SCR/VSC as a CCR Bailout Method

diverklondike

This is not directed at you personally or your post but in general I would suggest proper training versus thinking that you can learn what you need to know from a book...

Just my two cents...

M

Dave Sutton

Likely because very few people really understand what's going on. The most usable way is the VSC method that was well described by Will Smithers on the old Rebreather List. I use understanding of this as one of my metrics to feel for if a RB diver really 'gets it' as opposed to diving these things by rote.

Here is part of the discussion including a good spreadsheet for use:

Will Smithers VSC calculations


And here is a repost of Will's old Rebreather List post in which he describes it's actual use. It's a long post, read with care. But note the total gas used for the described dive.... and then tell me that this has no value for the *expert* RB diver.

++++++++++++++++++


Well, I finally had the opportunity to try out
Depth-modified Semi-closed operation (name
shortened to Variable SC).

For those who don't remember it, with VSC, rather
than doing a 5:1 breathe purge ratio like with straight SC,
you increase the number of breaths before purging as depth
increases. Generating the proper tables for depth/number
of breaths gets a bit complicated, and you can look
in the archives to find it, but the upshot is that you
can easily get a ratio of 75:1 at depth, provided you're
willing to put up with a varying PO2.

Well, About a year ago, I cut some tables for the various
ratios, depths, and gasses, and devised a rule of thumb for
padding my standard deco tables for use with VSC.

I practiced it repeatedly, and it worked pretty well,
*with the sensors on*, so I stopped trying to perfect
no-instrument CC mixing on the fly, which is tricky,
and adopted VSC as my bailout of last resort.

A few days ago I had the opportunity to use it in
a real situation (and as I'm writing this, it obviously
worked . Here's what happened:

I was on a 270fsw trimix dive looking for sea-caves, and all
was going well, when all of a sudden I heard a small pop.
To save batteries
(I only had one set with me on the trip), I was flying
"manually", so I flipped on the electronics and looked
at the primary electronics display. It was
telling me at least one sensor was out. I looked
at the raw PO2 display, and one sensor was showing
0.4, another .8, and the third was off
the high end of the scale! I'd seen very similar
behavior before when I had a total electronics flood,
so I assumed that's what had happened. A total pod
flood is the Achilles heel of the MK15.5, due to
the way the isolation of the secondary display works (BTW,
the same is true of the CCR1000, MK15, and BMR500).

The other handy item on my small set of laminated cards
is the PO2 of trimix 7/67 in 10ft increments down to
450fsw, so I did a diluent flush, checked the PO2, and
well, *none* of the sensors were right, although one
was fairly close.

I was diving with a couple of OC people on a deep
reef with no current and 150fsw of vis, so I'd opted
to not bring along full OC bailout - all I had was
an AL40 of trimix and an AL13 of air (for doing
a dil flush on ascent), plus my on-board gas.

So, at this point I could have bummed OC, but there
was no way I was going to stoop to sucking some
bubbler's hose for 90 minutes if I could avoid it!

So I shut off the electronics, took out my tables
and started doing Variable SC. I used my on-board 7/67 up to
100fsw, then purged to air, and used that up to
20fsw, where I switched to pure O2.

Upon surfacing, the bubblers didn't know anything had happened,
except that they'd seen me give the thumbs-up and stuck close.

Total SC gas consumption for the 90 minute bailout:

5 cu ft trimix
5 cu ft air (this includes the purge at 100ft, which was most of gas used)
(O2 was done fully-closed).

The AL40 of mix wasn't touched.

As it turned out, the electronics didn't flood - that small pop
I heard was an O2 sensor blowing it's electrolyte all over the
inside of my loop. It's still unclear why this affected
another sensor on the analog display. An interesting side-note
is that the effect of the electrolyte being gone was that
the sensor pumped out the equivalent voltage of a 2.2 PO2.
This continued on the surface, gradually tapering off to
nothing as the sensor died over the period of a couple hours.

All my other sensors of that type are being used in the
sensor burn tests, so I ended up diving for a couple
of days on just two sensors. Happily, the digital control
electronics worked just fine with this configuration.

I've been meaning to get to it for some time, but I guess
I really should get around to redesigning the original
isolation circuits of the unit.

