50/50 vs 80/20

[GDI]

I was asked about the advantages of using a 50/50 mix vs a 80/20 mix for a deco gas. I ran several dive scenarios using the different mixtures. When looking at the total dive time and the deco time. There really is a small difference in the amount of time that a diver would spend it in the water on a 50/50 mix vs a 80/20 mix. What is your take on what gas you would use for deco? I would use the 50/50 and maximize deep stops, then go to pure O2 at 20 feet. Opinions?

 

[boomx5]

I was asked about the advantages of using a 50/50 mix vs a 80/20 mix for a deco gas. I ran several dive scenarios using the different mixtures. When looking at the total dive time and the deco time. There really is a small difference in the amount of time that a diver would spend it in the water on a 50/50 mix vs a 80/20 mix. What is your take on what gas you would use for deco? I would use the 50/50 and maximize deep stops, then go to pure O2 at 20 feet. Opinions?

Well...I'm not Dr. Deco but it seems to me that 50/50 would be a better gas for the reason you gave. Also, you are "cleaning up" sooner in the dive. So if anything got out of hand at the shallower stops and you needed to get out for any reason and ahead of schedule you'd take less of a chance of getting bent.

 

[DepartureDiver]

There really is a small difference in the amount of time that a diver would spend it in the water on a 50/50 mix vs a 80/20 mix. What is your take on what gas you would use for deco? I would use the 50/50 and maximize deep stops, then go to pure O2 at 20 feet. Opinions?

There are mixed opinions on this. Going to EAN50 would maximize the offgassing at the deep stops and would reduce ingassing while at depth also. However, you never get completely off the nitrogen. There is also an issue with counterdiffussion depending on the profile and bottom mix you are using. But in general, I do not have a problem with one choosing EAN50. Another consideration is the amount of deco gas you need. Can you carry enough EAN50 since you will probably need more of it than O2 due to using it at deeper depths? Your method of using 2 gases is optimal of course by using EAN50 and then O2.

 

[salty]

Its apples to oranges

If your only using one gas O2 is the best. Once you get to the point where having two gases is worth while adding the 50% complements the O2 at the last stop. Each gas ( O2 and 50%) provides specific advantages when used to their fullest potential.

I dont see any explanation as to which is better 50% or 80% because 50% is a 70 foot mix and 80% is a 30 foot mix for people who cant maintain buoyancy properly. The question should be O2 or 80%, and I personally believe the correct answer would be O2.

 

[akscubainst]

It's funny, My group includes 4 trimix divers. We each had a different instructor and we each have similar but different thoughts on deco gas based on what we used in our class.

I was taught 40% at 100' and 80% at 30'.
Another was taught 40% at 80' all the way up, another 60% then 100% and still another 100% at 20'.

My thinking is switch to something deep to start clearing the He out and go a little rich towards the end to finish up. I was taught to compare V-planner with Decoplanner and adjust variables until my M-Values are always smooth. No huge changes to the M-values on the way up and finish in the low 80's.

Still others like to keep the HE in the deco-gases. So there is no hard and fast plan. I'll use whatever combination is most suitable for the dive at hand.

Dave

 

[DA Aquamaster]

I think 50% is an ideal gas for a single deco gas dive, particularly if you are using a profile cut with software using a bubble gradient or variable permeability model with relatively deep initial stops. You can get on it at 70 ft and with moderate profiles with run times in the 45-60 minute range, it will get you out of the water faster than 100% O2 and will get you out of the water as fast or only a few minutes later than 80%. If you are still diving a table using a bend and treat model, 80% and 100% probably make a lot more sense as the vast majority of the deco will be in the 10-30 ft range.

Using 50% can also ease the back gas requirement significantly as you switch to 50% at 70 feet rather than doing all of the stops below 20-30 ft on your backgas. This leaves you a little more bottom time without compromising your 1/3 reserve.

Plus only 1101 psi of O2 is needed to fill a 3000 psi deco tank with 50%, so it is a very freindly mix if you don't have access to a boost pump and have to transfill from an O2 bottle or cascade system.

For air or nitrox dives utilizing two deco gases, 50% and 80% are my preferences as the 80% is at least as effective as 100% but with fewer OTU's. And in most profiles I have played with, the gas consumption ends up about the same for each deco gas so a pair of 30's or 40's work fine. As a practical matter 70% is more realistic for a second gas given my lack of access to a boost pump. It's hard to even come close to transfilling a full 3000 psi deco bottle with 80% on anything other than a freshly filled O2 cylinder.

With He mixes and the generally deeper stops involved I think 36%-40% may make more sense than 50% for the first deco gas with 80% for the second deco gas.

 

[grazie42]

I think the gas you use also depends alot on where/how you dive (I know how boring that sounds).

Where I dive most of the time wave action and current make 3m/10ft stops really unpractical and so I´ll only use pure O2 on really long/deep dives and stay with 50% for the majority of the dives (in 1hr range @ 120-150ft).

For the dives I do using 100% on the last stops will only get me out of the water a minute or so earlier than staying on 50% the whole way. Also, as DA Aquamaster said, 50% saves backgas allowing you to use the expensive (here) He on the wreck instead of in OW.

To me practical considerations carry far more weight than "theoretical" optimums....maybe thats just me though...

 

[pt40fathoms]

For many reasons, most divers around here have standardized with 50 and 100 for deco. It gets us out of the water sooner, and when the water temp is 3c, the less hang time the better.

By standardizing we eliminate confusion and reduce risks.

