02 Flushes and Changing Dil on your Onboard Computer

Dave Faye · 14th August 2008, 13:17

When you do your 02 flush at 20 feet/6 meters after a deep dive with loads of deco, does anyone change the dil programed into their Vision or other computer to 100% 02 (or something similar). If not, are you just leaving the previous dil programed in.

The issue is on deeper dives where you are coming off high helium mixes. You have 1-3 hours of deco in the shallows. You flush thoroughly and have no helium left in the loop. Now your only adding 02 for the remainder of your dive. what do you do?

Stephane · 14th August 2008, 13:30

Hello Dave,

Why would you want to change the diluent type when the deco is based on the actual PpO2? If at 3m your controller displays 1.3, you're on pure O2.
You are not saturating helium or nitrogen at these shallow depths.

Dave Faye · 14th August 2008, 17:22

Quote: (Originally Posted by Stephane)

Hello Stephane,

Why would you want to change the diluent type when the deco is based on the actual PpO2? If at 3m your controller displays 1.3, you're on pure O2.
You are not saturating helium or nitrogen at these shallow depths.

To track my decompression more effectively. If the computer thinks there is helium in the loop, it will calulate your off gassing differently. If I am at 6 meters flush with 02 and now only have 02 and exhaled gas in my lungs and the loop, my deco time is different. If my dive computer still thinks I have a 10/50 or 8/60, etc in the loop, it will give me a differnt deco time. S.P. would not matter. My on board computer knows my pp of 02 but it does not that I am off my helium rich mix dil.

I'm just curious what other do. It is really more relevant on dives where you have significant deco time in the shallows. If you change your dil from say 10/50 to 02 on the computer at 6 meters you will see the deco time drop on long duration dives.

dave t · 14th August 2008, 17:52

Quote: (Originally Posted by Dave Faye)

To track my decompression more effectively. If the computer thinks there is helium in the loop, it will calulate your off gassing differently. If I am at 6 meters flush with 02 and now only have 02 and exhaled gas in my lungs and the loop, my deco time is different. If my dive computer still thinks I have a 10/50 or 8/60, etc in the loop, it will give me a differnt deco time. S.P. would not matter. My on board computer knows my pp of 02 but it does not that I am off my helium rich mix dil.

I'm just curious what other do. It is really more relevant on dives where you have significant deco time in the shallows. If you change your dil from say 10/50 to 02 on the computer at 6 meters you will see the deco time drop on long duration dives.

Are you talking about the vision computer?

If you are then it will base your deco on the gas you have entered, the po2 that the cells read and ambient pressure. Because it knows all of that information it will know that if you have a po2 of 1.6 at 6m there will be no helium or nitrogen present in the loop (execept what you offgass and that is a tiny amount.

If you are talking about a non-linked computer you are right it will not know unless you have changed it as it doesnt have all the info (no actual po2)

HTH

Dave

Dave Faye · 14th August 2008, 20:27

Are you talking about the vision computer?

I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop.

So, do you input your dil as 02 or just leave it on your last dil which is more conservative.

dave t · 14th August 2008, 20:56

Quote: (Originally Posted by Dave Faye)

Are you talking about the vision computer?

I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop.

So, do you input your dil as 02 or just leave it on your last dil which is more conservative.

yes it does know the pp of the other gasses because it knows the dil you have input and it knows the po2 and ambient pressure. the rest is just maths

Hanssing · 14th August 2008, 21:07

Quote: (Originally Posted by Dave Faye)

I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop.

So, do you input your dil as 02 or just leave it on your last dil which is more conservative.

Dude, thats pretty scary

Anayway I'm assuming its misunderstandings through my written english, so here goes nothing. But if you have a deco-computer (Inline with Cells) or offline, and you do a O2-flush you should offcourse keep the diluent settigns for the computer.

The computer will calculate the inert partial pressures of the loop based upon the PPO2 meassured (or set), and the makeupgas from the diluent. So to get "correct" deco you should offcourse still set the dilunt to what ever is/was in your loop.

