Diluent question

[charlesml3]

Gang,

First of all, I am not a CCR diver. I'm just the curious sort who likes to know how things work.

I was in Palau a few weeks ago and there was a really cool diver there from Ohio who showed me his rig. I think I drove him nuts with questions.

Anyway, I understand the O2 loop, solenoid, triple guages and sensors. I also understand why the diluent tank is there. As you go down, you have to maintain a consistent pressure so you add in air, Nitrox, trimix or whatever is in this tank. Once you're at depth, only O2 is being added in as you metabolize CO2 out and that's absorbed by the scrubber. No problems there.

So here's my question. Let's say the diluent tank has plain air in it and you're going about your dive at 70 feet. Throughout the dive, Nitrogen is being dissolved into your bloodstream and therefore at least some of it is not be exhaled back into the loop. Could this also be a reason why you'd have to add diluent back into the loop?

Thanks in advance!

-Charles

 

[Jimmer]

Well I think that would depend on how much nitrogen was absorbed. The rebreather balances and maintains a constant partial pressure of O2, so if that partial pressure started to rise because of a loss of N2, the only way to balance it back down would be to inject some diluent. I don't know if the body absorbs enough N2 on a dive to actually reduce the fraction of N2 in your breathing mix or not. Very interesting question.

Edit: Now that I think about it, I don't think there would be a reason to be too concerned with that. You're metabolizing the O2 all the time, and having to inject O2 to raise the PPO2 back up to your setpoint. Since you're injecting O2 to raise the setpoint, there shouldn't be much need to inject diluent to lower it.

 

[JeffG]

The reality is that you would lose more Diluent through bubbles escaping your mask than what your body absorbs.

 

[almity1]

Here is chart on what happens with Air Diluent with a constant PO2

Depth || Pressure (BAR) || P02 || PN2
50 || 6 || 1.3 || 4.7
40 || 5 || 1.3 || 3.7
30 || 4 || 1.3 || 2.7
20 || 3 || 1.3 || 1.7
10 || 2 || 1.3 || 0.7
5 || 1.5 || 1.3 || 0.2
3 || 1.3 || 1.3 || 0


And further to this with reference to the chart above

Using a Setpoint of 1.3 with AIR Diluent…..

@ 3m you are breathing 100% 02
@ 5m you are breathing 87% 02
@ 10m you are breathing 65% 02
@ 20m you are breathing 43% 02
@ 30m you are breathing 32% 02
@ 40m you are breathing 26% 02
@ 50m you are breathing 21% 02

So when you are at 3m you do your checks and bring your loop to 1.3, which at the same time verifies your cells, you can do the same thing at 5m to get 1.5.

With your loop at 1.3 at 3m, you begin your descent, and as you descent your P02 will increase and loop volume will decreas, and if you have set your controller to 1.3, effectively there will be no 02 injection, so you inject Diluent to maintain the loop and the constant P02.

As you can see from the chart you will breath more Nitrogen on the way down, but less on the way up.

Once you are at at constant depth, your P02 will lower as you breath it down but your unit will inject the 02 required to bring the P02 up, you can do this manually as well.

Hope the maths helps

 

[jepuskar]

Hey Charles,

A very wise and intelligent person I know posted the same question a little over a year ago...check it out.

http://www.scubaboard.com/forums/rebreathers/168084-basic-ccr-question.html

 

[Gill Envy]

that is a great thread! thanks for pointing it out J.

g

Hey Charles,

A very wise and intelligent person I know posted the same question a little over a year ago...check it out.

http://www.scubaboard.com/forums/rebreathers/168084-basic-ccr-question.html

 

[jepuskar]

Give me some green...oh no wait..wrong forum.

 

[wedivebc]

An estimate of dissolved gas in the body for a recreational diver is about 1 liter or so. It is enough to effect my PPO2 on deco. I notice a drop in PPO2 after doing an O2 flush so even though I start with 1.6 at the 20ft stop it soon drops to 1.5-1.4 due to inert gas coming out of my body tissue.
Since my dil tank hold about 600 liters that 1 liter of dissolved gas is a pretty small percentage of the total.

 

[charlesml3]

Got it. Thanks everyone!

-Charles