Current best practice for deco gas: 75% vs 100%

[Ryan Mcshane]

100%

 

[rjack321]

If you run the numbers, there is little gained between 50 and 75 being used. My classes just a few years back (NAUI. but I believe it to be my instructor) basically listed 50 and 100 as the choices. If you aren't "spot on" with holding the stop, or conditions are adding risk, my learning said 50%.....

Now, with adding He, it was 100 or nothing....

Curious as to what others will chime in with...

Why would helium in your backgas matter? The gradient to eliminate helium is as big as it can possibly get regardless if you switch to 32%, 50%, 80% or 100% or any other non- trimix gas. You dont need 100% to get rid of helium, you need it to get rid of nitrogen in slow tissues where it wants to persist despite the fairly shallow depth. Maybe I misunderstood what you meant.

 

[Ryan Mcshane]

I dont like 50%, it does most things poorly.

Wut?

 

[Germie]

50% is not efficient in most cases. For an easy dive to 45m on air as backgas you can use it. But if you do a normoxic trimixdive or full trimixdive you will need at least 2 decogases. A gas with 40-50% oxygen and a 80-100% oxygen. The difference between ean80 and 100% is almost zero in efficiency if you run it in a planner.
100% need gasbreaks officially, ean80 not. CNS is almost half with EAN80 instead of 100%. With 100% your body seems to close the small capillars so that offgassing after a while is less efficient as with EAN80. But 100% has no inert gases anymore, so has the biggest delta P for offgassing. With this, make your choice. For me it depends on the dive and what is available. Most times I will use 100%, but if not, then ean80. Never used ean75.

 

[ajduplessis]

As a deco gas you will not spend a lot of time at 21m, 18m and 15m on OC normoxic dives in OW, 3-4min at best. Anything shallower where you will be spending time during deco, you want something better than 50% IMO.

50 works better on multi level dives suited to its range or on hypoxic dives where He is added for travel/deco purposes.

 

[Ryan Mcshane]

There’s more to think about when choosing deco gas than how much deco it’s gonna shave off (few minutes at best) - how quick you can get into that next gas source if you end up sharing gas being one... it’s a lot quicker to get onto 50% at 21m than it is to get to 80% at 9m

 

[RainPilot]

There’s more to think about when choosing deco gas than how much deco it’s gonna shave off (few minutes at best) - how quick you can get into that next gas source if you end up sharing gas being one... it’s a lot quicker to get to onto 50% at 21m than it is to get to 80% at 9m

Agreed. The dives I am certified for at the moment allow one deco gas so I spend a fair bit of time trying to figure out which will be the most efficient use of that tank. In MY case, 100% almost never results in a big saving on deco time, but the lost gas scenarios tend to reduce my bottom time quite dramatically since I need those reserves to get to 6m. In almost every scenario I run (for MY 50m, 30 min deco dives) a 40 or an 80 of 50% ends up being the best choice.

I haven't bothered running other percentages after a few experiments since the "standard" deco gases are the most freely available and what most of us tend to have on hand. Mixing up a 60 or a 70 just isn't really worth the hassle, since everyone in the team would have to do the same.

 

[Alastor]

EAN80 when could be difficult mantain 6m because rough sea else 100%

 

[Searcaigh]

Always used 100% for the final gas with 50% from 21m to 6m.

As Alastor mentions above 80% is good if the surface is rough and it's difficult to maintain a 6m stop compared to a 9m stop.

 

[tbone1004]

100%.

It has no inert gas.

That’s what’s good about it.

added benefit of being a one-step fill

@Germie EAN50 is perfectly fine as a deco gas for most normoxic profiles. Run the profiles, there is usually no benefit in using 100% alone, and using both is usually of negligible benefit