continues for
some time. It is not a matter of not-taking breaks, rather it
is for him a matter of "can't tolerate" so he uses what works
for him. Even the *screwball* (and I say that with the utmost respect for the man and his accomplishments) Haldane found
that he eventually had low tolerance to O2 as a result of his experiments on self.

My question regarding the 50% at 70ft is related to the practice
of some folks of stopping for a 5-minute window when the fact
of the matter is their BT and depth was not such that it makes
a tinkers-da** bit of difference. Many are not carrying TWO
bottles, they carry one, of 50%. Would it not make more sense
to switch, if you *must* at 70 and then continue on up to
your first mandatory stop, rather than switch and hang around
at 70 ft for 5 minutes? Or, carry one bottle of 80% and switch at
30 feet; or 100% and switch at 20ft, etc.

To sum it up, the deco is occuring DEEPER with a switch at
70ft to 50%, but is it ultimately, all things factored in, occuring
BETTER?

 

[100days-a-year]

I often amuse myself at night running tables of planned up-coming dives using all the tools at my disosal.They are GAP,Vplan,Zplan,ProPlanner.Using all the above mixes I've found little difference in most out times.I'd really like to try the heliox method,gotta run it past my buddies tho.Everyone I dive with has thousands of dives to get set in thier ways.But of all of them ,no -ones been bent (slight pain onlys)no matter the mix.I'd like to believe at 38 I can get away with being very liberal with deco obligations within reason.I find dives to between 150-180 only add a few minutes on straight 50.I'll run some at 50 heliox.I try to worry about what I'm diving rather than try to convince everyone they are idiots.32 and 80 will be around for a while good or bad.If you get out unbent then I assume you did it "right"It's good to see you guys all be civil tho.