trimix on scr

**Freef** · 7th January 2008, 16:43

Quote: (Originally Posted by Humuhumu)

Personaly I can live with a PpO2 of 2.1, it will not be a problem if the situation is not lasting: this means if this happens you will have to go on the bail out, that's it ! That's why I have a 80 cft AIR, with me.

The problem is that the pO2 spike IS long lasting. Realistically, you won't hit 2.1 as there is still a proportion of 'used' gas in the loop [hoses and scrubber].

Let's say you end up with a pO2 of 1.9 after a flush-at 60m an fO2 of 27%. In a loop of 10.5L volume, that is 2.85L of O2-surface litres. If you are at 60m it is equivalent to 19.95L because of the pressure at that depth. If you were injecting 28% into the loop at a flow rate of 10.4 L/min you will be injecting 2.9 L/min of O2 into the loop. With your low vO2 the pO2 spike will last a long time. The only thing you could do to reduce it, other than ascend, is to turn off your supply gas and wait for the pO2 to drop, and the deeper you are the longer it will take.

Best not to forget to turn the tank back on!

Humuhumu · 8th January 2008, 19:28

Hi,

I have tried that as an exercice in a depth of 30 meters (my instructor asked me to close the tank). We have stopped after 15 minutes and the mix in the loop was still breathable, so this is not a good solution in a depth of 60 meters :-) to reduce the PpO2...

If you forget to open the tank in the beginning of a dive you will be in deep trouble ! But the good thing is that you will die without pain :-))))

In fact SCR is not a good solution for deep dives but for long dives (not too deep) without decompression and a maximum of confort. The other point is that you can for the first time in your life kiss a fish :-) (if you like)

Cheers

Bertrand

Hanssing · 8th January 2008, 20:13

Quote: (Originally Posted by Freef)

The problem is that the pO2 spike IS long lasting. Realistically, you won't hit 2.1 as there is still a proportion of 'used' gas in the loop [hoses and scrubber].

Hmm Warning: pulling some newbie theory out of my arse here:

A loop with PPO=2.1 contains the same amount of O2 at depth and shallow.A loop with PPO2.1 will breathe down faster at depth than a loop with 2.1 wich is shallower, asuming flow into loop is stopped, and all other things equal, since the relative fraction of N2 is bigger.

------------------

So using constant mass-flow SCR at depth, means you'll have to increase the mass-flow and lower the Fo2 of the gas (Basic, huh). But can you still get significant savings over OC-trimix (say 5:1), however you can end up with a flow so large that you save nothing or even use more gas at the shallower parts of the dives, though you'll allso need some bigger tanks.

I see no limit depth for this type of SCR, but they end up being only usefull (economic) at the deep end of the pool, for single jet dives. To have significant saving at the shallower parts allso, you then need multiple dosage-devices, say one per FO2-mix, or something to scale the mass-flow with absolute pressure.... Wich leads to pSCR.

Nicolai

**Freef** · 9th January 2008, 14:18

Quote: (Originally Posted by Hanssing)

A loop with PPO=2.1 contains the same amount of O2 at depth and shallow.A loop with PPO2.1 will breathe down fast at dpeth than a loop with 2.1 wich is shallower, asuming flow into loop is stopped, since the relative fraction of N2 is bigger.

Not quite true, in 10m a pO2 of 2.1 equals an fO2 of 105%-something you obviously can't get.

2.1 at 20m = fO2 of 70.0% = 22.05L of O2.

2.1 at 30m = fO2 of 52.5% = 22.05L of O2.

2.1 at 60m = fO2 of 30.0% = 22.05L of O2.

So although you have the same surface volume of O2 in the loop, if your vO2 is 1L/min you still have to get through a lot of O2. To get to a loop pO2 of 1.3 [18.5%/13.65L] would take 8 minutes 24 seconds at a vO2 of 1.

At 20m and looking for a pO2 of 1.3 you will have 43.3% O2 in the loop-13.65L So it will still take 8 minutes 24 seconds at a vO2 of 1 to breathe the loop down from 70%.

Quote: (Originally Posted by Hanssing)

So using constant mass-flow SCR at depth, means you'll have to increase the mass-flow and lower the Fo2 of the gas (Basic, huh). But you still get significant savings over OC-trimix (say 5:1), however you can end up with a flow so large that you save nothing or even use more gas at the shallow parts of the dives, though you'll need so bigger tanks.Nicolai

On a trimix dive you will be swapping to a deco mix as you ascend anyway. You are right that you need a larger flow rate the deeper you go because of the problems of getting an accurate gas mix-see

http://www.rebreatherworld.com/semi-...-reposted.html

for examples of how high and low vO2's affect gas choice, and

http://www.rebreatherworld.com/semi-...ix-diving.html

for trimix theory.

Quote: (Originally Posted by Hanssing)

I see no limit depth for this type of SCR, but they end up being only usefull at the deep end of the pool. To have significant saving at the shallower parts you then need multiple dosage-devices, per FO2-mix, or something to scale the mass-flow with absolute pressure.... Wich leads to pSCR.

