SCR Gas Switching

bgpartri

I am in the process of implementing SCR support for the Pursuit, but I have some questions.

Do SCR divers switch gases during a dive?

If you do, how do you decide when to switch?

Do SCR divers use trimix?

I actually had a Dolphin about 10 years ago, but I suspect I am out of touch with today's practices.

Bruce

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EBT

The guys using scr for cave do bruce, both gas switching and trimix.

Its a Po2 dependent switch with a variety of different strategies. Probably the most common is switching based on the standard gases (billy deans, lately having acquired the DIR label)

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gerstl_ossi

Hi Bruce,
I use also a pSCR homemade. (not often as I dive more with eCCR with Shearwater electronic) Gas switching is a normal practice on this kind of rebreather. Every gas has a MOD deep, NX in shallow water, Trimix in deep water. Some DIR diver use the RB80 but they use no computers at all and have no O2 gauge on there units (Ratiodeco and Standartgases for different deep - and normally switch to open in in shallow part of the dive)


<edited out horrible table> - ebt

Cheers Ossi

PS: The Pursuit with software version 32 works great, in 4 weeks will be in the red see for 14 days so I will do a lot of dives. Let You know how it works.

gerstl_ossi

uuups the table copied from the EDO4 homepage in my first replay didn't work. I can't even edit it to make a correction, sorry, so here the table where You can see that using a pSCR in different deeps means using different gases with different MOD.

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bgpartri

Excellent.

Now that I know it's happening, I can make sure we do the right stuff.

The question I'm really trying to answer is:

When I am doing the TTS prediction, at what depths should I predict the diver is going to switch gases? This would also be the depth at which I would start flashing the mix to remind the diver to switch.

Since the FO2 would give a worst case for MOD, and you would typically like to switch as soon as possible, would it be reasonable to assume that the diver would switch when the PPO2 of the next gas is less than 1.6?

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bgpartri

Gerstl,

Your chart seems to imply that you would switch when the expected FiO2 is 1.3.

I am continuously calculating the FiO2 from the measured PPO2 and since the difference between FiO2 and FO2 is depth independent, it should be possible to calculate the depth at which the PPO2 would be 1.3 for a given FO2.

Basically, I am using the PPO2 and the depth to calculate the FiO2, then using the FiO2 and the inert gas in the selected gas to calculate the FN2 and FHe. I then use that gas as an OC gas to predict the ascent.

Does that all make sense?

Bruce

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gerstl_ossi

Hi Bruce, it's not my chart but from the guys who make the EDO4 pSCR Fonctionnement .I used it because the ratio of my homebuild is very similar to the swiss pSCR.
I said I'm not diving often my pSCR and I think that there are many people her on the forum witch knows much, much more about SCR. But what You said I think should work.

What I do (or did) in my dive is changing gas depending on deep.

Example:
Deep 40 Meter
NX50 MOD 21
change to 32/30
on ascent change to NX50 at 21 M
(all gas are respirable OC with PPO2 less than 1.6)
So the Pursuit should flash at 21 to indicate the gas switch to 32/30 and coming back from deep it should indicate the switch to the NX50 gas.

At 40 M the Pursuit should indicate a PPO2 something like 1,25 (1.25 / 5 Bar = 25% Oxygen) So the gas in the counterlung-bellow should be 1.25Bar Oxygen, and 3,75 bar inert gas (He/N2) - As the gas in the tank is TX32/He30/N38 the computer would calculate 25%(PP 1,25)Oxygen 33% He(PPHe 1,6544) and 42% N2 (PPN2 2,0955) . For the TTS it should use this for calculation and NX50 as OC gas, not knowing what the exactly inert gas would be from 21 to surface.

Example 2
Deep 55 Meter
NX50 or TX 50/25 MOD 21
change to TX 25/35
on ascent change back to TX 50/25 at 21 M
(all gas are respirable OC max. PPO2 1.62)
For long dive Oxygen at 6 Meter

Example 3
Deep 70 Meter
TX 50/25 MOD 21
change to TX 32/30 MOD 36
change to TX 21/55
on ascent change back to 32/30 at 36M
Deco 6 M on Oxygen

The Guys from DIR, as I know, use standard mix and switch deep and use to go OC in the shallow part in there slow ascent (on pSCR you have to exhale a lot from the nose in the shallow part on ascent to avoid PPO2 drop, because the expanding air in the counterlungs will not permit the injection system to work proper and to add enough fresh mix)
Deco Mix 6 M - 100% Oxygen
21M 50NX or 50/25
36M 35/25
57M 21/35

Bottom mix:
1-30 M 32 NX
33-45M TX 21/35
48-60M TX 18/45
63-75M TX 15/55
78 - 121 TX 10/70


and 50/25 at 21 M


...but as I said, there are many guys diving SCR here in the forum which can help You more.

Cheers
Ossi

kiwi_diver

Hi Bruce, I've contemplated diving my SCR with multiple mixes but have not yet done it. I believe I would use 1.4 as a switch point since that would give me a margin during which I might do a gas-switch on descent without a flush. With a 7 litre counterlung, it takes a while to dilute the loop. If I were to switch at 1.6, I think I would have to flush pretty fast.

I may never go there. But it's nice to know that you are including full monty SCR support in the Pursuit. Looking forward to air calibration!

Cheers.

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bgpartri

Air calibration is definitely part of it.

Bruce

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Good judgment comes from experience. Experience comes from bad judgment.

se2schul

All my comments are relevant to the RB80 (which is pretty much the only Rebreather I know, and I'm still pretty new with it).

Yes. It's not a constant PPO2 like CCR, but rather a gas switch which is more akin to an OC gas switch. On the RB80, you just take whatever gas you want to switch to, plug it into the switchblock, unplug the current gas, flush a couple breaths out your nose, and that's an SCR gas switch.

Same as for OC. You have bottles premixed with whatever you want. When you get to the appropriate depth for next mix, you switch as I described above. For example, I often use EAN50 for deco, and because I don't want to exceed a PO2 of 1.6, I switch to it as 21m, just as an OC diver would do.

SCR divers can use any mix that an OC diver can, including trimix.


FWIW, there is a drop in FO2 when on pSCR. At depth, the drop is about 2%, but it can be much higher in the shallows. For the most part, I just plan dives as though I'm on OC, and I don't really take the FO2 drop into account. If I were doing a dive where I felt that the FO2 drop would be a concern, I'd just pad my deco a little.

Hope that helps with your software.