A couple of finer points:

1) When offgassing from Trimix one could argue that the PP_Inert mix is unknown, to counter this you could flush with diluent to bring back the reletive FN2 and FHe to known values - I dont think this is needed, but perhaps it is practised by some?

2) Setting you computer to a dill og 100% does not make sense unless the PPO2 is excatly the sorounding pressure (minus some water vapor etc). So if you were to set the dil as pure O2, then what happens when the loop drops (as you write), during a stop at constant depth? The fact that the PPO2 is dropping is an indication that the F_O2 is dropping proportionately, so some other gas is present in the loop - in order for the DeCo-computer to calculate this it needs to know what the inert-gasses are present, and more importantly in what fractions.

I reality it does not matter for the Deco is you enter your Dil as Air, EAN32 or even EAN99, as this causes the computer to calculate deco with Nitrogen only as inert gas - again based upon the PPO2!

YOu could also use different Trimix'es and get the same results:
Tx50/25 or Tx40/30 or Tx20/40, would give the same decoprofile as the F_N2/F_He is the same.

Some Examples:

Code:

Depth   Dil    P_Total   PPO2     P_N2    P_He 6m     Air     1.6 bar   1.0bar   0.6bar  0.0bar 6m     Air     1.6 bar   1.6bar   0.0bar  0.0bar <-- After flush 6m     Tx20/40 1.6 bar   1.0bar   0.3bar  0.3bar 6m     O2      1.6 bar   1.0bar   ????    ???? <-- Illegal some inert gas must be present...

Did this clear up your question?

Nicolai

Dave Faye · 15th August 2008, 00:40

"I reality it does not matter for the Deco is you enter your Dil as Air, EAN32 or even EAN99, as this causes the computer to calculate deco with Nitrogen only as inert gas - again based upon the PPO2!"

Agreed with air or nitrox the only other gas is nitrogen. The computer knows this is the only other gas and applies the remaining pp as nitrogen.

"You could also use different Trimix'es and get the same results:
Tx50/25 or Tx40/30 or Tx20/40, would give the same decoprofile as the F_N2/F_He is the same."

This is my issue. If you are running a dil mix with helium in it and you flush thoroughly at 20 feet with 02 you have now removed the helium. The mix contains only 02 and exhaled co2,
water vapor etc. If you don't tell the computer that the helium is gone, the computer sees the current p02 calculates it and assumes that the remaining pp is still helium and nitrogen. On a
helium rich mix this will effect the way deco calculates out.

So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone. Your loop only contains 02 and whatever is produced as
a byproduct in you lungs as you matabolize the gas. Co2, water vapor, unused 02, etc. What do you tell your computer your dil is?

Hanssing · 15th August 2008, 09:27

Quote: (Originally Posted by Dave Faye)

So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone.

Its not, nearly but some is still left. Besides this you offgas Trimix into the loop (Ie. the PPO2 decays over time). So some inert gas IS in the loop.

Quote:

Your loop only contains 02 and whatever is produced as
a byproduct in you lungs as you matabolize the gas. Co2, water vapor, unused 02, etc. What do you tell your computer your dil is?

IF the PPO is cloase to a pure O2-loop, then the computer will factor this in automatically. If it is far from pure O2, say 1.3bar at 6msw, then you need to tell the computer what the relative fractions are of F_N2 and F_He. You do this by setting the Dil.

Again the fractions due to different speeds in offgassing, and amounts dissolved in the tissues (He is faster but less soluble) could over time deviate from the Dil mix. Therefor if you want to be really anal about it, you cold do a dil-flush periodically and then bring up the PPO afterwards. As I'm talking out of my ass here (I'm not a Tx-diver) - someone wiht extensive Tx-experience will hace to chime in if this is done in practice?

COncerning running the loop on PURE O2 this never really happens, due to the fact that you offgass into the loop, and never really flush out all the traces of inert gas. Therefor you keep the Dil-setting and the Deco-computer will calculate just how much N2 and He is actually in the loop from this setting. If you flush the loop several times, and clean it out completely then the PPO wil indicate this, and the computer will therefor calculate with no Tx in the loop (In that case you could use any dil-setting - as there is no dill in the loop it would not affect the deco-calculations).