The depth limit for CMF Rebreather's [both SCR and CCR] is determined by the critical ratio of pressures on the upstream [tank] and downstream [loop] side of the jet. Of this ratio is not maintained the flow rate becomes subsonic and will vary with depth. The Dolphin is diveable to 81m while maintaining critical flow, beyond this depth the flow of gas into the loop decreases.

Hanssing · 9th January 2008, 17:13

Quote: (Originally Posted by Freef)

Not quite true, in 10m a pO2 of 2.1 equals an fO2 of 105%-something you obviously can't get.

2.1 at 20m = fO2 of 70.0% = 22.05L of O2.

2.1 at 30m = fO2 of 52.5% = 22.05L of O2.

2.1 at 60m = fO2 of 30.0% = 22.05L of O2.

So although you have the same surface volume of O2 in the loop, if your vO2 is 1L/min you still have to get through a lot of O2. To get to a loop pO2 of 1.3 [18.5%/13.65L] would take 8 minutes 24 seconds at a vO2 of 1.

At 20m and looking for a pO2 of 1.3 you will have 43.3% O2 in the loop-13.65L So it will still take 8 minutes 24 seconds at a vO2 of 1 to breathe the loop down from 70%.

Jeez freef, your kinda the SCR-guru here, so I don't want to challenge.... But here goes anyway:

About 10m - goes without saying! But your'e forgetting that you change the ratio of FO2/N2 but the pressure remains constant (remember flow to loop is inhibited).

But lets do simple one in the head first:
Loop-volume 10,5 l. (as in your examples)
PPO = 2.1 @ 11msw ie. FO2 = 100%
How long until you have reduced PPO to 1.3?
Its not the same as reducing the PPO at depth, thats all I wrote.

You forget that the PPO does not scale linearly with "amount" of O2 in a closed loop.

From you examples directly (plus one ekstra to really bring out the difference):
2.1 at 15m = fO2 of 84.0% = 22.05L of O2. + 4,2 l N2
1.3 at 15m = fO2 of 52,0% => Metabolise 17,5 l O2

2.1 at 20m = fO2 of 70.0% = 22.05L of O2 + 9,45l N2.
1.3 at 20m = fO2 of 43,3% => Metabolise 14,8 l O2

2.1 at 30m = fO2 of 52.5% = 22.05L of O2 + 19,95l N2.
1.3 at 30m = fO2 of 32,5% => Metabolise 12,4 l O2

2.1 at 60m = fO2 of 30.0% = 22.05L of O2. + 51,45l N2
1.3 at 60m = fO2 of 18,5% => Metabolise 10,3 l O2

The change in PPO at depth becomes nearly lienar with amount metabolised, but at shallows it does not. What makes it worse at shallows are, that you have to add fresh diluent because you reduces you loop-volume significantly.

Am I wrong?

Quote:

The depth limit for CMF Rebreather's [both SCR and CCR] is determined by the critical ratio of pressures on the upstream [tank] and downstream [loop] side of the jet. Of this ratio is not maintained the flow rate becomes subsonic and will vary with depth. The Dolphin is diveable to 81m while maintaining critical flow, beyond this depth the flow of gas into the loop decreases.

Yes, that I thought was implicit. I was writing that the constant-mass flow concept in theory works well at depth (If you can make a constant flow offcourse), but then can become wastefull when the same flow is used shallow with a new mix. Therefor its not enough to scale the flow only to the deep-part, you also need something to reduce it with for you decomixes.

Regards
Nicolai

**Freef** · 10th January 2008, 13:30

Quote: (Originally Posted by Hanssing)

Am I wrong?

You're not wrong, just not quite right. As you said in the first post, for a given pO2 there is a certain volume of O2 in the loop-22.05L for a pO2 of 2.1. You should have gone on to see that this is also true for the lower pO2 I used in my calculations [1.3] which gives a loop volume of O2 of 13.65L.

So at whatever depth you are at, to get down from 2.1 [22.05L] to 1.3 [13.65L] will take the 8 min 24 sec at a vO2 of 1. The amount of N2 in the loop makes no difference in this case, and you have to use 8.4 L of O2 to drop the pO2.

When you were looking at the lower pO2/fO2 figure did you remember to multiply back up to get the depth? I forgot when I first started running the figures

Of course, you need to remember that the CO2 produced will be absorbed, reducing lung volume as you allude to.

Quote: (Originally Posted by Hanssing)

Yes, that I thought was implicit. I was writing that the constant-mass flow concept in theory works well at depth (If you can make a constant flow offcourse), but then can become wastefull when the same flow is used shallow with a new mix. Therefor its not enough to scale the flow only to the deep-part, you also need something to reduce it with for you decomixes.

I tend to go onto OC for deco. I haven't done a deco trimix dive on the Dolphin yet as I need more time in the shallows on a weak trimix before progressing. It is possible to switch gas mixes on the loop [an adaptor from Tecme to allow 2 mixes into the bypass is needed] by turning one tank off and another on for deco, but I prefer to know exactly what I am breathing on deco, and vO2 is never a constant.

