tank size question

[naimis]

I was thinking about rebreathers again today (at my experience level, that's about all I can do right now , and something occurred to me: every rebreather I've seen has the same size tank for diluent and oxygen. Why is this? I would have expected that the absorption rate of the diluent would be *much* lower than the consumption rate of the O2. Am I way off base, here?

Is it just a matter of keeping the unit in balance?

 

[netmage]

I was thinking about rebreathers again today (at my experience level, that's about all I can do right now , and something occurred to me: every rebreather I've seen has the same size tank for diluent and oxygen. Why is this? I would have expected that the absorption rate of the diluent would be *much* lower than the consumption rate of the O2. Am I way off base, here?

Is it just a matter of keeping the unit in balance?

A few thoughts.... and generalizations...

Most ccr units follow sort of a similar look, that is a central scrubber with 2 cylinders on each side. This layout lends itself to the balance aspect.

The new Apoc which is aimed at the recreational market opted to put a small (~2L I believe) O2 cylinder underneath, with Dil on one side, and if desired, a dedicated bailout cylinder on the other.

The REALLY new REALLY small unit from ISC has a single 6cuft bottle on the side for O2, w/ the intent of a single shared Dil\Bailout being side-mounted.

If your only considering a single dive, you certainly do have overkill when it comes to dil (for most dives...) Thru the course of a full day of recreational diving, I find that 3L bottles are just about perfect w/o ever having to swap any tanks. If I start ~3000psi - I'm in the 500-1000psi range on both O2 and Dil after 4-5 hours underwater w/ 4 dives.

-Tim

 

[Mr.X]

Size of the bottles depends. The 20 cu. ft bottle are a nice blend of small enough with enough gas for several dives. Over the years the sizes have varied - Cis Lunar MK 5P (20cu & 13CuO2), Megalodon where you can strap on different sized bottles. As you said - balance of the unit is important because you don't want oversize bottles tipping you to one side. That being said, you're always slinging bailout which usually has a minimal effect on trim and balance.

X

 

[rigdiver]

I was thinking about rebreathers again today (at my experience level, that's about all I can do right now , and something occurred to me: every rebreather I've seen has the same size tank for diluent and oxygen. Why is this? I would have expected that the absorption rate of the diluent would be *much* lower than the consumption rate of the O2. Am I way off base, here?

Is it just a matter of keeping the unit in balance?

The "Dil" is really just to take up space in the loop and BC inflation and/or maybe drysuit inflation. As stated it is a balance issue with the same size cylinders on each side of the unit.
Tim and I were on a trip recently where one Meg diver had 45 cu ft cylinders. I don't think he needed a fill all week long.
I use 3L cylinders and generally have at least 1/2 left of O2 and dil at the end of the day. One 4 hour dive in Devil's I used about 7 cu ft of O2 and 7 cu ft of dil. Max D was 96ffw. They do have benefits.
Know your ppO2!
Bill
BTW: Diluent is not really absorbed. Some of the oxygen in it is metabolized, but the Nitrogen or Helium is not used by the body and is "100% recoverable."

 

[ScubaDadMiami]

Also consider that the diver will go through a fair amount of diluent when doing dives other than square profile. The ups-and-downs of a typical cave dive will often consume a significant amount of diluent.

 

[Camerone]

Let's not forget that most modern rebreathers also integrate a BOV into the mouthpiece, which divers often plumb inboard to the diluent for sanity breaths when on initial stages of a bailout operation. Having a reasonable sized dil tank to give you those two or three breaths after using some of your dil to inflate the BC during the dive is important. Compound this with a cave profile and you can see where larger dil is an advantage.

FWIW, you don't need to balance - I often run a 19 on one side and a 13 on the other, and tanks like that are so small that the weight differential is not noticable. What I use depends on the scenario, the dives, and what tanks in my garage are filled with (different mixes - 18/45, 21/35, air, etc)

We have a saying in aviation - the three most useless things are altitude above you, runway behind you, and fuel back on the ground. Similarly in diving, you'll never regret _extra_ gas.

 

[naimis]

Also consider that the diver will go through a fair amount of diluent when doing dives other than square profile. The ups-and-downs of a typical cave dive will often consume a significant amount of diluent.

Why is that? I understand that diluent is really not "consumed" per se, and "absorbed" might not be the right term either, that it's dissolved into the tissue and comes back out of solution.

So why would going up and down end up going through more diluent than an "equivalent" square profile? Isn't the rate that noble gasses are dissolved into tissues a function only of time and pressure?

 

[kidspot]

Because even if you do not use your diluent bottle for BCD or Drysuit you still have the gas expansion and release from depth at work on the gas in the counterlungs. You go down and it compresses, so you add some to make up the volume, then you ascend and have to vent that same gas as it expands, then you descend and have to add volume to the loop again ... repeat several times and you've used up a lot of gas. Add in a BCD or Drysuit and the issue just grows. It's not really due (at least in any large part) to the absorption of gas.

If you were on a truly square profile (which is rare on most dives) then you would only fill the loop, BCD and Drysuit once, then vent the gas as it expanded once on the ascent.

Hope that helps explain it.

Aloha, Tim