Yet another twin question

[JohnK]

OK - so I got my twinset from Andy at Dalesdiver and he was great at explaining the set up and why etc. This is how it looks (viewed as if I`ve got them on)

Left post - SPG, Secondary reg to right, drysuit inflator to right

Right post - Primary reg, BC inflator to left.

One thing that I dont understand, if I isolate the LH cylinder or isolate at the manifold, how will I know what is left in the RH cylinder apart from adding another SPG.
I`m assuming that I cant reuse the LH cylinder due to SPG blow out

My answer would be abort and surface immediately (incl safety stops etc) Does anyone have any other views???

[Mark Chase]

Have you dived this rig yet?

If I put my secondary reg on the my left cylinder a la DIR the reg hose is too short to comfortably sit neutrol in the neckles. It's OK ish in the mouth but dont try to look left

As a result I have primary 2m hose on the left and secondary hose on the right (Left being my parallel to my left hand)

I have the presure gauge on the primary reg left side. There are a two reasions for this.

1: The inflator hose for my wing is on the left and that meens my dry suit hose comes from the right. If you put a small rubber loop on the end of the PG you can hold the gauge in position by putting the dry suit inflator through the loop then snaping it to the chest dump/inflator valve. End result is glance down and there is your gauge

2: I want to know the presure on the tank I am breathing

Apart form that its all bad news. I suspect your primary reg is the most likley to fail during a dive as it is the one in use. Then after shutting down you have no indication of remaining gas and therefore no indication of how much time you can take getting to the surface. I generaly dont worrey about this coz if I am doing Deco I will carrey a stage so I have some form of bail out and If I am not I recon I will make surface no mater what heppens. Knowing that I havent got enough air to make it, is not going to help.

So reality check.

30 -40m no deco dive on twin set. Lets say 35m for 30mins Nitrox 32 no deco. SAC 20 air consumption 2700ltrs at end of dive.  You have a failure in the 30th min of the dive and loose half of your reserve gas shutting down. That leavs you 1470 ltrs to get home. Lets say you cant re open your manifold to redistribute gas so you have 735ltrs left

Your at 35m so at 20 sac you have 8.17 mins to get the #### out of the water. Bearing in mind you have panicked lets say your SAC shoots to 40 you have 4mins to get out of the water bassed on a SAC of 20 (fairley average). Even with a conservitave 10m/min ascent rate you could do that. Taking into account the ever decreasing air consumption to the surface I would be tempted to attempt a 3m safety stop on the asumption if my reg stoped delivering air half way through I could fin up to the surface and manualy fill my BCD.

So the risk is farley small

If I were doing deco in the UK I would take a stage even if it was just a rich mix in a 3ltr. It can get you out of a hole heap of trouble. and even give you time to signal for a drop tank.

Having said all that I dive two presure gauges when diving independants and it is no problem. I have them tethered together across my midrif nose to nose and I glance down and there they are. No big deel just another hose to rout and another bit of kit to maintain.

Mark Chase

[Dominic]

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Originally Posted by [b

Quote[/b] ]One thing that I dont understand, if I isolate the LH cylinder or isolate at the manifold, how will I know what is left in the RH cylinder apart from adding another SPG.
I`m assuming that I cant reuse the LH cylinder due to SPG blow out

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The basic thinking is: If you have to isolate and you're unable to re-open the isolator for whatever reason, the dive is over and you're ascending.

At this stage, you either have enough gas to get you back, in which case you don't need a gauge; or you don't have enough gas to get you back, in which case you don't need a gauge.

There might be some psychological comfort in knowing you have enough gas, but practically, the gauge won't add anything: You either have the gas or you don't.

And don't forget, a lot of the time, people on the twin setup will be carrying a stage or two as well, so you only need the gas to get you up to the first stage depth...

However: Why are you assuming that a blown SPG means permanently closing the isolator? The high pressure hose doesn't leak terribly fast, it's very small bore. If you turn off the cylinder valve, no air will be lost, and you can re-open at your leisure. You still won't have a working gauge, but you'll at least have access to all your air..

[Ian Murray]

The only difference to your set up is that I have the drysuit inflator to the left (from the left post). If you then make sure that the connections to the wing and the drysuit are the same then the wing and drysuit hoses become interchangable for emergencies. You then only need to carry one low pressure hose in your spares box as well. If you get some of the ap valves low pressure connections then the hoses can be cut down to the exact size required.

You only need one spg and this comes from the left post. If you have a problem from behind the head and you can't work out what it is then you shut down the isolator. A quick glance at the spg will tell you which post the problem is with. If the spg is moving then it's the left post. If it's not then it's the right post and you act accordingly. The spg being on the left post can also tell you if you've rolled off the left post or forgot to open the isolator before you've got in (yes it does happen) as it doen't move during the dive (you're breathing the long hose from the right post).

If you have to shut down any of the posts or isolate the cylinders then the dive is over and therefore you don't need to know what gas you have left. You should have been planning the dive on the rule of thirds and therefore have enough gas to complete any deco requirements and get out ok without knowing what you've got left.

If everything doesn't go to plan and you manage to blow all your gas then you always have a completely independant supply at hand - your buddy, they are also part of your equipment and dive plan.

[JohnK]

Thanks folks - your replies confirm what I was thinking but its always good to get reassurance from experienced folk. Also thought that the HP hose would lose more air but again, thanks for clarifying things.

I`m not at stages (yet) - still got to get the hang of the twins in use etc but will probably convert the pony for shit or bust bailout.

Thanks again