It is also possible to deco on an OC reg connected to an H valve tank [when using nitrox] but the likelyhood is that the 'step' in fO2 is too small to be useful-see the bottom of this post:

http://www.rebreatherworld.com/semi-...tml#post121362

Hanssing · 10th January 2008, 14:17

Quote: (Originally Posted by Freef)

You're not wrong, just not quite right. As you said in the first post, for a given pO2 there is a certain volume of O2 in the loop-22.05L for a pO2 of 2.1. You should have gone on to see that this is also true for the lower pO2 I used in my calculations [1.3] which gives a loop volume of O2 of 13.65L.

So at whatever depth you are at, to get down from 2.1 [22.05L] to 1.3 [13.65L] will take the 8 min 24 sec at a vO2 of 1. The amount of N2 in the loop makes no difference in this case, and you have to use 8.4 L of O2 to drop the pO2.

When you were looking at the lower pO2/fO2 figure did you remember to multiply back up to get the depth? I forgot when I first started running the figures

Sorry Freef, but my calculations are correct, I dont know if we misunderstand each otherbut......

The whole point is that the amount of O2 you have to metabolize is not canstant, it is dependant on the FIXED AMOUNT of N2 in the loop, since this is a "diluter" of the O2 and therefore also the PO2.

We agree on the the PPO-calculations for the high loop, wich contains the same amount of O2 at each depth. For illustration I added the N2 (Could also just be Total inertgasses, makes no difference). This is very simple, since we have the amount of O2:

2.1 at 15m = fO2 of 84.0% = 22.05L of O2. + 4,2 l N2
2.1 at 20m = fO2 of 70.0% = 22.05L of O2 + 9,45l N2.
2.1 at 30m = fO2 of 52.5% = 22.05L of O2 + 19,95l N2.
2.1 at 60m = fO2 of 30.0% = 22.05L of O2. + 51,45l N2

Now we can calculate the fO2 at PPO=1.3 easily enough, but the main point is that this does not scale linearly with volume of O2.

Note: You cannot just cut the amount of O2 by half and then exspect the drop in PPO to be halfed also. It nearly works when the FO2 your reducing is very low (at depth), but it does not work when the mix is rich! Case in point: using 11 l. of a pure O2 loop, does not affect the loop-PPO at all.

For at given FO2 with a FIXED AMOUNT of N2 you actaully have to do a little math. I use the formula FO2 = VO2 / Vol_Total = VO2 / (VO2+VN2).

So we start by calculating Fo2 at PPO=1.3:

1.3 at 15m = fO2 of 52,0%
1.3 at 20m = fO2 of 43,3%
1.3 at 30m = fO2 of 32,5%
1.3 at 60m = fO2 of 18,5%

For each depth the following will hold true:
FO2 = VO2 / (VO2+VN2), where FO2 and VN2 is known, you therefor solve for VO2:
VO2 = FO2*(VO2+VN2) <=> VO2 = FO2*VN2 /(1-FO2)

So you end up with:
1.3 at 15m = fO2 of 52,0% => 4,55l O2 + 4,2 LN2 (ie. FO2=52%)
Therefor you have to Metabolise 17,5 l O2

1.3 at 20m = fO2 of 43,3% => 7,25l O2 + 9,45l N2 (ie. FO2=43,3%)
Therefor you have to Metabolise 14,8 l O2

etc.

Still think you are wrong freef, but perhaps we are misunderstanding each other?

Its actually quite logical when you think about it...

Best regards
Nicolai

**Freef** · 10th January 2008, 18:06

Here's my thought process, starting with Dragers spec for the Dolphin loop being 10.5L:

At a fixed pO2, for any depth the surface volume of O2 in the loop is the same.

Therefore, for a loop pO2 of 0.5, at the surface the loop will need 50% or 5.25L of O2 in it.

At 10m the loop would need 2.625L of O2 for the same pO2-or with the same fO2 you would get a pO2 of 1

Examples:

pO2 of 2.1 at 30m:

fO2 = 2.1 / 4 = 52.5% O2. At 30m the pressure is 4 bar, therefore the O2 content of the loop is 10.5 x 0.525 x 4 = 22.05L

pO2 of 2.1 at 50m:

fO2 = 2.1 / 6 = 35% O2. At 50m the pressure is 6 bar, therefore the O2 content of the loop is 10.5 x 0.35 x 6 = 22.05L

And to find the loop O2 volume at 1.3 bar for the same depths:

pO2 of 1.3 at 30m:

fO2 = 1.3 / 4 = 32.5% O2. At 30m the pressure is 4 bar, therefore the O2 content of the loop is 10.5 x 0.325 x 4 = 13.65L

pO2 of 1.3 at 50m:

fO2 = 1.3 / 6 = 21.7% O2. At 50m the pressure is 6 bar, therefore the O2 content of the loop is 10.5 x 0.217 x 6 = 13.65L

So therefore you would have a constant surface volume of O2 to metabolise.

