How many gas switches?

[allen]

As my gas consumption improves, I find it almost essential to switch to a deco gas at the end of the dive. So far, one deco mix is enough. Does there come a time when a diver who sticks to nitrox and doesn't go below 110 feet needs two deco mixes?

[FW]

As my gas consumption improves, I find it almost essential to switch to a deco gas at the end of the dive. So far, one deco mix is enough. Does there come a time when a diver who sticks to nitrox and doesn't go below 110 feet needs two deco mixes?

I never do, on shallow dives (above 110').

[CaveBuddy95]

it would be a bit overkill to have 2 deco gasses diving to 110/130 feet

[donlabbruzzo]

Deco time required is a function of depth, time at depth and breathing gas. If you sned enough time at 110' even while on Nitrox (32%), you could need a more efficient deco gas and/or 2 gases.

[Bobby]

I have done over 4 hours on OC at those depths and have only used O2. I suppose 50% could be thrown in but it does not make that much of a difference. With long BT using 70% or 80% will cut down on the CNS load, just too lazy to mix it unless I really needed it.

[gschaut]

In my incredibly short tec experience, I have found I prefer 80% as a deco gas. It allows me to stay below 1.4 at 20', and if needed would allow me to deco at 30' without breaking 1.6. When crunching numbers on my deco planning software, EAN-80 never causes more than a 3 minute "penalty" when compared to 100% O2 for dives of less than 130'. I prefer the safety margin over speed.

Mixing 80% (more or less) has not been a problem. A 3-to-1 mix of O2 and air gives a 80.25% mix, and a 2-to-1 mix of O2 and EAN-32 gives a 77.33% mix. So simple, I can do the math in my head, and come out within 3% of EAN-80 every time I do my own mixes. I just make sure my computer is set for the exact mix I ended up with.

[DA Aquamaster]

Two factors driving the use of a second deco gas are 1) deco efficiency and 2) lost gas contingency planning. Once you reach the point where losing/losing access to your single deco gas puts you in a position where you can no longer complete your deco on back gas, it's time to bring a second bottle, and it just as well be a second mix to improve efficiency.

100% is common in cave country and is well suited to use in many caves, but I prefer 80% offshore when diving trimix as it works well with a 30-32% travel/first deco mix in terms if both deco and gas planning, lowers the total CNS load and is better suited to rough conditions with large swells.

However in the 110-150' range 50% and 100% are fairly standard deco gas choices.

[Bobby]

Larry,
I believe you misread the OP. He specified dives 110 ft. and shallower only.

Two factors driving the use of a second deco gas are 1) deco efficiency and 2) lost gas contingency planning. Once you reach the point where losing/losing access to your single deco gas puts you in a position where you can no longer complete your deco on back gas, it's time to bring a second bottle, and it just as well be a second mix to improve efficiency.

100% is common in cave country and is well suited to use in many caves, but I prefer 80% offshore when diving trimix as it works well with a 30-32% travel/first deco mix in terms if both deco and gas planning, lowers the total CNS load and is better suited to rough conditions with large swells.

However in the 110-150' range 50% and 100% are fairly standard deco gas choices.

[kwinter]

In my incredibly short tec experience, I have found I prefer 80% as a deco gas. It allows me to stay below 1.4 at 20', and if needed would allow me to deco at 30' without breaking 1.6. When crunching numbers on my deco planning software, EAN-80 never causes more than a 3 minute "penalty" when compared to 100% O2 for dives of less than 130'. I prefer the safety margin over speed.

Mixing 80% (more or less) has not been a problem. A 3-to-1 mix of O2 and air gives a 80.25% mix, and a 2-to-1 mix of O2 and EAN-32 gives a 77.33% mix. So simple, I can do the math in my head, and come out within 3% of EAN-80 every time I do my own mixes. I just make sure my computer is set for the exact mix I ended up with.

Ditto. 80% reduces OxTox risk while barely extending deco time.

[DA Aquamaster]

Larry,
I believe you misread the OP. He specified dives 110 ft. and shallower only.

No. I addressed 110' as well, but I think it's important to consider the big picture, why you choose what you choose, what you are accomplishing and how it will adapt to other diving.

In this case, if I were diving to 110' offshore and needed a single deco gas, 80% would be a serious contender.

However on a repetitive dive or with a long bottom time (80-90 minutes or more) to the same depth, where your back gas alone would no longer support your deco obligation, a second deco gas becomes important for both gas planning and for reducing the run time.

Regardless of the profile the same basic considerations drive the selection of 1 versus 2 deco gasses - lost gas planning and deco efficiency.