Diving with Independent Doubles

[LiteHedded]

seems like a reg failure would cause you to lose half your gas right away since you can't shut that valve down and access the gas via a manifold

 

[DA Aquamaster]

Correct. That is the downside of independent doubles - if the reg fails and you have to shut it down, you lose access to the gas in that tank - which is why the gas management strategy outlined above is important as it ensures you always a reserve in the other at least equal to the gas you used going in.

The upsides are that failing to close a valve or isolator does not threaten all your remaining gas, and the tanks are fully redudant with no risk of losing all of the gas in both tanks through an isolator/crossbar failure.

 

[hobodiver]

So how do you route the hoses for independent doubles if they are back mounted?

 

[DA Aquamaster]

I used to route them the same way I would with manifolded doubles. The only difference is the second spg. I routed it along the right shoulder to just above the d-ring. It kept it free of the long hose, can light etc.

 

[Doubler]

When I first started cave diving in 1980 my buddy and I used the independent system which we were taught was called the "European System". We were also taught to breath down each tank 100psi at a time until thirds were reached. This didn't make sense to me then nor does it make sense to me now. You really couldn't enjoy the dive because you were constantly checking your SPG's. We switched to manifolds about a year later and things were much simpler there after.

 

[ianr33]

100 psi is overdoing it! With a 3600 psi Cave Fill I will typically switch regs when one tank is around 600 psi below the other. i.e breathe first tank down to 3000,switch. Breathe second tank to 2400, switch and so on.
Thats all of 4 reg switches on a dive planned to turn at 1/3rds. Maybe one switch every 15 minutes at 100 feet.
Many sidemount divers have the spg's pointing forwards from the tanks. The gauge can be read simply by glancing down.Checking it every couple of minutes takes 2 seconds.
If I am in a spooky cave then I will switch more often,but certainly not every 100 psi.

 

[DA Aquamaster]

4 reg switches is still overkill.

Look at the diapgram DaleC has as a visual aide.

If you use 1/3rd from one tank, then use 2/3rds from the other tank (you turn the dive once you use the first third but stay on that tank until you finish the second third) it becomes obvious that you still have 1/3rd of the total gas you started with left in the first tank at the turn point. And when half way back out (when you switch back to the first tank) you still have an amount of gas equal to what you need to exit left in the second tank.

2 Gas switches is all you need to ensure an adequate reserve in either tank for you and your buddy.

 

[ianr33]

Yes,but,changing more often gives you greater gas reserves for a failure anywhere other than at maximum penetration. Your method will give adequate gas reserves,mine gives more than adequate. Its a trade off between maximizing reserves and number of reg switches. Switching every 600 psi works for me. YMMV.

 

[DA Aquamaster]

Your argument is very seductive but the logic is flawed.

As you indicate at the point of maxmium jpenetration where the reserve is most critical, there is no difference.

At any other point in the dive more frequent reg switches keep the differential between tanks smaller, but the reserve is only larger half the time or in half the failures. The other half of the time and in the other half of the failures, the opposite is true - due to the larger differential between tanks. So it is 50-50 whether 4 gas switches help or hurt you in terms of increased reserves compared to two gas switches.

And that makes sense as whether you swap four times or two times, you don't grow more gas once yuo start the dive.

Statistically, more switches probably leaves you at a very slightly more risk of failure - if the reg you are breathing is working, keep it in. Also if you switch to 4ths or 6ths on a dive, 4 reg swaps starts to requiring annoyingly frequest switches, for no gain in reserve. In contrast switching twice works anywhere equally well and lets you develop the same gas switch habits regardless of whether you dive thirds, fourths or sixths.

 

[wedivebc]

Yes,but,changing more often gives you greater gas reserves for a failure anywhere other than at maximum penetration. Your method will give adequate gas reserves,mine gives more than adequate. Its a trade off between maximizing reserves and number of reg switches. Switching every 600 psi works for me. YMMV.

Actually the 1/3rds switch method gives you more than enough gas to make an exit at any point in the dive except in the event you turn at 1/3rds (deepest point of penetration). Any other time you will have more gas to exit than you did going in so your argument is faulty.