How to configure independent doubles

[MikeFerrara]

I have been diving independants for alot longer than I have been on scubaboard. This is the first time I have seen this thread.
Toodles you would be proud of me in class tonight.
student: so let's say you had a freeflow at the turn point of the dive and lost your left tank gas supply
wedivebc: no problem I still have 1/3 remaining in my other tank, enough to complete the exit point.
student: OK so what if I was OOA at this point you would not have enough to get me out
wedivebc: if you were diving independants you wouldn't be OOA

I would have explained to the student that the gas in that tank is still useable by feathering the valve even if the free flow won't stop. Featherg the valve is especially easy if the independant tank is sidemounted. An OOG diver could easily feather the valve on your back as long as you don't have any restrictions to go through. A little free flow is no reason to act as if the tank is empty (or to let it empty in the first place) if you need the gas.

 

[padiscubapro]

Is ANDI the only agency that teaches how to dive either independants or manifold safely?

Does every other agency say "independats suck" and not explain why?

Of the people that trash independats, who has actually tried diving in this configuration for an extended period of time?

I am interested in hearing from people that have dove both for more than a couple of dives about their experiences.

I have heard from too many people that repeat what they have read without trying it themslefs. [To this someone could respond "I have read that jumping off a bridge is a bad idea, that does not mean I am going to try it." Don't tell me that. Diving independants a couple of times is not 99.98% certain to kill you.]

ANDI does have a specific protocol when using independent twins for gas use and how to configure the gear..

When using independednt swins the diver's left cylinder has an Extra second stage on a long ose so that Gas can be shared in an emergency..

We teach start on right cylinder, use 1/3 of gas, switch to left cylinder use 1/3 of gas, switch back to right and use 1/3 by the end of this use you should be near your gas switch point in most situations.. if a failure was to happen at this point, you would still have 2/3 of the left cylinder with 2 second stages on it (this is the equivalent of 1/3 the starting supply) plus whatever is stil in the right cylinder (1/3), so if your buddy had a failure you should have enough gas for the both of you..

 

[wedivebc]

We teach start on right cylinder, use 1/3 of gas, switch to left cylinder use 1/3 of gas, switch back to right and use 1/3 by the end of this use you should be near your gas switch point in most situations.. .

Actually the protocol is 1/3 from the side with the long hose, (reg switch), 1/3 from the other side (turn dive) 1/3 from the same side (reg switch) 1/3 from 1st side, end dive. This method involves only 2 reg switches and is less task loading than the method you outlined. It may be different in the ANDI trimix manual but the guy who introduced the protocol to Ed, (Harry) teaches it this way.

 

[wedivebc]

I would have explained to the student that the gas in that tank is still useable by feathering the valve even if the free flow won't stop. Featherg the valve is especially easy if the independant tank is sidemounted. An OOG diver could easily feather the valve on your back as long as you don't have any restrictions to go through. A little free flow is no reason to act as if the tank is empty (or to let it empty in the first place) if you need the gas.

I agree but the whole point of this methodology is that a failure does not have to be life threatening if the diver is in a restriction or involved in an important task. I once saw a trimix student abandon his lift bag and reel when a gas failure was introduced just as he was preparing to deploy them. A highly unlikely scenario, yes but it was designed to show that diving independant can take away the urgency of managing a potential failure. Locating a guideline in a wreck is another example of a jeopordizing situation if you have to manage a gas failure. By the time the mass of bubbles have blown to the ceiling of a rusty old wreck how well will you be able to see the line?

 

[DA Aquamaster]

I was REQUIRED to have a two-valve manifold in order to get my cave certification.

I also dive with side mounts and do not have any difficulty monitoring the air in two tanks. I watch the gauges of course, but the tanks themselves remind me to keep them even. If I don't they will float me sideways.

The manifold keeps things simpler because there is only one gauge to watch. But, I don't think that the manifold is more safe.

I have had to shop around a bit to find an instructor that is ok with independent doubles in a given course. If they are not rigidly opposed to independent doubles they are just concerned, like they would be with any other tech diver/student, that your configuration is clean, well ordered and that you know how to use it. Agency acceptance is normally not the problem, but specific instuctors can be.

 

[padiscubapro]

Actually the protocol is 1/3 from the side with the long hose, (reg switch), 1/3 from the other side (turn dive) 1/3 from the same side (reg switch) 1/3 from 1st side, end dive. This method involves only 2 reg switches and is less task loading than the method you outlined. It may be different in the ANDI trimix manual but the guy who introduced the protocol to Ed, (Harry) teaches it this way.

No the standard is VERY specific..

The left post has 2 second stages.. This Is the tank with the long hose..

the right post has a single second stage.. We always start on the right post.. No exceptions..

The following is right out of our text.. If Harry taught you otherwie well he made a mistake..

Start Of Text (from wreck text):

The procedure is to begin the dive on the​

[font=TimesNewRoman,Italic]Right Cylinder [/font]until 1/3 of the gas​

from [font=TimesNewRoman,Italic]Right Cylinder [/font]is used. Then switch to the [font=TimesNewRoman,Italic]Left Cylinder[/font]. Breathe 1/3​

of the gas from the [font=TimesNewRoman,Italic]Left Cylinder[/font]. At this point you have used 1/3 of the ​

total gas supply, therefore this is the turning point of the dive. Switch to​

the [font=TimesNewRoman,Italic]Right Cylinder [/font]and continue to use this gas for the return (or to the ​

first deco stop). Should additional gas be required beyond this point, ​

switch backto the [font=TimesNewRoman,Italic]Left Cylinder[/font].​

If prior to or at the turn point, a catastrophic gas loss should occur
the diver will still have [font=TimesNewRoman,Bold]2/3 of either cylinder [/font]with which to breathe from.
Additional gas should not be required on a well executed dive. At this point
the team should be breathing surface Air discussing the great dive.

End..​

The same text appears in the ERD/TMD manual which Harry co-authored, so could you have mis-understood him?? There ARE questions on the final exam that you should have gotten wrong if you didn't understand this.

 

[scubafool]

Good thread, lots to think through.