trimix on scr

jacquesm · 9th April 2008, 06:11

Hello everibody,
I am a new incomer, but had a pair of Trimix dives using an SCR, Azimuth in these cases. And I set an article on such "deep SCR ?" on a french forum. I"ll try to sum it up here.

Basically to be safe without complex computations, your max depth is directly deduced from the bottle content, same way than OC : if you decided not to go over 1.3 ATA you will reach 50 meters with à 22 % O2 mix, never mind the rest of it being N2, He or any mix. The deepest you dive, the poorer your mix.

Using an SCR with the same mix and flow rate, you then will be limited on the shallower part of the dive by the need to keeep enough O2 in your loop. The poorer your mix, the faster you inject.
Let say that you take the french legal limit of 0.16 ATA (yes we have a law on this) and want to protect yourself in case of heavy efforts near the surface, rising your metabolic consumption to 1.2 l/min, that make you compute the flow rate (SCR formula), here about 16.5 l/min. nearly as much as OC ... at surface.

You may of course freely change the input figures above to test your case, but somewhere between 50 and 65 meters you will find the needed flow rate raise dramatically, negating any benefit to rebreather.

This led to a new need : alter the flow rate between shallow and depth, e.g. with double injection (manual switch) of Azimuth AF, or manually reduce the injection to reduce it at depth ... but then why not also upgrade your mix taking a richer one, or even pure O2. Oh, that needs a diluent add ... and you specified an mCCR.

The trouble to use an SCR at big depth is not He, it is the SCR. If you want to ascend to surface with it, on same settings.

**Freef** · 9th April 2008, 14:42

Quote: (Originally Posted by jacquesm)

Let say that you take the french legal limit of 0.16 ATA (yes we have a law on this)

How does that work with OC or dil? Min 16% O2 in any mix? naximum dive depth of 80 odd metres?

jacquesm · 9th April 2008, 18:54

The mix you breath at each depth must have more than 0.160 ATA O2 in it. That can be a 17% near surface, or a 4% mix below 31 meters.

Another article limits the max O2 you breathe, here to 1.60 ATA max.
So playing open circuit you may dive very deep but must change your mix while descending or ascending. In fact this a common practice in Trimix dives worldwide, and is the basis of CCR diving.

**Freef** · 10th April 2008, 18:40

I did wonder about the limits, thanks for clearing that up.

9th April 2008, 06:11	#21 (permalink)
jacquesm New Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jan 2006 Location: Meudon (near Paris) Posts: 12	Re: trimix on scr Hello everibody, I am a new incomer, but had a pair of Trimix dives using an SCR, Azimuth in these cases. And I set an article on such "deep SCR ?" on a french forum. I"ll try to sum it up here. Basically to be safe without complex computations, your max depth is directly deduced from the bottle content, same way than OC : if you decided not to go over 1.3 ATA you will reach 50 meters with à 22 % O2 mix, never mind the rest of it being N2, He or any mix. The deepest you dive, the poorer your mix. Using an SCR with the same mix and flow rate, you then will be limited on the shallower part of the dive by the need to keeep enough O2 in your loop. The poorer your mix, the faster you inject. Let say that you take the french legal limit of 0.16 ATA (yes we have a law on this) and want to protect yourself in case of heavy efforts near the surface, rising your metabolic consumption to 1.2 l/min, that make you compute the flow rate (SCR formula), here about 16.5 l/min. nearly as much as OC ... at surface. You may of course freely change the input figures above to test your case, but somewhere between 50 and 65 meters you will find the needed flow rate raise dramatically, negating any benefit to rebreather. This led to a new need : alter the flow rate between shallow and depth, e.g. with double injection (manual switch) of Azimuth AF, or manually reduce the injection to reduce it at depth ... but then why not also upgrade your mix taking a richer one, or even pure O2. Oh, that needs a diluent add ... and you specified an mCCR. The trouble to use an SCR at big depth is not He, it is the SCR. If you want to ascend to surface with it, on same settings. __________________ See you soon under surface Jacques
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9th April 2008, 14:42	#22 (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 jacquesm) Let say that you take the french legal limit of 0.16 ATA (yes we have a law on this) How does that work with OC or dil? Min 16% O2 in any mix? naximum dive depth of 80 odd metres? __________________ David. Currently owner of two differently sized ankles.
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9th April 2008, 18:54	#23 (permalink)
jacquesm New Member Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Jan 2006 Location: Meudon (near Paris) Posts: 12	Law and PpO2 (not FiO2) The mix you breath at each depth must have more than 0.160 ATA O2 in it. That can be a 17% near surface, or a 4% mix below 31 meters. Another article limits the max O2 you breathe, here to 1.60 ATA max. So playing open circuit you may dive very deep but must change your mix while descending or ascending. In fact this a common practice in Trimix dives worldwide, and is the basis of CCR diving. __________________ See you soon under surface Jacques
(Offline)

10th April 2008, 18:40	#24 (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 I did wonder about the limits, thanks for clearing that up. __________________ David. Currently owner of two differently sized ankles.
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