Quote: (Originally Posted by
Audimaniac)
Hi all, I am very new to DIR and have my fundies booked with Brian in August. I have already got my kit and twins set up in acceptable DIR config and am happily getting used to it but I am constantly having arguments with one of my dive buddies about the correct set up for twins. Basically, without opening up a huge can of worms (hence posting here and not on YD) I need a definitive reason why we use twins fixed “upright” instead of inverted. The problem is my buddy is Ex Royal Marine and is a very accomplished recreational diver. He is also wanting to go down the twin route but insists that he will use his twins inverted as that is the correct way. He cites the Royal Navy divers as an example and also make a very good case for bio-mechanics (in that operating valves on inverts is far easier and “natural” for our limbs than with uprights). He states that entanglement issues are the same with both and that hoses will route over the shoulder in any case. I can’t seem to find a satisfactory and definitive answer to his question as to why DIR divers set them up they way we do and not inverted. I just need that convincing reason to give him.....Can anyone help? As with most DIR questions I expect the answer to be blatantly obvious once I know it and I promise to go stand in a corner with the Dunce hat on….!
Thanks 1) You can use "upright" cylinders with any size of tank. If you invert twin 18s, the diver will be massively top-heavy.
2) Doing an air-sharing exit through a restriction, you may be one behind the other. The recipient will be in front. (If the donor gets snagged/held up on something, the recipient is not going to out-swim his/her gas source. Put the recipient behind and if he/she gets snagged and the donor doesn't realise instantly, there is a high chance that the reg will get pulled out of the snagged diver's mouth and be out of his reach. If it's a tight restriction, the donor may not be able to turn round to get the reg back to the recipient. So the recipient goes in front). This therefore requires that the donated hose has sufficient length to run from the pillar valve along the full length of the recipient diver...which works out about 7' long.
This great length of hose could be stowed bungeed against the tank...which some will recommend and which works fine until it has to be deployed and ties itself in knots (seen it happen when an advocate of this method was trying to demonstrate its superiority). At which point you can't donate the full length and you now have a lovely set of loops sticking out from the side of your tanks...just right to get snagged. So we "wrap" the long hose. This takes up a good couple of feet running down to the waist and under the light/waist belt before it comes back up across the divers chest.
If you now invert cylinders you a) need an extra 2' of hose so that it can get from the tanks to the OOG diver and b) have lost 2' of "storage space". You've now got 9' of hose starting at your right hip...how do you store it?
3) If you get stuff tangled around your valves, it's easy for your buddy to see if they're behind your head. If the valves are behind your butt, your legs are in the way. Not insurmountable, but more of a PITA.
RN Ship's Divers work in very different conditions. They are, in general, shallow, no restrictions, no overheads (other than the bottom of the ship). They are (I think) solo and I'm not sure whether they use contents gauges or whether they still plan their dives with the isolator closed, opening it when the first tank is exhausted (50% left), then closing it, then repeating (25% left), then coming up. AFAIK they also use the same size of tanks every time. It's a configuration that works well enough for that particular environment. It doesn't scale up or down to other types of diving, though.
Your friend is correct in that it's much easier to turn valves on typical inverts. However, by taking a little time to set up one's kit (and it may take a few dives to tweak it), almost everyone can get their upright tank valves in a place where it is sufficiently easy to get at them that the embuggerance level is minimal. And you get the advantages outlined above. And there are probably others that haven't immediately come to my mind.
HTH.
Iain