The only difficulty is getting full fills of O2 into an AL80. If the lift pump is down, then we end up with short fills and have to adjust accordingly.

 

[JS1scuba]

I was asked about the advantages of using a 50/50 mix vs a 80/20 mix for a deco gas. I ran several dive scenarios using the different mixtures. When looking at the total dive time and the deco time. There really is a small difference in the amount of time that a diver would spend it in the water on a 50/50 mix vs a 80/20 mix. What is your take on what gas you would use for deco? I would use the 50/50 and maximize deep stops, then go to pure O2 at 20 feet. Opinions?

-------------

The question leaves out the most important part of the divep plan.

What's the initial dive depth and the bottom time ? What is the bottom gas ?

One mix is an intermediate decompression gas (50/50 oea) the other is a final decompression gas and is normaly a replacment for 100% oxygen. (filling reasons).

Rarely will you have a need on a nitrogen based dive to use more than one decompression gas. That does not mean that people don't do it to be cool or because they have not fully understood the gas planning process.

Here's a few examples. This is a sample dive 150' Air as bottom gas, 30 minutes.(this example is for discussion purposes and does not take into consideration the narcotic effect of nitrogen at 150 fsw) - Buhlman schedule

150 fsw 30 min air (deco on air)

DECO Stops
Depth Time

40 2
30 5
20 10
10 27

Total Run time = 76 minutes

Max PO2 = 1.16 atm
CNS % at end of dive = 14.33%
----------------------------------------

150 fsw 30 min air (ascend on air to gast shift depth )
shift at 70 fsw for 50/50 OEA

DECO Stops
Depth Time
40 1
30 4
20 7
10 15

Total Run Time = 60 min

Max Po2 = 1.56 atm at 70 fsw
CNS % at end of dive = 21.08%

This took 16 minutes off of the decompression over the AIR only
=========================================================

150 fsw 30 min air (ascend on air to gast shift depth )
shift at 30 fsw for 80/20 OEA

DECO Stops
Depth Time

40 3 air
30 3
20 6
10 12

Total Run Time = 56 minutes

Max PO2 = 1.53 atm at 30 fsw
CNS % at end of dive 26.73%

4 minutes less decompression over the Air / 50/50
21 minutes less decompression over the air / air

==========================================
COMMENT = 4 min advantage is not significant enough to warrant limiting the depth range of a decompression gas as a bail out.

Keep in mind that this profile never requires much more than a 40 foot stop even if on air. So on a dive like this the 80/20 or pure oxgen would be a "best" choice but not necessarily the wisest choice. Operational simplicity would vote for the 50/50 since it can be used to bail out significantly deeper than its MOD and it gives a better "range of depth"

From a decompression standpoint the true savings in decompression when using OEA to accelerate decompression from air is usually in the area where the decompression time is the greatest which for the most part is in the shallow areas (10-30 fsw)
===========================================

Taking the same profile and doubling the gases to use BOTH 50/50 and 80/20 is an exercise in futility in that it only saves 2 more minutes of decompression but increases the equipment load and limits mobility.

These examples are just a VERY small look at what these deco mixes do. On significantly longer dives they will each play a significant role in decompression and oxygen toxicity levels. You need a good set of software or tables and start to run the comparisons. Learn what the effects are on the overall dive and THEN work on operational simplicity.

Having been gas diving for about 15 years now I have standardized on certain mixes for decompression. They are all set up in dedicated cylinders and never get mixed otherwise

EAN 36% 110 fsw MOD
EAN 50% 70 fsw MOD
100% oxygen 20 fsw MOD

On occasion for 300 fsw dives I will use an EAN of 32% so i can get on it at 130 fsw MOD and open up that oxygen window a little sooner. +/- 2% on mixes is more than acceptable for me.

I'm not a fan of 80/20 all the reasons the people have for using it really come down to the fact that they or the place they get thier gas from does not have a gas booster to make 3000 psi oxygen fills. I can see its use over 100% oxygen when you are in the field or ofshore and want to stretch gas. But that's about it.

When I shift to helium based dives my formula is even simpler. If a bottom mix has more than 40% helium in it 100% oxygen will always be used. If less than 40% helium (i like 18/35 a lot) I will carry 50/50 only and use optional surface supply O2. I dont need the O2 but if its there I will use it. If a gas dive will be a repet within 3 hours of the previous dive 100% oxygen is mandatory.

Regards,

 

[DepartureDiver]

Taking the same profile and doubling the gases to use BOTH 50/50 and 80/20 is an exercise in futility in that it only saves 2 more minutes of decompression but increases the equipment load and limits mobility.

I agree this occurs when using a Buhlmann/Haldanian model when the stops don't start until 40 feet. Of course when there is only a brief stop at 40’, the EAN50 won’t reduce the time much when another switch is then made at either 30’ or 20’. But when using a bubble based model with stops starting around 90 for the 150'/30 min air dive, the differences are much greater. With a bubble model, using both gases will save about 10 minutes of deco over using just a single deco gas. This is a significant percentage drop in the deco time as well as letting the diver get the benefit of the deeper gas and finishing off the deco with a low nitrogen gas. The time savings may warrant carrying a second deco mix, but of course some may opt for the longer deco while needing less equipment. The other nice advantage is that if 1 deco gas is lost, there is still another deco gas left and the dive can be completed with just one deco gas as compared to none if the diver only carried one deco gas and then lost it. This is for an example only since there should be other contingencies in place if this did in fact occur.