This is as far as my thinking went, and where I went wrong. I see what you mean about the changing ratio of gasses the deeper you are, so I ran through the figures again-and it gets worse, not better. For example:

If you take the decrease in loop volume into account, at 40m, in one minute intervals you would get the following, if you assume a vO2 of 1 L/min, 100% CO2 capture and a loop volume of 10.5L that can collapse as the gas is used.

time = 0 pO2 = 2.100 vol [O2] = 22.05 vol [inert] = 30.45 loop fO2 = 0.42

time = 1 pO2 = 2.045 vol [O2] = 21.05 vol [inert] = 30.45 loop fO2 = 0.409

time = 2 pO2 = 1.985 vol [O2] = 20.05 vol [inert] = 30.45 loop fO2 = 0.397

time = 3 pO2 = 1.925 vol [O2] = 19.05 vol [inert] = 30.45 loop fO2 = 0.385

time = 4 pO2 = 1.860 vol [O2] = 18.05 vol [inert] = 30.45 loop fO2 = 0.372

time = 5 pO2 = 1.795 vol [O2] = 17.05 vol [inert] = 30.45 loop fO2 = 0.359

time = 6 pO2 = 1.725 vol [O2] = 16.05 vol [inert] = 30.45 loop fO2 = 0.345

time = 7 pO2 = 1.650 vol [O2] = 15.05 vol [inert] = 30.45 loop fO2 = 0.330

time = 8 pO2 = 1.580 vol [O2] = 14.05 vol [inert] = 30.45 loop fO2 = 0.316

time = 9 pO2 = 1.500 vol [O2] = 13.05 vol [inert] = 30.45 loop fO2 = 0.300

time =10 pO2 = 1.420 vol [O2] = 12.05 vol [inert] = 30.45 loop fO2 = 0.284

time =11 pO2 = 1.330 vol [O2] = 11.05 vol [inert] = 30.45 loop fO2 = 0.266

If you take the decrease in loop volume into account, at 60m, in two minute intervals you would get the following, if you assume 100% CO2 capture and a loop volume of 10.5L that can collapse as the gas is used.

time = 0 pO2 = 2.100 vol [O2] = 22.05 vol [inert] = 51.45 loop fO2 = 0.300

time = 2 pO2 = 1.960 vol [O2] = 20.05 vol [inert] = 51.45 loop fO2 = 0.280

time = 4 pO2 = 1.820 vol [O2] = 18.05 vol [inert] = 51.45 loop fO2 = 0.260

time = 6 pO2 = 1.666 vol [O2] = 16.05 vol [inert] = 51.45 loop fO2 = 0.238

time = 8 pO2 = 1.505 vol [O2] = 14.05 vol [inert] = 51.45 loop fO2 = 0.215

time =10 pO2 = 1.330 vol [O2] = 12.05 vol [inert] = 51.45 loop fO2 = 0.190

So I was wrong to assume a fixed volume, and my figures gave a shorter time to metabolise the pO2. Looks like it's you who is the SCR guru!

Appropriate green sent and humble pie in the oven. If you find these figures wrong I'll sulk though!

The real problem with attempting to rely on the pO2 dropping in this way is that the loop will be collapsing by 2.2L at 40m to drop the pO2. I think that this is about the volume of the exhale CL. It will become difficult to breathe as the loop volume decreases, and when the gas is turned back on the ADV will be likely to fire, adding more O2 to the loop.

Hanssing · 10th January 2008, 19:00

Quote: (Originally Posted by Freef)

So I was wrong to assume a fixed volume, and my figures gave a shorter time to metabolise the pO2. Looks like it's you who is the SCR guru!

Appropriate green sent and humble pie in the oven. If you find these figures wrong I'll sulk though!

The real problem with attempting to rely on the pO2 dropping in this way is that the loop will be collapsing by 2.2L at 40m to drop the pO2. I think that this is about the volume of the exhale CL. It will become difficult to breathe as the loop volume decreases, and when the gas is turned back on the ADV will be likely to fire, adding more O2 to the loop.

The conclusion in part could be that running comparably hot diluents at differents depths, then its will take longer to breathe it down at shallows. Even more so since the loop-redution is bigger and therefor even more O2 gets added. This is however offset by the fact that the relative pressure drops faster when you ascend, and allso that you dont have any advantage to that hot diluents when shallow, unless for deco-mixes (It's really a matter of how high a PPO2 you willing to run when flushing, and then how low a flow you can safely use)....

About the guru-part I respectfully decline, with only 20mins on a dolphin, and only 5mins SCR on my rEvo. I may find the theory intersting, but I normally have little practical idea what I talking about (yeah I'm an engineer

).
I tend to go out on a theoretical limb, just see the SCR-bailout-thead......

Thanks for the green - I will however still judge impartially in the caption-contest

Regards
Nicolai

**Freef** · 10th January 2008, 20:48

Here are some graphs that show the reduction in pO2 vs time at 20, 40, 60m with a pO2 starting point of 2.1 and a vO2 of 1.

