What happens when you descend on a CCR?

[onthetrain]

Originally Posted by Mal Bridgeman

You sure you want the answer to this?

OK....Firstly, trying to understand how a CCR works with regard to "gas mixing on the fly". The only one I have seen close up is the Dude's (Inspiration) and I am sure that's not a shining example More seriously I allow my mind to wander sometimes and was thinking that in a loop the sensors are in one place, the O2 is added in a different place, and the Diluent at a different place so as Diluent and O2 are being added there are different concentrations of O2 at different places in the loop and I wondered if that had a material effect.

Clearly the descent is the point at which this would be at it's greatest.

I was also interested to see the selection of 1.3 as a desirable ppO2 throughout the dive. As you probably realise the DIR way is to use 1.2 on the bottom and use 1.6 on deco so was interested in that too.

Now I am sure you just wanted me to say I was thinking of buying one, but it'll be a while before my OC kit goes up for sale

In short, idle curosity.
Mal

BTW Thanks to all for your answers .... Greens have been liberally distributed .... even D-J got one

Actually its a great question.

Yes the sensors etc are all in one place, but dont forget its a one way flow of gas and the sensors are on the exhale side. TBH in normal operation the fluctuations are really minimal. So small you barely notice it.

I'm not sure I agree that its on descent when the unit works hardest.

In my experience its on ascent.

The po2 is constantly dropping and the unit continually injecting o2 to maintain setpoint.

This means the diver is alsoat his or her most task loaded during this phase. You are trying to maintain stable depth, a near constant po2 and venting gas from up to 3 spaces whilst the unit is injecting gas. I find I use as much o2 on an ascent as I do during the bottom portion of the dive, mostly due to almost constant venting of the loop.

Once you can work out how to maintain minimum loop and have a nice slow ascent the easier CCR becomes and the less danger of a runaway ascent.

Alan

[milldog]

Originally Posted by Mal Bridgeman

What happens when you descend on a CCR?

TIA
Mal

errrrr you go down?????

hope that helps mate and when you ascend you come up

Graham

[Mark Chase]

Originally Posted by onthetrain

I'm not sure I agree that its on descent when the unit works hardest.

In my experience its on ascent.

The po2 is constantly dropping and the unit continually injecting o2 to maintain setpoint.

This means the diver is alsoat his or her most task loaded during this phase. You are trying to maintain stable depth, a near constant po2 and venting gas from up to 3 spaces whilst the unit is injecting gas. I find I use as much o2 on an ascent as I do during the bottom portion of the dive, mostly due to almost constant venting of the loop.

Once you can work out how to maintain minimum loop and have a nice slow ascent the easier CCR becomes and the less danger of a runaway ascent.

Alan

I used to think this, then I got a MCCR. The decent is usualy fast 20m/min+ and that is the problem. The ascent is slow and its easy to maintian PP02 but bloody hard to get right on the way down. I am still strugling to get the decent right ofer about 22 goes at it. Ascent is fine.

Runaway ascents are a diferent story but at 5m/min or less you have a shead load of time to sort out PP02.

Ascent if your using oversize counterlungs can be a problem as the loop gas expands but with the tiny counterlungs on a KISS you have no choice but to vent the loop or you cant breath out and your cheeks explode


ATB

Mark Chase

[Mark Chase]

Originally Posted by Mal Bridgeman

You sure you want the answer to this?

OK....Firstly, trying to understand how a CCR works with regard to "gas mixing on the fly". The only one I have seen close up is the Dude's (Inspiration) and I am sure that's not a shining example More seriously I allow my mind to wander sometimes and was thinking that in a loop the sensors are in one place, the O2 is added in a different place, and the Diluent at a different place so as Diluent and O2 are being added there are different concentrations of O2 at different places in the loop and I wondered if that had a material effect.

Clearly the descent is the point at which this would be at it's greatest.

I was also interested to see the selection of 1.3 as a desirable ppO2 throughout the dive. As you probably realise the DIR way is to use 1.2 on the bottom and use 1.6 on deco so was interested in that too.

