Open circuit at first sign of trouble?

schford

Having read fairly extensivly about rebreathers, modus operandi, theories and training techniques I have a question and a poll.

Why do Training agencies teach O/C as the solution for a problem with MOD 1 and then change the solution as you advance...

Surely you want muscle memory to kick in if you have a problem - especially at deeper tri mix depths when you have incurred serious deco.

Open Loop - would seem to be a much better solution, with, in my view the following benefits.

1 - As a beginner if you come off the loop you are going to think of it as the death loop and not want to go back on there - potentially seriously increasing your in water time if you had say just 10 mins deco on CC this could increase substantially depending on your bail out gases.

2 - As an advanced diver you want muscle memory to kick in and not start going open circuit and then thinking oops not supposed to do that any more.

3 - It is much much quicker to go Open Loop than Open circuit - crush counter lungs purge and then breathe in through mouth out through nose letting ADV do work - potential life saver this one lets just say you have a CO2 hit and were breathing so fast you just could not go Open circuit without taking a mouthful of water.

4 - It can save lives especially on SCR - time taken to go from on the loop to OCR can be the time it takes to die.

5 - Its a one stop solution, assuming you have the right diluent mix for your dive, Open Loop will give you time to analyse the problem - It works for Hyperoxia, Hypoxia, Hypercapnia then you have time to decide best solution, be it continue on Open loop, go SCR or move off to OC if for instance you think scrubber has gone completely west...

I know this might be contentious but I think its good to debate these issues.

Stuart

ROB DAVIE

Stuart,

Actually, while it is a good discussion to have, I really don't believe it IS contentious.

In another discussion, I thought there was a mis-understanding about intent when it came to OC and remaining on the loop, but it turns out that it was only terminology.

As Phi and Stefan said, abandoning the loop for any little problem is counter-productive, and constitutes bad practice. As Seb said, going OC and remaining that way is using OC thinking on a CCR, when we should be using CCR thinking on on a CCR. These are excellent points all.

The point of using a momentary bail-out to OC as the first part of your procedures when something goes wrong is simple. Given the subtle nature of some of the problems that can occur with our magic boxes, the "Best Practice" answer is to switch to OC, and get a nice fresh breath of sweet gas to clear our heads. Then our carbon-based life-form unit logic circuits can kick in, and we can analyse the problem, and take steps to cure it. Failing that, we can resort to non-standard operations procedures (manual injection, SCR procedures, valve control, etc.), and, if the unit remains within tolerances, continue on our merry way.

So, it is my firm belief that this constitutes the safest practice. Now I also am a firm believer in having a good DSV, so that your bail-out, whether temporary or not, is easy and smooth.

That last is just my PPO2's worth.

Cheers all!

Rob

schford

Good post Rob -

Although I am of the opinion that going open circuit is not as simple as going open loop. I also do not think it is the safest to teach.

You need to - Pull our your O/C bail out, make sure its on, close your DSV, replace DSV with O/C, purge reg with purge button.

If you had a bad CO2 hit which you had not noticed due to increased work rate - then your rate of breathing might be such that you could not hold yoru breath long enough to switch.

Where as Open Loop is just, Crush and Purge CLs in one movement and breathe.

Stuart

Mdemon

Nice post Stuart - I tried to green blob you for it! (Given you too much apparently???)

It's nuts that MOD1 doesn't cover open loop - in fact I've heard MOD3 not covering it in some courses.

I think instructors should assess the equiment a user has/is intending to get and adapt accordingly. VERY good points on muscle memory BTW.

I now have the OC/DSV, but still plan and and have trained to use the ADV as a first resort onto open loop... Still testing though. Will post in v. near future with pictures all my new hardware toys...

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ROB DAVIE

Stuart,

You are right. That is certainly quick, and it does need to be taught. Just reaching up and flipping the switch to OC on your mouthpiece is even quicker though!

Rob

schford

DOH - got to give you that one!

Stuart

nl_carey

I'm pretty new at all this (so perhaps those of you with advanced training have more info), but my opinion is that if it's anything really serious (more than a little bit of gurgling or some bubbles for example) then get the hell off the loop, quick as you can. Don't neccesarily rely on your Rebreather sustaining you to get a bit of a deco advantage. That's what your bailout gas is for, right?
Then, after catching some sanity breaths, checking guages and instruments, you can tentatively get back on the loop if everything looks ok. Perhaps even doing this at a shallower stop if the suspect is the scrubber. At the very least, if it's CO2 related, it might give the scrubber a chance to 'catch up'.
Making this easy was a prime concern when I got my Rebreather, which is a major reason why I bought a KISS. If I had bought any other RB, I would definitely have been getting a KISS OC/DSV and plumbing it in. I can definitely understand the reasoning behind not wanting to get off the loop due to time constraints - so I never wanted to have to make that choice.

Andy M

I agree. Thats another reason why i bought a KISS. Any problems that might be due to a bad loop, turn the knob, get some nice clean known good gas down you, check everything out and go back on loop. If any dud scrubber symptoms reappear SCR your way home. I assume SCR is what is meant by "open loop"? dump CLs out your nose every few breaths?

Andy M

MOD 1 -can- cover open loop, but obviously that depends who your instructor is... Is there a prescriptive list of skills issued by each of the agencies on exactly what should be covered during a course? I've never seen one. The manuals are crap and have no mention of anything other than very basic techniques in them.

caveseeker7

Flying a CCR as an SCR means dumping every 4th to 5th breath, depending on who you ask.
Open loop refers to dumping every breath, you basically turn the ADV into your 2nd stage (or refill the loop manually).

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Cheers
Stefan



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