7th January 2008, 16:43	#11 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,356	Re: trimix on scr Quote: (Originally Posted by Humuhumu) Personaly I can live with a PpO2 of 2.1, it will not be a problem if the situation is not lasting: this means if this happens you will have to go on the bail out, that's it ! That's why I have a 80 cft AIR, with me. The problem is that the pO2 spike IS long lasting. Realistically, you won't hit 2.1 as there is still a proportion of 'used' gas in the loop [hoses and scrubber]. Let's say you end up with a pO2 of 1.9 after a flush-at 60m an fO2 of 27%. In a loop of 10.5L volume, that is 2.85L of O2-surface litres. If you are at 60m it is equivalent to 19.95L because of the pressure at that depth. If you were injecting 28% into the loop at a flow rate of 10.4 L/min you will be injecting 2.9 L/min of O2 into the loop. With your low vO2 the pO2 spike will last a long time. The only thing you could do to reduce it, other than ascend, is to turn off your supply gas and wait for the pO2 to drop, and the deeper you are the longer it will take. Best not to forget to turn the tank back on! __________________ David. Currently owner of two differently sized ankles.
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8th January 2008, 19:28	#12 (permalink)
Humuhumu New Member Current Rebreather/s: Inspiration Vision Other Rebreather/s: Not Bought Yet Join Date: Jan 2007 Location: Blue Planet Posts: 79	Re: trimix on scr Hi, I have tried that as an exercice in a depth of 30 meters (my instructor asked me to close the tank). We have stopped after 15 minutes and the mix in the loop was still breathable, so this is not a good solution in a depth of 60 meters :-) to reduce the PpO2... If you forget to open the tank in the beginning of a dive you will be in deep trouble ! But the good thing is that you will die without pain :-)))) In fact SCR is not a good solution for deep dives but for long dives (not too deep) without decompression and a maximum of confort. The other point is that you can for the first time in your life kiss a fish :-) (if you like) Cheers Bertrand __________________ Humuhumu Nukunuku Apua'Ha
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8th January 2008, 20:13	#13 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 350	Re: trimix on scr Quote: (Originally Posted by Freef) The problem is that the pO2 spike IS long lasting. Realistically, you won't hit 2.1 as there is still a proportion of 'used' gas in the loop [hoses and scrubber]. Hmm Warning: pulling some newbie theory out of my arse here: A loop with PPO=2.1 contains the same amount of O2 at depth and shallow.A loop with PPO2.1 will breathe down faster at depth than a loop with 2.1 wich is shallower, asuming flow into loop is stopped, and all other things equal, since the relative fraction of N2 is bigger. ------------------ So using constant mass-flow SCR at depth, means you'll have to increase the mass-flow and lower the Fo2 of the gas (Basic, huh). But can you still get significant savings over OC-trimix (say 5:1), however you can end up with a flow so large that you save nothing or even use more gas at the shallower parts of the dives, though you'll allso need some bigger tanks. I see no limit depth for this type of SCR, but they end up being only usefull (economic) at the deep end of the pool, for single jet dives. To have significant saving at the shallower parts allso, you then need multiple dosage-devices, say one per FO2-mix, or something to scale the mass-flow with absolute pressure.... Wich leads to pSCR. Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear Last edited by Hanssing : 9th January 2008 at 17:44. Reason: Spelling
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9th January 2008, 14:18	#14 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,356	Re: trimix on scr Quote: (Originally Posted by Hanssing) A loop with PPO=2.1 contains the same amount of O2 at depth and shallow.A loop with PPO2.1 will breathe down fast at dpeth than a loop with 2.1 wich is shallower, asuming flow into loop is stopped, since the relative fraction of N2 is bigger. Not quite true, in 10m a pO2 of 2.1 equals an fO2 of 105%-something you obviously can't get. 2.1 at 20m = fO2 of 70.0% = 22.05L of O2. 2.1 at 30m = fO2 of 52.5% = 22.05L of O2. 2.1 at 60m = fO2 of 30.0% = 22.05L of O2. So although you have the same surface volume of O2 in the loop, if your vO2 is 1L/min you still have to get through a lot of O2. To get to a loop pO2 of 1.3 [18.5%/13.65L] would take 8 minutes 24 seconds at a vO2 of 1. At 20m and looking for a pO2 of 1.3 you will have 43.3% O2 in the loop-13.65L So it will still take 8 minutes 24 seconds at a vO2 of 1 to breathe the loop down from 70%. Quote: (Originally Posted by Hanssing) So using constant mass-flow SCR at depth, means you'll have to increase the mass-flow and lower the Fo2 of the gas (Basic, huh). But you still get significant savings over OC-trimix (say 5:1), however you can end up with a flow so large that you save nothing or even use more gas at the shallow parts of the dives, though you'll need so bigger tanks.Nicolai On a trimix dive you will be swapping to a deco mix as you ascend anyway. You are right that you need a larger flow rate the deeper you go because of the problems of getting an accurate gas mix-see http://www.rebreatherworld.com/semi-...