Pony or Octopus

[Jay]

Ammers - just for a bit of clarification, I dive with a 15l steel cylinder and a side slung 7l steel bottle on a BCD. I am working up to a twin set and I understand your argument that if you need a side slung then you should really be on a twin set and I agree with you so far as if you are using your side slung as a secondary source of gas for 'extended bottom time'. Is there any problem with me using my current system for diving to 35m max (using only my 15l for bottom gas - nitrox blend to suit) and then using my side slung with 40% for accelerated deco / safety stop off-gassing?

My rational is:

1. I plan my dives to always be within the range suited to my main nitrox mix and always with sufficient gas reserves in my main cylinder to complete the dive.
2. When I dive with my computer / tables, even though I finish the dive off on 40%, my dive plan is still base on the lower nitrox mix (so I am not cutting corners on my deco / safety stops but am buying additional safety
3. I have re-adjusted my weights to take account of the extra weight of my side slung (and have checked my BC has sufficient lift capacity at depth)
4. I am constantly aware of the max ppO2 limits and keep within 1.4
5. I am aware that in the unlikely event of having to use my side slung at a ppO2 greater then 1.4, it is NEVER greater than 1.6 and I would always immediately be ascending to within limits within 1 - 2 breaths?

Your input please.

Regards

[Ammers]

Aloha.

Couple of points. I don't know what courses you have done, I assume that to be doing deco stuff you have done a course such as TDI Deco Procedures. Breakdown as follows..remember these are only my opinions and as such you might think them to be complete nonsense.

TANKS

If you are using a 15l tank as your bottom/travel gas (not sure what mix you're using for 35m, probably EAN32%) then you have no redundancy at this depth because you can't breathe the 40% 7l bottle - that would equate to a PPO2 of 1.8. So if you get a blow out at 35m, your only option is to ascend to the depth where you can start using your deco bottle and extend the time.  You could fit a J valve and have two regs on the 15 bottle but you only have one o-ring and one tank and one gas supply in there. All potential failure points.
If the 7l bottle goes bang then you would have to deco on your back/bottom/travel gas, thereby extending your deco time because it will be a weaker mix than the 40% bottle. On this point - this is probably irrelevant but if it's a really long bottom time on the dive then you have to consider the capacity of the 7l bottle to complete full deco schedule and you also have to consider the amount of remaining back gas that you might end up decoing on in the event of the 7l dying. So there is a remote possibility of running out of air and not being able to ascend! Remote, I know, but possible. I like to be aware of EVERY possible scenario that could occur and then mitigate the risk.
On the PPO2 front, I would plan the dive to try not to exceed a PPO2 of 1.2 if conditions are strenuous or extra cold. Exertion and cold are factors which can increase the risk of a DCI hit and O2 tolerance is unquantifiable on any given day. Therefore 1.2 is a better safety margin because if you DO accidentally exceed, you have more to play with than if you are planning at 1.4 or indeed 1.6

On your point "I am aware that in the unlikely event of having to use my side slung at a ppO2 greater then 1.4, it is NEVER greater than 1.6 and I would always immediately be ascending to within limits within 1 - 2 breaths" - if the max depth for your dive plan is 35 and you plan to hit 35, (some people plan by adding a few metres on), if you hit 35 m and had to breathe from the deco bottle at 40% then the calculation is .40 multiplied by (35 divided by 10 + 1) bar, i.e. .40 X 4.5 = 1.8. PPO2 should not exceed 1.6. If you exceed 1.6 even on a couple of breaths then you run the risk of oxygen toxicity - nobobdy can predict how you will react on a few breaths past accepted PPo2.

40% isn't a great choice for deco, really 50% would be around the minimum. Perhaps you are using 40% as that is the max you can use with basic Nitrox ticket? Not sure if you have done Adv. Nitrox or not?

RIG/SUPPORT/REGS

A BCD isn't ideal for the kind of weight associated with a 15l and 7l bottle. Perhaps it is one of the bigger Buddy BCDs in which case you would only just get away with it. Apart from the lift issues, the weight could strain the BCD. Also, the camband is a potential failure. If it's a wreck dive then regs with a strippable face plate and hand tight are a good idea in the event of debris entering the second stage - it's likely that you only have one reg on each tank therefore you have no redundancy should one fuck up and you have two different bottles with differing depth considerations. Not to mention laying a line. etc etc blah blah please stop droning on now!

Hope this makes sense, for that kind of dive I woulnd't touch it without twins for the travel gas and a stronger deco mix. That way if the deco bottle dies you have more back gas to play with in the associated longer deco schedule as you're then on a weaker mix.