-Will


++++++++++++++++++



Dave

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__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

undrh2o

Found this in the web archive at (http://web.archive.org/web/200112061...wsletter3.html)

It's from the journal of the Australian Amateur Rebreather group. It gives some calculations behind the theory.


Manual Semiclosed Operation
by William Smithers 1998

The following algorithm assumes one exhale-through-the-nose, inhale-through-the-mouth cycle, instead of assuming a full loop purge.

The trick here is that you have to know the ratio of a single deep breath to your *total* loop volume, including the volume of hoses, intergranualr space, the whole nine yards. There's two ways to do this, 1) flood your entire loop with water, while scrubber is installed, drain it into a pail, and measure the amount of water, then figure out the volume of a big exhale, or 2) see how much one purge cycle affects a known loop PO2 as measured against a full loop purge.

So the method is:

I = V*B*((((D/33+1)*F)-M)/R)

Where:
I = Inspirations between loop flushes.
F = FO2 of your mix.
R = Max measured O2 metabolic rate in PO2 per minute
B = Breathing rate at R
M = Minimum acceptable PO2 at any point.
D = Depth in feet seawater
V = Ratio of a single breath to total loop volume.

Here's how I calculated V. Using my EANx70 dil, I filled the loop, then breathed it down to a .52. Then I exhaled one breath and squirted in one breath of diluent. The loop recharged to .595 PO2, after quickly hyperventilating a few times to mix the gas.

That's .105 PO2 below what a full flush would have done, which works out to be about 58% of the expected PO2.

In other words, V = (g-a)/(g-d)

where:

V = breath to loop ratio.
g = PO2 of diluent you are using in the test. (.7)
d = How far you breathed the PO2 down before stopping. (.52)
a = actual PO2 in the loop after depleting from g to d and recharging with a single breath dump-and-pump. (.595)
I did this a couple of more times, and the results came out between 58 and 68%, so I called it 60% to be conservative.

So V got set to .6. Obviously, it's best to do this while at rest, so that you're metabolic consumption won't skew the results.

Plugging this in, and setting a .6 PO2 as the minimum acceptable PO2, I calculated that I should get 30 workload-independent breaths between single-breath purges at 1 ATA on EANx70.

The figures looked good, but thinking I might be off on a bit of an intellectual bender by this point, I hit the treadmill for two 30-minute on-the-loop sessions with EANx70 dil. Each session started with slow walking, worked up to full-sweat running, then back to walking again for the last five minutes. I checked the PO2 just before each 30-breath purge, and lowest PO2 I saw in either session was .58, and the highest was about .63 - with the average spot-on the target zone at .61.

Needless to say, two tests at 1 ATA with a .1 PO2 range are nowhere near conclusive, but it's looking pretty good so far.

For comparison, here's the new EANx40 table:

Depth Breaths before single-breath-purge
----- -----------------------------------
10 66
20 72
30 110
40 145
50 182
60 218
70 255
80 290
90 327
100 363

(Editors note: Authors procedures and calculations not checked by the AARG, use at your own risk)

decoweenie

Can you expand on that method ?

Hanssing

I dont see why we're making this so complicated.

Scenarioa a) Loss of O2 addition to loop.
Run SCR with constant PPO based upon depth and available diluent.

Scenario b) Loss of PPO-monitoring
Run SCR with constant FO2 based upon available diluent (easy)
.. or ..
Run SCR with constant PO2 based upon available diluent (harder)
.. or ..
Run SCR with fixed ratio (easiest)

Constant FO2 is achieved by scaling no. of breaths to absolute pressure:
EAN28 3:1 @ 0msw
EAN28 6:1 @ 10msw
EAN28 9:1 @ 20msw
EAN28 12:1 @ 30msw
EAN28 15:1 @ 40msw
This means the absolute flow into the loop is constant (like a ray/dolphin) and therefor you get a "constant" FO2 in the loop. Selecting the base-rate depends on metabolism, likewise the resulting FO2 - testing goes a long way.
For mCCR with a working O2-addition (easy to check), the FO2 will not drop nearly as much, and the rate can be exstended further.

Running constant PO2 without electronics is harder. Some claim to be able to do this on boyancy (consumed O2) alone for fixed depth even at 40msw.... Alternatively you can use the referece Dave pointed out. I however see this as a more dangerous, and if OC-bailout is planned, then the added risk of f**king up the count etc, does not seem worth it. If your'e able to SCR (in any form) you have plenty of gas, even with a marginal OC-bailout.