Now I am sure you just wanted me to say I was thinking of buying one, but it'll be a while before my OC kit goes up for sale

In short, idle curosity.
Mal

BTW Thanks to all for your answers .... Greens have been liberally distributed .... even D-J got one

Minor confusion here i think.

On decent its harder to maintain a stable set point because your going down asap. On ascent its easy because your coming up slow. However the unit is injecting 02 on the ascent to keep the pp02 up and diluient is sucked in on decent to take the PP02 down. If your using a fairley rich diluient this often cant compensate for the rate of decent. So say you had a diluient that was 1.3 at max depth and you started on the surface with 100% 02 then it will not flush as efectivly once you get deep. This can result in spikes.

If you have a low mix on a shalow dive (say 10/50 on a 50m dive) you will find that accidental firein of the ADV will cause big downward spikes folowed by frantic injection of 02 to compensate. If you have say a 1.2mix on a dive running 1.3 then the spikes will be nominal. Prety simple realy.

Point is the spikes arent particularly relevent. Think of it in terms of average depth calculations. This is more like average PP02 calks.

As for the set point of choice?? with the short duration (IE less than 120min) dives most divers do then 1.3 is fine. Its deco eficient and the CNS will be negligable over a 120min run. If you were planning a six hour dive then you would probably be running 1.2 or less on the bottom.

GUE use 100% for deco and this puts the CNS numbers past 100% even on relitivly short dives. 40mins at 65m would put you on 100% CNS if you ran 1.3 or 1.2 for the bottom mix it wouldent make any diference you would still hit 100%. Running 1.4 for deco @ 6m would only put you on 56% CNS


As a result there is little or no need to run 1.2 on short dives. On shalow stuff (max 50m) I usualy run 1.4 for the whole dive

ATB

Mark Chase

[Paul Evans]

Why worry about the spike on decent, do a Scamahorn slide

[Janos]

The ppO2 will always go up as you descend.

Consider a rebreather and diver. Let's say that on the surface lungs and RB combined have a volume of 10 litres.

Let's also say (to make the numbers easy) that on the surface the RBer is breathing 60% and diluent is Nitrox 20%

At 10m, there is now 20 litres of gas in the loop. (2 bar x 10 litres). So to get there the RBer must have added 10 litres of diluent (Nitrox 20%)

Thus the mixture in the loop will be 1/2 (60% + 20%) = 40%
But we're at 20m. So the ppO2 is 0.8, even though the Nitrox mix is weaker. Ie it's gone up from 0.6 on the surface by 0.2.
So to put it another way: Every 10m the ppO2 increases by the ppO2 of the Diluent on the surface

At 20m, the ppO2 will be 1.0
At 30m, the ppO2 will be 1.2
etc. etc.

On the KISS the diluent is added through an ADV. I suck the gas in the loop into my lungs, then suck some more and the ADV fires like a stiff reg and adds gas.

I choose to run a ppO2 of between 1.1 and 1.2 (but I don't get upset if it's between 1.0 and 1.3) as for long dives the extra .1 or so makes a difference, and the difference between doing a 61m dive on 1.2 gives you the same gas loading as a 60m dive on 1.3. (The maths is easy on this one too).

I boost the ppO2 on ascent and deco.

Janos

[Diving Dude]

Feck me, l amaze myself that l manage to descend on my unit.

[Janos]

Originally Posted by Diving Dude

Feck me, l amaze myself that l manage to descend on my unit.

You and me both mate

Janos

PS- Since I was made an Knight of the Lobster Realm, I'm now seeing loads of lobster. Three in fifteen minutes on the Iolanthe, and even one on the Countess.

[Mick F]

Originally Posted by Diving Dude

Feck me, l amaze myself that l manage to descend on my unit.

Originally Posted by Janos

You and me both mate

I'm even more surprised when he manages to ascend!!

[Janos]

Oh, Mal,

Here's a trace of one of my dives. The green line is the ppO2.

Janos