-reposted.html for examples of how high and low vO2's affect gas choice, and http://www.rebreatherworld.com/semi-...ix-diving.html for trimix theory. Quote: (Originally Posted by Hanssing) I see no limit depth for this type of SCR, but they end up being only usefull at the deep end of the pool. To have significant saving at the shallower parts you then need multiple dosage-devices, per FO2-mix, or something to scale the mass-flow with absolute pressure.... Wich leads to pSCR. The depth limit for CMF Rebreather's [both SCR and CCR] is determined by the critical ratio of pressures on the upstream [tank] and downstream [loop] side of the jet. Of this ratio is not maintained the flow rate becomes subsonic and will vary with depth. The Dolphin is diveable to 81m while maintaining critical flow, beyond this depth the flow of gas into the loop decreases. __________________ David. Currently owner of two differently sized ankles.
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9th January 2008, 17:13	#15 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 350	Re: trimix on scr Quote: (Originally Posted by Freef) Not quite true, in 10m a pO2 of 2.1 equals an fO2 of 105%-something you obviously can't get. 2.1 at 20m = fO2 of 70.0% = 22.05L of O2. 2.1 at 30m = fO2 of 52.5% = 22.05L of O2. 2.1 at 60m = fO2 of 30.0% = 22.05L of O2. So although you have the same surface volume of O2 in the loop, if your vO2 is 1L/min you still have to get through a lot of O2. To get to a loop pO2 of 1.3 [18.5%/13.65L] would take 8 minutes 24 seconds at a vO2 of 1. At 20m and looking for a pO2 of 1.3 you will have 43.3% O2 in the loop-13.65L So it will still take 8 minutes 24 seconds at a vO2 of 1 to breathe the loop down from 70%. Jeez freef, your kinda the SCR-guru here, so I don't want to challenge.... But here goes anyway: About 10m - goes without saying! But your'e forgetting that you change the ratio of FO2/N2 but the pressure remains constant (remember flow to loop is inhibited). But lets do simple one in the head first: Loop-volume 10,5 l. (as in your examples) PPO = 2.1 @ 11msw ie. FO2 = 100% How long until you have reduced PPO to 1.3? Its not the same as reducing the PPO at depth, thats all I wrote. You forget that the PPO does not scale linearly with "amount" of O2 in a closed loop. From you examples directly (plus one ekstra to really bring out the difference): 2.1 at 15m = fO2 of 84.0% = 22.05L of O2. + 4,2 l N2 1.3 at 15m = fO2 of 52,0% => Metabolise 17,5 l O2 2.1 at 20m = fO2 of 70.0% = 22.05L of O2 + 9,45l N2. 1.3 at 20m = fO2 of 43,3% => Metabolise 14,8 l O2 2.1 at 30m = fO2 of 52.5% = 22.05L of O2 + 19,95l N2. 1.3 at 30m = fO2 of 32,5% => Metabolise 12,4 l O2 2.1 at 60m = fO2 of 30.0% = 22.05L of O2. + 51,45l N2 1.3 at 60m = fO2 of 18,5% => Metabolise 10,3 l O2 The change in PPO at depth becomes nearly lienar with amount metabolised, but at shallows it does not. What makes it worse at shallows are, that you have to add fresh diluent because you reduces you loop-volume significantly. Am I wrong? Quote: The depth limit for CMF Rebreather's [both SCR and CCR] is determined by the critical ratio of pressures on the upstream [tank] and downstream [loop] side of the jet. Of this ratio is not maintained the flow rate becomes subsonic and will vary with depth. The Dolphin is diveable to 81m while maintaining critical flow, beyond this depth the flow of gas into the loop decreases. Yes, that I thought was implicit. I was writing that the constant-mass flow concept in theory works well at depth (If you can make a constant flow offcourse), but then can become wastefull when the same flow is used shallow with a new mix. Therefor its not enough to scale the flow only to the deep-part, you also need something to reduce it with for you decomixes. Regards Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear Last edited by Hanssing : 9th January 2008 at 17:46. Reason: spelling
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10th January 2008, 13:30	#16 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,356	Re: trimix on scr Quote: (Originally Posted by Hanssing) Am I wrong? You're not wrong, just not quite right. As you said in the first post, for a given pO2 there is a certain volume of O2 in the loop-22.05L for a pO2 of 2.1. You should have gone on to see that this is also true for the lower pO2 I used in my calculations [1.3] which gives a loop volume of O2 of 13.65L. So at whatever depth you are at, to get down from 2.1 [22.05L] to 1.3 [13.65L] will take the 8 min 24 sec at a vO2 of 1. The amount of N2 in the loop makes no difference in this case, and you have to use 8.4 L of O2 to drop the pO2. When you were looking at the lower pO2/fO2 figure did you remember to multiply back up to get the depth? I forgot when I first started running the figures Of course, you need to remember that the CO2 produced will be absorbed, reducing lung volume as you allude to. Quote: (Originally Posted