Bit of info about doubles for a technical noob :)

[Griff..]

Right.. So I'm looking to broaden my diving ability/knowledge and start diving doubles and get into more the technical side of diving.

So trouble is.. I've dived with people on twinsets.. I know theres a Manifold, and two second stages.. but thats about all I know..

It would be greatly appreciated if someone could help explain to me.. how it works...?

does each tank have an individual first stage with the manifold connecting the two so either stage is breathing from both tanks.. then there is 3 valves.. what does each do?
turn the stage off?

I'm just getting myself abit confused and having people say different things..

would be great if someone could explain it to me..

Cheers
James

 

[Dr Wu]

Right.. So I'm looking to broaden my diving ability/knowledge and start diving doubles and get into more the technical side of diving.

So trouble is.. I've dived with people on twinsets.. I know theres a Manifold, and two second stages.. but thats about all I know..

It would be greatly appreciated if someone could help explain to me.. how it works...?

does each tank have an individual first stage with the manifold connecting the two so either stage is breathing from both tanks.. then there is 3 valves.. what does each do?
turn the stage off?

I'm just getting myself abit confused and having people say different things..

would be great if someone could explain it to me..

Cheers
James

Do you plan on taking the information gathered from this question as your sole instruction on technical diving?

Because there are people who post on this site that insist using a bungee necklace on their long hose is a "personal preference".

 

[Griff..]

No I plan to take a course I just wanted a bit of information first.. Was just chatting to mate about them really and was wondering..

I was aware you had a long hose primary and donated that in OOA and the short hose bungee'd round your neck..
I was more wondering about the tanks and the valves, etc.

Cheers
James

 

[LiteHedded]

the two manifolded tanks act like one big tank no matter which regulator you're breathing from until you close the isolator. then the tanks are two independent gas sources.

the two outside valves shutdown each regulator and the knob in the middle is your isolator

 

[rjack321]

The left and right valves just turnoff their respective outlets. E.g. gas for the right tank can still go to the left outlet when the right valve is off. The isolator is a bit more complicated but at the most basic level it only blocks gas flow between the two tanks. It does not close either outlet.

Its really important you understand the isolator, how the tanks were filled, how they were analyzed, or screwed up isolators can kill you pretty fast. Best to take a course or at a minimum get a worthy mentor.

 

[iztok]

I have a follow up question.

Valve drill.

What is the logic behind valve drill procedure?

From what I've seen it is

right post close (while signaling for attention)
breathe down right post reg (long hose)
switch to second reg (necklace one)
close isolator
open isolator
open right post
check and switch to long hose reg
close left post (while signaling for attention)
empty backup regulator regulator
open left post
verify backup regulator

 

[Blackwood]

I have a follow up question.

What is the logic behind valve drill procedure?

It's just a drill - it's meant to generate familiarity with your valves. Reach them easily, shut them expediently, understand what does what. It's not a primary problem solving algorithm (yes, the right post is probably the most likely thing to fail hence it being first, but if you hear bubbles from the left, don't go for the right).

right post close (while signaling for attention)
breathe down right post reg (long hose)
switch to second reg (necklace one)
close isolator
open isolator
open right post
check and switch to long hose reg
close left post (while signaling for attention)
empty backup regulator regulator
open left post
verify backup regulator

I do this:
Test purge necklace.
Shut right post, breathe down.
Switch to necklace, clip off primary.
Open right post. Unclip primary. Test purge. Switch back.
Shut isolator.
Open isolator.
Shut left post. Purge necklace. Open left post. Test purge necklace.
Flow check.

But it really doesn't matter. You could start from the left if you wanted to. What's important in practice is that you (and your teammates if applicable) understand the system, and can problem solve. I find dryruns to be extremely valuable.

 

[TSandM]

As Richard says, a manifold is a way of making two tanks into one big tank. You have valves installed on each tank, and the tube which connects the two valves comes off BEFORE the gas gets to the shutoff on either side (therefore, shutting the valve on one side still allows gas to flow from one tank to the other). This tube is interrupted by an isolator valve, which "isolates" the tanks from one another, so that if you have a leak you can't stop with the valve (for example, a blown tank o-ring) you can keep that leak from emptying BOTH tanks.

The merits of that setup can be and are argued. The advantage of a manifolded doubles setup is that it allows you access to all the gas you are carrying, even though you have had to shut one of your valves because of a regulator or o-ring problem. The disadvantage is that, with manifolded doubles, you can lose more than half of your gas from any single leak (whereas, if you dive sidemount or independent doubles, you can only lose half of your gas from any one leak). The isolator is there to limit this, but introduces its own issues, in that if it is closed and the tanks are filled, you run the risk of having completely different (and potentially unbreathable) mixes in the two sides.

Virtually all the advantages of using manifolded doubles require that you learn to manipulate your valves, and do it quickly and calmly. Otherwise, you are diving one big tank with a bunch of failure points!

The purpose of a valve drill is to practice accessing and closing all of your valves. Although some people make arguments about the precise sequence having some educational value, it is my belief that what's important is that you can shut them quickly and without perturbing your buoyancy or trim, and without losing awareness of team, position, or line. I've been taught two different sequences, and the only thing that's really important about the differences is remembering which sequence to use in front of which instructor

 

[Griff..]

Ok. I think I get it.. So if I were to turn the Isolator off then each outlet would be for the actual individual tank its attached too. but then how do you actually turn the tanks on and off if the valve jsut turns the outlets on/off.. or am I just being stupid now?

 

[rjack321]

Ok. I think I get it.. So if I were to turn the Isolator off then each outlet would be for the actual individual tank its attached too. but then how do you actually turn the tanks on and off if the valve jsut turns the outlets on/off.. or am I just being stupid now?

The left and right valves turn on gas to the reg attached to that valve. Just like with a single tank.

The isolator stays open at all times and especially when filling. The only time it is closed is UW when you have a damn good reason to close it. So the doubles act like one big tank 99+% of the time.