If you want to REALLY do pure O2, then you should breathe it Open Circut and set the computer accordingly.

Disclaimer: I'm talking out of my ass here (I'm not a Tx-diver)

Regards
Nicolai Hanssing

Tino de Rijk · 15th August 2008, 10:20

Quote: (Originally Posted by Dave Faye)

So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone.

No, absolutely not "nearly everyone". In fact, I doubt if it is a majority. The advantage of doing that at 6 meters is minimal, because:

a perfect flush, giving you REALLY 100% O2, is almost impossible, so you'll end up with around 90% or so. Leaving it at 1.3 and let the computer do its injection thing gives you 1.3/1.6 = 81% O2. It makes almost no difference in deco, while on the other hand raising the PO2 to above 1.4 does raise your risk on CNS toxicity, especially at the end of a long dive (with subsequent prolonged high PO2 exposure). So why do it?
as explained by someone else here, your loop will "dilute" right after the flush, by a.o. your body gassing off inert gasses (N2 and/or HE) into the loop. So you need to repeat flushing multiple times to keep it high - which costs a lot of valuable O2.

To make sure I wasn't talking bollocks, I ran a CCR profile through both V-Planner (+2 conservatism) and GAP (Buhlmann-GF-15/85). Here's the diveplan:

bottom: 60 meters, 45 minutes, bottom & ascent dil 10/52
switch to air dil (N2-only) at 6 meters: total runtime 142 (Vplan) or 144 (GAP)
switch to 20/80 dil (He-only) at 6 meters: total runtime 149 (Vplan) or 152 (GAP)
switch to 20/40 dil (50/50 N2 & He) at 6 meters: total runtime 146 (Vplan) or 147 (GAP)
switch to 100% O2 as "dil" at 6 meters: total runtime 136 (Vplan) or 138 (GAP)
switch to 90 O2 as "dil" at 6 meters: total runtime 142 (Vplan) or 144 (GAP)
don't switch dil, leave it at 10/52 at 6 meters:total runtime 146 (Vplan) or 148 (GAP)

So switching to air might save you a total of 4 minutes, which on an almost 2,5 hour dive to me is almost neglectible. Switching to 20/40 or 20/80 makes things worse. Switching to pure 02 gains you 10 minutes, BUT assumes you indeed have 100% all the time, which is quite unlikely, as discussed above.
If I assume that after a flush you'll only have 90% O2 in the loop or so, the advantage is only 4 minutes.

for me, it isn't worth the hassle. I'll just stick to my bottom dil, as it is effectively an 81% O2 mix anyway at 6 meters, and let the system do its thing.
But then again, I'm old & lazy.....

ciao,

Tino.