by Hanssing) Yes, that I thought was implicit. I was writing that the constant-mass flow concept in theory works well at depth (If you can make a constant flow offcourse), but then can become wastefull when the same flow is used shallow with a new mix. Therefor its not enough to scale the flow only to the deep-part, you also need something to reduce it with for you decomixes. I tend to go onto OC for deco. I haven't done a deco trimix dive on the Dolphin yet as I need more time in the shallows on a weak trimix before progressing. It is possible to switch gas mixes on the loop [an adaptor from Tecme to allow 2 mixes into the bypass is needed] by turning one tank off and another on for deco, but I prefer to know exactly what I am breathing on deco, and vO2 is never a constant. It is also possible to deco on an OC reg connected to an H valve tank [when using nitrox] but the likelyhood is that the 'step' in fO2 is too small to be useful-see the bottom of this post: http://www.rebreatherworld.com/semi-...tml#post121362 __________________ David. Currently owner of two differently sized ankles.
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10th January 2008, 14:17	#17 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 350	Re: trimix on scr Quote: (Originally Posted by Freef) You're not wrong, just not quite right. As you said in the first post, for a given pO2 there is a certain volume of O2 in the loop-22.05L for a pO2 of 2.1. You should have gone on to see that this is also true for the lower pO2 I used in my calculations [1.3] which gives a loop volume of O2 of 13.65L. So at whatever depth you are at, to get down from 2.1 [22.05L] to 1.3 [13.65L] will take the 8 min 24 sec at a vO2 of 1. The amount of N2 in the loop makes no difference in this case, and you have to use 8.4 L of O2 to drop the pO2. When you were looking at the lower pO2/fO2 figure did you remember to multiply back up to get the depth? I forgot when I first started running the figures Sorry Freef, but my calculations are correct, I dont know if we misunderstand each otherbut...... The whole point is that the amount of O2 you have to metabolize is not canstant, it is dependant on the FIXED AMOUNT of N2 in the loop, since this is a "diluter" of the O2 and therefore also the PO2. We agree on the the PPO-calculations for the high loop, wich contains the same amount of O2 at each depth. For illustration I added the N2 (Could also just be Total inertgasses, makes no difference). This is very simple, since we have the amount of O2: 2.1 at 15m = fO2 of 84.0% = 22.05L of O2. + 4,2 l N2 2.1 at 20m = fO2 of 70.0% = 22.05L of O2 + 9,45l N2. 2.1 at 30m = fO2 of 52.5% = 22.05L of O2 + 19,95l N2. 2.1 at 60m = fO2 of 30.0% = 22.05L of O2. + 51,45l N2 Now we can calculate the fO2 at PPO=1.3 easily enough, but the main point is that this does not scale linearly with volume of O2. Note: You cannot just cut the amount of O2 by half and then exspect the drop in PPO to be halfed also. It nearly works when the FO2 your reducing is very low (at depth), but it does not work when the mix is rich! Case in point: using 11 l. of a pure O2 loop, does not affect the loop-PPO at all. For at given FO2 with a FIXED AMOUNT of N2 you actaully have to do a little math. I use the formula FO2 = VO2 / Vol_Total = VO2 / (VO2+VN2). So we start by calculating Fo2 at PPO=1.3: 1.3 at 15m = fO2 of 52,0% 1.3 at 20m = fO2 of 43,3% 1.3 at 30m = fO2 of 32,5% 1.3 at 60m = fO2 of 18,5% For each depth the following will hold true: FO2 = VO2 / (VO2+VN2), where FO2 and VN2 is known, you therefor solve for VO2: VO2 = FO2(VO2+VN2) <=> VO2 = FO2VN2 /(1-FO2) So you end up with: 1.3 at 15m = fO2 of 52,0% => 4,55l O2 + 4,2 LN2 (ie. FO2=52%) Therefor you have to Metabolise 17,5 l O2 1.3 at 20m = fO2 of 43,3% => 7,25l O2 + 9,45l N2 (ie. FO2=43,3%) Therefor you have to Metabolise 14,8 l O2 etc. Still think you are wrong freef, but perhaps we are misunderstanding each other? Its actually quite logical when you think about it... Best regards Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear Last edited by Hanssing : 10th January 2008 at 14:25.
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10th January 2008, 18:06	#18 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,356	Re: trimix on scr Here's my thought process, starting with Dragers spec for the Dolphin loop being 10.5L: At a fixed pO2, for any depth the surface volume of O2 in the loop is the same. Therefore, for a loop pO2 of 0.5, at the surface the loop will need 50% or 5.25L of O2 in it. At 10m the loop would need 2.625L of O2 for the same pO2-or with the same fO2 you would get a pO2 of 1 Examples: pO2 of 2.1 at 30m: fO2 = 2.1 / 4 = 52.5% O2. At 30m the pressure is 4 bar, therefore the O2 content of the loop is 10.5 x 0.525 x 4 = 22.05L pO2 of 2.1 at 50m: fO2 = 2.1 / 6 = 35% O2. At 50m the pressure is 6 bar, therefore the O2 content of the loop is 10.5 x 0.35 x 6 = 22.05L And to find the loop O2 volume at 1.3 bar for the same depths: pO2 of 1.3 at 30m: fO2 = 1.3 / 4 = 32.5% O2. At 30m the pressure is 4 bar, therefore the O2 content of the loop is 10.5 x 0.325 x 4 = 13.65L pO2 of 1.3 at 50m: fO2 = 1.3 / 6 = 21.7% O2. At 50m the pressure is 6 bar, therefore the O2 content of the loop is 10.5 x 0.217 x 6 = 13.65L So therefore you would have a constant surface volume of O2 to metabolise. This is as far as my thinking went, and where I went wrong. I see what you mean about the changing ratio of gasses the deeper you are, so I ran through the figures again-and it gets worse, not better. For example: If you take the decrease in loop volume into account, at 40m, in one minute intervals you would get the following, if you assume a vO2 of 1 L/min, 100% CO2 capture and a loop volume of 10.5L that can collapse as the gas is used. time = 0 pO2 = 2.100 vol [O2] = 22.05 vol [inert] = 30.45 loop fO2 = 0.42 time = 1 pO2 = 2.045 vol [O2] = 21.05 vol [inert] = 30.45 loop fO2 = 0.409 time = 2 pO2 = 1.985 vol [O2] = 20.05 vol [inert] = 30.45 loop fO2 = 0.397 time = 3 pO2 = 1.925 vol [O2] = 19.05 vol [inert] = 30.45 loop fO2 = 0.385 time = 4 pO2 = 1.860 vol [O2] = 18.05 vol [inert] = 30.45 loop fO2 = 0.372 time = 5 pO2 = 1.795 vol [O2] = 17.05 vol [inert] = 30.45 loop fO2 = 0.359 time = 6 pO2 = 1.725 vol [O2] = 16.05 vol [inert] = 30.45 loop fO2 = 0.345 time = 7 pO2 = 1.650 vol [O2] = 15.05 vol [inert] = 30.45 loop fO2 = 0.330 time = 8 pO2 = 1.580 vol [O2] = 14.05 vol [inert] = 30.45 loop fO2 = 0.316 time = 9 pO2 = 1.500 vol [O2] = 13.05 vol [inert] = 30.45 loop fO2 = 0.300 time =10 pO2 = 1.420 vol [O2] = 12.05 vol [inert] = 30.45 loop fO2 = 0.284 time =11 pO2 = 1.330 vol [O2] = 11.05 vol [inert] = 30.45 loop fO2 = 0.266 If you take the decrease in loop volume into account, at 60m, in two minute intervals you would get the following, if you assume 100% CO2 capture and a loop volume of 10.5L that can collapse as the gas is used. time = 0 pO2 = 2.100 vol [O2] = 22.05 vol [inert] = 51.45 loop fO2 = 0.300 time = 2 pO2 = 1.960 vol [O2] = 20.05 vol [inert] = 51.45 loop fO2 = 0.280 time = 4 pO2 = 1.820 vol [O2] = 18.05 vol [inert] = 51.45 loop fO2 = 0.260 time = 6 pO2 = 1.666 vol [O2] = 16.05 vol [inert] = 51.45 loop fO2 = 0.238 time = 8 pO2 = 1.505 vol [O2] = 14.05 vol [inert] = 51.45 loop fO2 = 0.215 time =10 pO2 = 1.330 vol [O2] = 12.05 vol [inert] = 51.45 loop fO2 = 0.190 So I was wrong to assume a fixed volume, and my figures gave a shorter time to metabolise the pO2. Looks like it's you who is the SCR guru! Appropriate green sent and humble pie in the oven. If you find these figures wrong I'll sulk though! The real problem with attempting to rely on the pO2 dropping in this way is that the loop will be collapsing by 2.2L at 40m to drop the pO2. I think that this is about the volume of the exhale CL. It will become difficult to breathe as the loop volume decreases, and when the gas is turned back on the ADV will be likely to fire, adding more O2 to the loop. __________________ David. Currently owner of two differently sized ankles.
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10th January 2008, 19:00	#19 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 350	Re: trimix on scr Quote: (Originally Posted by Freef) So I was wrong to assume a fixed volume, and my figures gave a shorter time to metabolise the pO2. Looks like it's you who is the SCR guru! Appropriate green sent and humble pie in the oven. If you find these figures wrong I'll sulk though! The real problem with attempting to rely on the pO2 dropping in this way is that the loop will be collapsing by 2.2L at 40m to drop the pO2. I think that this is about the volume of the exhale CL. It will become difficult to breathe as the loop volume decreases, and when the gas is turned back on the ADV will be likely to fire, adding more O2 to the loop. The conclusion in part could be that running comparably hot diluents at differents depths, then its will take longer to breathe it down at shallows. Even more so since the loop-redution is bigger and therefor even more O2 gets added. This is however offset by the fact that the relative pressure drops faster when you ascend, and allso that you dont have any advantage to that hot diluents when shallow, unless for deco-mixes (It's really a matter of how high a PPO2 you willing to run when flushing, and then how low a flow you can safely use).... About the guru-part I respectfully decline, with only 20mins on a dolphin, and only 5mins SCR on my rEvo. I may find the theory intersting, but I normally have little practical idea what I talking about (yeah I'm an engineer ). I tend to go out on a theoretical limb, just see the SCR-bailout-thead...... Thanks for the green - I will however still judge impartially in the caption-contest Regards Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear
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10th January 2008, 20:48	#20 (permalink)
Freef Custom Title Disallowed! Current Rebreather/s: Dolphin Other Rebreather/s: Dolphin Join Date: Jan 2006 Location: Land of the Freef, UK. Posts: 1,356	Re: trimix on scr Here are some graphs that show the reduction in pO2 vs time at 20, 40, 60m with a pO2 starting point of 2.1 and a vO2 of 1. __________________ David. Currently owner of two differently sized ankles.
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