One other point, I would have tables and a spare watch so that in the event of your computer failing you can still calculate your bottom time and work out your according deco obligation. Also, a spare mask is a good idea in any dive with a "ceiling" i.e. you can't directly ascend to the surface. Helps to see deco time!

Hope that's not too much waffle.

(Edited by Ammers at 11:14 pm on Mar. 13, 2002)

[Jay]

Cheers for that - to clarify a few points, yes I only have basic nitrox hence the 40% max. When I talk about deco, I mean that I have only strayed into deco though spending an extra minute or so at 35 mins that accumulates 7 mins or so deco - this was wiped out by ascending and continuing the dive on a shallower profile (which almost immediately wiped the deco requirement). I do not have any TDI certs and therefore have no official deco training (a no-no I know!) but I NEVER plan any intentional deco dives.

Good points raised though about the risk of o rings bursting etc. I do have an octo attached to my primary 1st stage so could use that in an emergency (given that it is a 2nd stage prob and not a 1st stage freeflow). It was based on a dive I completed in Jan where I 'touched' deco for the first and only time. I will be going 'twins and wings' ASICAI!! (As soon as I can afford it)!!

Cheers

[Ammers]

Noworriesmate! Sadly I can't afford a rebreather or the course or I would be going down that route. Prices mental!
If anyone wants to contribute to the "Ammers wants a rebreather" fund then make cheques payable to...

[Dominic]

A Rebreather? Surely you're not going over to a YBOD? ;)
Where r u getting your figures for stab jacket lift? The only wings I know of that have significantly more than an average-sized Buddy are the bungeed 100lb ones. I take it those aren't the ones you're recommending?
Incidentally, in JJ's DIR book, even JJ himself states that one or two breaths of a too-rich-Oxygen mix isn't enough to cause toxicity. So long as you headed instantly up if you switched to a high Nitrox mix, you'd almost certainly get away with it. I wouldn't want to gamble it myself tho..
(I'm reminded of a guy I know who uses a single 15, a pony and a 7l deco cylinder on deep dives - if his primary goes POP, he uses the pony to get himself to the MOD of his stage. He didn't mamage to convince me that I should go that route rather than using my twinset ;o)
I'd agree that you need 50% plus for deco - did you see the latest answer to your O2 question on UKRS?

[Ammers]

Hi Dom. On the lift thing, I don't recommend a BCD for this amount of weight, Heads is moving towards the wing route, so I mentioned the Buddy jackets as an example that he MAY get away with. Obviously it would be pertinent to check this out with whoever manufactures his jacket if it's not a Buddy.
Good point about JJ - but a book can't quantify or take into account what factors you have introduced to your dive on a certain day. Throw some tiredness into the mix or cold and that few breaths of a too rich mix at the wrong depth could have a different effect today than tomorrow and a different effect on Tom than Jerry. Also it's reliant on having no stress/other things go wrong and actually managing to get up to the right depth. So I agree with you - I wouldn't want to gamble on it myself!  The initial symptoms of ox tox include the "ultraviolet" visual disturbance and or auditory disturbance although there are others. The very second that the ultraviolet feeling comes on, the person has literally seconds to get themself up to a better depth before they convulse.
Haven't had a chance to look at UKRS, will check it out, cheers.

[Hobby]

Hi,I'm skipping back to page 3(ooer)to the discussion between Heads Up and Divedog.Divedog's right.The reason why valves/regs were mounted between the shoulders is a throwback to the old twin hoses which did'nt work well any other way really.As gear developed industry,military and others(?)who use BA etc.developed into the inverted position after much R&D etc.The Amateur field remained with their gear the wrong way up and continue to do so.This is reflected in valves etc.If you look at an industrial valve eg.a Sabre as used by RN Fire Services etc.you'll see they're designed to take the weight of a twinset on their rubber knobs.In the valve is a filter tube going into the tank to prevent ingress of debris in any position.Here's a poor link to illustrate some of this. http://www.sabrenett.com/centpaq1.htm#cylinder. Cheers.

[Hobby]

Sorry,I can only do short posts!Anyway,it follows that yes a "slob-winder" is indeed another pressurised component and is thus a potentisl failiure point of the worst type - and you don't need it really if you config.as inverted.A slob winder is like ankle weights,in that it's something born out of compromise for inefficient design or config.Not because it's actually good gear! A question I feel we should sometimes consider is:-If the pros etc.do it differently to the amateur field then why do we do it differently? With inverts should we ask "Why don't we invert?" rather than "Why should I?". I can try and get the articles on Octopus rigs if you're interested. Regards,Hobby.