14th August 2008, 13:17	#1 (permalink)
Dave Faye Know the wreck below? Current Rebreather/s: Evolution Other Rebreather/s: Inspiration Classic Evolution Megalodon Join Date: Oct 2005 Location: Boston Posts: 169	02 Flushes and Changing Dil on your Onboard Computer When you do your 02 flush at 20 feet/6 meters after a deep dive with loads of deco, does anyone change the dil programed into their Vision or other computer to 100% 02 (or something similar). If not, are you just leaving the previous dil programed in. The issue is on deeper dives where you are coming off high helium mixes. You have 1-3 hours of deco in the shallows. You flush thoroughly and have no helium left in the loop. Now your only adding 02 for the remainder of your dive. what do you do? __________________ Dave http://www.Bostondeepwrecks.com http://www.Fayelaw.com Last edited by Dave Faye : 15th August 2008 at 00:54.
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14th August 2008, 13:30	#2 (permalink)
Stephane Stéphane Acounis Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Nantes - France Posts: 788	Re: 02 Flushes and Changing Dil on your Onboard Computer Hello Dave, Why would you want to change the diluent type when the deco is based on the actual PpO2? If at 3m your controller displays 1.3, you're on pure O2. You are not saturating helium or nitrogen at these shallow depths. __________________ Stéphane
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14th August 2008, 17:22	#3 (permalink)
Dave Faye Know the wreck below? Current Rebreather/s: Evolution Other Rebreather/s: Inspiration Classic Evolution Megalodon Join Date: Oct 2005 Location: Boston Posts: 169	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Stephane) Hello Stephane, Why would you want to change the diluent type when the deco is based on the actual PpO2? If at 3m your controller displays 1.3, you're on pure O2. You are not saturating helium or nitrogen at these shallow depths. To track my decompression more effectively. If the computer thinks there is helium in the loop, it will calulate your off gassing differently. If I am at 6 meters flush with 02 and now only have 02 and exhaled gas in my lungs and the loop, my deco time is different. If my dive computer still thinks I have a 10/50 or 8/60, etc in the loop, it will give me a differnt deco time. S.P. would not matter. My on board computer knows my pp of 02 but it does not that I am off my helium rich mix dil. I'm just curious what other do. It is really more relevant on dives where you have significant deco time in the shallows. If you change your dil from say 10/50 to 02 on the computer at 6 meters you will see the deco time drop on long duration dives. __________________ Dave http://www.Bostondeepwrecks.com http://www.Fayelaw.com
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14th August 2008, 17:52	#4 (permalink)
dave t RBW Member Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 674	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Dave Faye) To track my decompression more effectively. If the computer thinks there is helium in the loop, it will calulate your off gassing differently. If I am at 6 meters flush with 02 and now only have 02 and exhaled gas in my lungs and the loop, my deco time is different. If my dive computer still thinks I have a 10/50 or 8/60, etc in the loop, it will give me a differnt deco time. S.P. would not matter. My on board computer knows my pp of 02 but it does not that I am off my helium rich mix dil. I'm just curious what other do. It is really more relevant on dives where you have significant deco time in the shallows. If you change your dil from say 10/50 to 02 on the computer at 6 meters you will see the deco time drop on long duration dives. Are you talking about the vision computer? If you are then it will base your deco on the gas you have entered, the po2 that the cells read and ambient pressure. Because it knows all of that information it will know that if you have a po2 of 1.6 at 6m there will be no helium or nitrogen present in the loop (execept what you offgass and that is a tiny amount. If you are talking about a non-linked computer you are right it will not know unless you have changed it as it doesnt have all the info (no actual po2) HTH Dave
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14th August 2008, 20:27	#5 (permalink)
Dave Faye Know the wreck below? Current Rebreather/s: Evolution Other Rebreather/s: Inspiration Classic Evolution Megalodon Join Date: Oct 2005 Location: Boston Posts: 169	Re: 02 Flushes and Changing Dil on your Onboard Computer Are you talking about the vision computer? I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop. So, do you input your dil as 02 or just leave it on your last dil which is more conservative. __________________ Dave http://www.Bostondeepwrecks.com http://www.Fayelaw.com Last edited by Dave Faye : 14th August 2008 at 20:35.
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14th August 2008, 20:56	#6 (permalink)
