50/50 vs 80/20

[DA Aquamaster]

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.

I agree with Brian that using a bubble model with deeper stops makes a difference.

For example a 150 for 30 profile on air with 50% for deco with DPlan gives:

Depth Time
80 for 1 (21%)
70 for 1 (50%)
60 for 1 (50%)
50 for 2 (50%)
40 for 3 (50%)
30 for 5 (50%)
20 for 6 (50%)
10 for 14 (50%)

Total Run Time = 65 minutes
Gas consumption (sac of .6 for bottom time and .4 for deco)
101 cu ft of air
22 cu ft of 50%


The same profile with 100% for deco:

Depth Time
80 for 1 (21%)
70 for 1 (21%)
60 for 1 (21%)
50 for 4 (21%)
40 for 5 (21%)
30 for 7 (21%)
20 for 6 (100%)
10 for 10(100%)

Total Run Time = 67 minutes
Gas consumption (sac of .6 for bottom time and .4 for deco)
117 cu ft of air
9 cu ft of 100%

The curves for each dive are substantially different with the 50% requiring shorter stops at depth but a longer 10 ft stop.

The O2 is in the end 2 minutes slower, but more significantly using O2 requires another 16 cu ft of backgas since most of the deco is done on backgas. So the first dive with 50% could be done with, for example, a set of double 80's while still maintaining a 1/3 reserve while the second dive using 100% could not be done without cutting into a 1/3 reserve.

The best of both worlds of course would be to use both 50% and 100%.

Depth Time
80 for 1 (21%)
70 for 1 (50%)
60 for 1 (50%)
50 for 2 (50%)
40 for 3 (501%)
30 for 5 (50%)
20 for 4 (100%)
10 for 9(100%)

Total Run Time = 58 minutes
Gas consumption (sac of .6 for bottom time and .4 for deco)
101 cu ft of air
11 cu ft of 50%
7 cu ft of 100%

It gets you out 7 minutes faster than 50% alone and is again still do-able on a set of double 80's. Whether getting out 9 minutes sooner is worth the extra bother of obtaining 100% and bringing along the extra deco bottle is a personal choice.

If it is an extended trip and I can top off the deco bottle with air but have no O2 available on the boat, bringing along deco bottles with 100% O2 may be worth while. If I transfill a 3000 psi AL 30 with O2 (to 2000 psi) and then use 700 psi on the first dive, without dumping much I can top off the tank with air and end up with a full 30 cu ft of 50% for subsequent dives. So in that case taking a deco bottle with 50% and a couple more with 100% starts to make a lot of logistical sense and allows the use of one or both gases over several dives while only having to drag along a few small deco bottles.