Running fixed SCR-rate gives neither constant FO2 nor constant PO2, infact the Rebreather is working like a RB80. This means increasing FO2 with depth, and more wastefull at depth. Again a perfectly good way if marginal OC-bailout is available. One should be carefull at shallows, so the rate does not get hypoxic.

Scenario c) SCR'ing on a dodgy scrubber.
Here the diver has access PPO-control, O2-addition and nearly a ***tioning Rebreather. Loop is run as constant PPO2, but with a high loop-flush frequncy. The intrest here is to keep the scrubber working, wich means lower depth if possible and being acutely aware of possible signs for it failing completely, this incedently means low taskloading....

An interesting question is: Can you SCR on a completely dead scrubber? Ie. can you run 1:2 ratio, and cut your bailoutgas-use in half?
Or even 1:1.5? Would still cut your consumption by 33%.

As a wreck-diver I dont think I would even try this unless at the upline, perhaps even not until at deco.

Nicolai - Who needs to do the walk, instead of talking the talk

__________________
Woohooo - I can change my rEvo!
Its going to be bitchin' tricked out piece of gear

decoweenie

No big deal, I was just curious and didn't want to miss out on any earth-shattering new ideas...

fireman

Bear with me here:
I'm sure there is a very analytical way to do this, a matrix with probable faults, their chances and their consequences. And if I was going to do anything at the ragged edge I would want to do it. In the absence of this we could do "gross overkill" ala WKPP. Or we could take a best quess and make a calculated risk. The problem with the first is its very expensive, the second is you are accepting some (unknow) risk of death injury.
My analysis runs thusly:
I want to think of all single events which might kill me and have a plan. I want to have a plan for any two events that are dependent or likely and could kill me.
The question that I would like to address is "minimum decompression" or maximizing your chance of survival when you have gas limited decompression. I was trying to force V-Planner to give omitted stop info and couldn't.
How can you best compensate for blown deep stops, if you have surface supplied gas at some level. How much shallow deco can you blow given the availability of a chamber x hours away.
This would have a direct bearing on the preceeding discussion. For example: Say you have enough gas between you and a buddy to make all your stops. Event one puts you on OC, and you and your bud towards the surface. Event two takes away half of the gas needed to complete your deep stops. Clearly more info is needed to solve this and after you have survived 1 min and 10 min, more data is required, some of which is what is the optimal way to truncate your deco.
Or you just have your gas caddy bring you a couple of 80's of bottom mix. and a dry martini. (Please no infinite resource solutions)
Oh and I realize that if you have your PADI get out of death free trimix CCR ultra deep card this wouldn't have happend to you in the first place...

__________________
Heres to you Capt. Bill
Never Forget, and stay safe everyone.

Dave Sutton

Nonsense.

The "survival objective" is to *stay alive* and not drown. Letting the PP02 slide down to 0.6 as the low end PP02? Damn.... I'd let it slide to 0.2 if it was either that or drown.... If you use reasonable PP02 limits, the deco will sort itself out. And, uhh..... my guess is that most of the guys at this level are using PP02 sensing computers of some sort. Run a few scenario tables, have a look at them for gut-check 'reasonableness', and then print them if you feel like it. I'll take my chances on a gut check of what my CCR planned deco 'was' as compared to my VR-3 that's seeing the actual loop gas on SCC bailout.

Actual milage may vary. Don't use this forum as a substitute for THINKING about it and PRACTICING IT YOURSELF.


Dave

__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com

Brainx3

Hi,


I absolutely agree with you, Dave.

Fly safe


Best Regards. Wael

Dave Sutton

The point is to be able to deal with loss of electronics or 02, and to be able to play "all the cards" on the intellectual table based on firm understanding of the physics and physiology.


But, to place us all on the same sheet of paper, why don't you share "Cedrics Technique" since I'll be pretty unlikely to buy his book... . My guess is that there's nothing new under the sun and I really doubt it varies much from the methods we were discussing on the NWD list 10+ years ago. If it's not on the table for us to discuss, we're shooting blanks here.


Best,


Dave


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__________________
"Silent Diving with No Bubbles and No Politics".... www.nobubblediving.com