dave t RBW Member Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 674	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Dave Faye) Are you talking about the vision computer? I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop. So, do you input your dil as 02 or just leave it on your last dil which is more conservative. yes it does know the pp of the other gasses because it knows the dil you have input and it knows the po2 and ambient pressure. the rest is just maths
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14th August 2008, 21:07	#7 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 430	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Dave Faye) I'm actually talking about both onboard Vision (or any other onboard computer) and your off board computer. If your running a 1.6 then the computer will know your mostly on pure 02. Running 1.6 in the shallows is not practical on long dives of course as you will have major cns issues. On long dives I run a 1.2 or 1.3 max at 20 feet/6 meters and 10 feet/3 meters. After I flush I allow my ppo2 to drop back to my lower setpoint. So, my onboard computer only knows what I tell it about my dil. It knows my ppo2 but not the remaining pp of the other gases in my loop. So, do you input your dil as 02 or just leave it on your last dil which is more conservative. Dude, thats pretty scary Anayway I'm assuming its misunderstandings through my written english, so here goes nothing. But if you have a deco-computer (Inline with Cells) or offline, and you do a O2-flush you should offcourse keep the diluent settigns for the computer. The computer will calculate the inert partial pressures of the loop based upon the PPO2 meassured (or set), and the makeupgas from the diluent. So to get "correct" deco you should offcourse still set the dilunt to what ever is/was in your loop. A couple of finer points: 1) When offgassing from Trimix one could argue that the PP_Inert mix is unknown, to counter this you could flush with diluent to bring back the reletive FN2 and FHe to known values - I dont think this is needed, but perhaps it is practised by some? 2) Setting you computer to a dill og 100% does not make sense unless the PPO2 is excatly the sorounding pressure (minus some water vapor etc). So if you were to set the dil as pure O2, then what happens when the loop drops (as you write), during a stop at constant depth? The fact that the PPO2 is dropping is an indication that the F_O2 is dropping proportionately, so some other gas is present in the loop - in order for the DeCo-computer to calculate this it needs to know what the inert-gasses are present, and more importantly in what fractions. I reality it does not matter for the Deco is you enter your Dil as Air, EAN32 or even EAN99, as this causes the computer to calculate deco with Nitrogen only as inert gas - again based upon the PPO2! YOu could also use different Trimix'es and get the same results: Tx50/25 or Tx40/30 or Tx20/40, would give the same decoprofile as the F_N2/F_He is the same. Some Examples: Code: Depth Dil P_Total PPO2 P_N2 P_He 6m Air 1.6 bar 1.0bar 0.6bar 0.0bar 6m Air 1.6 bar 1.6bar 0.0bar 0.0bar <-- After flush 6m Tx20/40 1.6 bar 1.0bar 0.3bar 0.3bar 6m O2 1.6 bar 1.0bar ???? ???? <-- Illegal some inert gas must be present... Did this clear up your question? Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear Last edited by Hanssing : 14th August 2008 at 21:13.
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15th August 2008, 00:40	#8 (permalink)
Dave Faye Know the wreck below? Current Rebreather/s: Evolution Other Rebreather/s: Inspiration Classic Evolution Megalodon Join Date: Oct 2005 Location: Boston Posts: 169	Re: 02 Flushes and Changing Dil on your Onboard Computer "I reality it does not matter for the Deco is you enter your Dil as Air, EAN32 or even EAN99, as this causes the computer to calculate deco with Nitrogen only as inert gas - again based upon the PPO2!" Agreed with air or nitrox the only other gas is nitrogen. The computer knows this is the only other gas and applies the remaining pp as nitrogen. "You could also use different Trimix'es and get the same results: Tx50/25 or Tx40/30 or Tx20/40, would give the same decoprofile as the F_N2/F_He is the same." This is my issue. If you are running a dil mix with helium in it and you flush thoroughly at 20 feet with 02 you have now removed the helium. The mix contains only 02 and exhaled co2, water vapor etc. If you don't tell the computer that the helium is gone, the computer sees the current p02 calculates it and assumes that the remaining pp is still helium and nitrogen. On a helium rich mix this will effect the way deco calculates out. So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone. Your loop only contains 02 and whatever is produced as a byproduct in you lungs as you matabolize the gas. Co2, water vapor, unused 02, etc. What do you tell your computer your dil is? __________________ Dave http://www.Bostondeepwrecks.com http://www.Fayelaw.com Last edited by Dave Faye : 15th August 2008 at 00:51.
