diluant

The choice of a diluant has to satisfy several surviving conditions and other practical "field" conditions :
- as low as possible EAD;
- minimum work of breath (specially at depth below 100 m);
- possibility to perform a diluant flush to reduce the PpO2;
- possibility to open-circuit breathe the diluant
- easy mix : heliair. Take full advantage of your CCR, let it do the correct mix for you at all depth. Other good point with heliair is that you only need an oxymeter to know the exact content of your tanks.

So, to fullfill these requirements, I decided to follow these principles :
- no EAD below 35 m. A CCR diver needs a very clean thinking all dive long. No bottom numbness is allowed;
- that also keeps a low fraction of nitrogen and thus reduces the work of breath;
- the diluant flush should bring back the ppO2 close to the set-point value of 1 bar at the maximum depth of use of this diluant. So in case of high PpO2, the flush creates a quick decrease of PPO2;
- all my mixes for CCR or bail-out, are heliair. I do not use heliox because I just can not easily reach 250 bars with helium and oxygen tanks; with heliair, no problem.

The following table is easy to understand. So you can guess that I would use air as diluant down to 40 meters, heliair 14/33 from 40 till 60 meters, 10/50 from 55 till 85 meters, 8/62 from 75 till 110 meters, and deeper, rather 6/72. Practically, using 8/62 instead of 10/50 and even instead of 14/33, will not lenghten a lot your deco; just a few minutes more. This is true considering the fact that I tend now to flush the loop during deco with nitrox 32 or air between 45 and 40 meters. So, I tend to always use 8/62 whenever I need helium in the diluant tank.

Lots of personnal beliefs and pseudo-theoretical concerns separate rebreather divers who are adept of a diluant switch during deco from the ones sticking to the "one-gas-all-the-way" faith. I have done both. I usually use now a diluant change when it will shorten the deco of at least 15-20 minutes in cold water. I switch then from a 8/62 bottom diluant to an air or a nitrox 32 deco diluant at 40 meters.

I may be wrong on all points, so ask your trimix CCR instructor.

EAD are read from V-planner software.

Stéphane Havard.
Mars 2003.

			HELIAIR
	AIR		14/33	10/50	8/62	7/66	6/72
Set P.	1	1,3	1	1	1	1	1
40 m	39	36	20
45 m	46	42	24
50 m	52	48	28
55 m			32	20
60 m			36	23
65 m			40	26
70 m			44	29
75 m				32	20
80 m				34	23
85 m				37	25	21
90 m				40	27	23
95 m				43	29	24
100 m					31	26
105 m					33	28	21
110 m					35	30	22
115 m					37	32	24
120 m					39	34	25
125 m					41	35	27
130 m					43	37	28
150 m							34

EQUIVALENT AID DEPTH (m)