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15th August 2008, 09:27	#9 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 430	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Dave Faye) So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone. Its not, nearly but some is still left. Besides this you offgas Trimix into the loop (Ie. the PPO2 decays over time). So some inert gas IS in the loop. Quote: Your loop only contains 02 and whatever is produced as a byproduct in you lungs as you matabolize the gas. Co2, water vapor, unused 02, etc. What do you tell your computer your dil is? IF the PPO is cloase to a pure O2-loop, then the computer will factor this in automatically. If it is far from pure O2, say 1.3bar at 6msw, then you need to tell the computer what the relative fractions are of F_N2 and F_He. You do this by setting the Dil. Again the fractions due to different speeds in offgassing, and amounts dissolved in the tissues (He is faster but less soluble) could over time deviate from the Dil mix. Therefor if you want to be really anal about it, you cold do a dil-flush periodically and then bring up the PPO afterwards. As I'm talking out of my ass here (I'm not a Tx-diver) - someone wiht extensive Tx-experience will hace to chime in if this is done in practice? COncerning running the loop on PURE O2 this never really happens, due to the fact that you offgass into the loop, and never really flush out all the traces of inert gas. Therefor you keep the Dil-setting and the Deco-computer will calculate just how much N2 and He is actually in the loop from this setting. If you flush the loop several times, and clean it out completely then the PPO wil indicate this, and the computer will therefor calculate with no Tx in the loop (In that case you could use any dil-setting - as there is no dill in the loop it would not affect the deco-calculations). If you want to REALLY do pure O2, then you should breathe it Open Circut and set the computer accordingly. Disclaimer: I'm talking out of my ass here (I'm not a Tx-diver) Regards Nicolai Hanssing __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear
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15th August 2008, 10:20	#10 (permalink)
Tino de Rijk Old, maybe one day wise Current Rebreather/s: Inspiration Vision Other Rebreather/s: Inspiration Classic Evolution Join Date: Mar 2006 Location: Amsterdam Posts: 375	Re: 02 Flushes and Changing Dil on your Onboard Computer Quote: (Originally Posted by Dave Faye) So my simple quetion is this. Nearly everyone flushes with 02 at 6 meters after a deco dive. Now all the trimix in the loop is gone. No, absolutely not "nearly everyone". In fact, I doubt if it is a majority. The advantage of doing that at 6 meters is minimal, because: a perfect flush, giving you REALLY 100% O2, is almost impossible, so you'll end up with around 90% or so. Leaving it at 1.3 and let the computer do its injection thing gives you 1.3/1.6 = 81% O2. It makes almost no difference in deco, while on the other hand raising the PO2 to above 1.4 does raise your risk on CNS toxicity, especially at the end of a long dive (with subsequent prolonged high PO2 exposure). So why do it? as explained by someone else here, your loop will "dilute" right after the flush, by a.o. your body gassing off inert gasses (N2 and/or HE) into the loop. So you need to repeat flushing multiple times to keep it high - which costs a lot of valuable O2. To make sure I wasn't talking bollocks, I ran a CCR profile through both V-Planner (+2 conservatism) and GAP (Buhlmann-GF-15/85). Here's the diveplan: bottom: 60 meters, 45 minutes, bottom & ascent dil 10/52 switch to air dil (N2-only) at 6 meters: total runtime 142 (Vplan) or 144 (GAP) switch to 20/80 dil (He-only) at 6 meters: total runtime 149 (Vplan) or 152 (GAP) switch to 20/40 dil (50/50 N2 & He) at 6 meters: total runtime 146 (Vplan) or 147 (GAP) switch to 100% O2 as "dil" at 6 meters: total runtime 136 (Vplan) or 138 (GAP) switch to 90 O2 as "dil" at 6 meters: total runtime 142 (Vplan) or 144 (GAP) don't switch dil, leave it at 10/52 at 6 meters:total runtime 146 (Vplan) or 148 (GAP) So switching to air might save you a total of 4 minutes, which on an almost 2,5 hour dive to me is almost neglectible. Switching to 20/40 or 20/80 makes things worse. Switching to pure 02 gains you 10 minutes, BUT assumes you indeed have 100% all the time, which is quite unlikely, as discussed above. If I assume that after a flush you'll only have 90% O2 in the loop or so, the advantage is only 4 minutes. for me, it isn't worth the hassle. I'll just stick to my bottom dil, as it is effectively an 81% O2 mix anyway at 6 meters, and let the system do its thing. But then again, I'm old & lazy..... ciao, Tino. Last edited by Tino de Rijk : 15th August 2008 at 10:23.
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