how to use doubles

[aprilgoddess1987]

I'm just about there cash wise to start diving with doubles, but I had a few questions,

1.) When do you swap from one tank to another?
2.) Are there setups for using one first stage or do you have to have 2
3.) just because you have two tanks do use use both tanks

[Sounder]

Originally Posted by aprilgoddess1987

I'm just about there cash wise to start diving with doubles, but I had a few questions,

1.) When do you swap from one tank to another?
2.) Are there setups for using one first stage or do you have to have 2
3.) just because you have two tanks do use use both tanks

Are you going to be diving singles and doubles or just doubles?

When you dive them you generally use both tanks but you can shut one down in the event of a failure. You CAN get 2 dives out of it them, but you should really hook up with a mentor (pick them carefully - the guy who speaks loudest isn't usually the best choice) to learn to dive them and learn drills and emergency procedures.

[aggie99]

Originally Posted by aprilgoddess1987

I'm just about there cash wise to start diving with doubles, but I had a few questions,

1.) When do you swap from one tank to another?
2.) Are there setups for using one first stage or do you have to have 2
3.) just because you have two tanks do use use both tanks

I would recommend taking an Intro to Tech class prior to diving doubles but here are the best answers I can give:

1) That is not the function of having doubles. The idea is to have both tanks act as one but in emergency situations they give the ability to shut off one side or one first stage and get back safely.

2) The manifold design would work with just one 1st stage (thus the safety factor) but it isn't recommended. If for some reason that valve was knocked on under water it would be dangerous.

3) Yes, the whole point of having doubles is to use both. Your primary is connected to one side and the secondary is connected to the other. In the event of an emergency you wouldn't want to be trying to turn one tank on.

If more air is your goal doubles might not be the route for you. Maybe a single HP120 would make mor sense, you basically get the same air as two LP80's doubles and negate the need for a pricer route of a manifold, bands and two tanks. If the technical diving route is what you want take an Intro to tech class, it is invaluable!!!

[doczerothree]

The 2 tanks you are purchasing are connected by a dual iso manifold. you will need 2 reg sets. I good plan of action would be to take an "intro to tech" class. What kind of experience do you have?

[CompuDude]

Originally Posted by aprilgoddess1987

I'm just about there cash wise to start diving with doubles, but I had a few questions,

1.) When do you swap from one tank to another?
2.) Are there setups for using one first stage or do you have to have 2
3.) just because you have two tanks do use use both tanks

Re 1):

Doubles are usually used with an "isolator manifold". This is a special valve that connects both tanks together, allows each valve to be turned on and off to flow out to the regular (or not), and has a middle knob which allows the connection between the two tanks to be cut, essentially splitting the tanks into two separate tanks. In normal use, however, the two tanks act as one giant tank.

As such, you don't "swap" from one tank to the other, unless there is an emergency requiring you to shut off and isolate one of the tanks.

There are a few manifold designs out there that do NOT isolate the tanks from each other, but they are not commonly used any more.

There is a school of people who set up what is called "independent doubles". This is literally two tanks held onto your back in some manner, with two fully independent regulator setups used. To change tanks, you change regulators, and have to carefully monitor gas levels in each tank, and keep the levels somewhat similar. There are special protocols that are used for this, but honestly, I'm not overly familiar with them since I doubt I'll ever use a set rigged in such a manner.

Re 2):

There are some "old school" manifolds floating around out there that allow the use of only one first stage, but they are far from common these days. The isolation manifold has pretty much been selected as the manifold of choice, although there are still some die-hard adherents to the old versions.

Re 3):

See #1. With the manifold connecting the two tanks, they share a gas pool, so unless they are isolated for some emergency reason, when you breathe off one you're breathing off both.

~~~

To get started in doubles, you're going to need a rig that can handle them (usually this means a backplate and a doubles wing), two matched tanks (not all tanks, even those of the same size, are matched perfectly enough to join together in a manifold), an isolation manifold valve to replace the independent valves the tanks [probably] have currently, and a pair of bands sized for the correct tank diameter. You'll also need a second first stage for the send tank post, if you don't have one already, and may need different hoses.

As a general rule, double tanks is getting ready for technical diving, so you'll need a technical diving setup with technical hose configutation: The long hose rig.

[aprilgoddess1987]

i am wanting to take the intro to tech in a few months, i just wrapped up my cavern class and was just curious, i figured i would ask here and save a trip to the shop. I know it seems dumb to here these questions from a newbie the only thing ive heard about doubles is from the book deep descent about the doria

[LCF]

If you think about the reasons people dive doubles, some of the answers will make a lot more sense.

Most sane people don't dive doubles for shallow, recreational dives. You don't need the gas, you don't need the redundancy, you don't need the weight, and you don't need the expense and complication of having a manifold and two first stages. So why DO people dive doubles? They dive them when they need a much larger gas supply, or when they feel they can't afford the risk of depending on a single regulator providing gas. So you see doubles in use for deeper dives, and for overhead (real or virtual) diving.

There, you want to preserve access to the entire gas supply in the event of almost all failures. Therefore, most of us dive with a manifold that connects the two tanks, and allows either first stage to provide gas from the entire supply. Most people nowadays use isolation manifolds, which have a valve in the middle that allows you to isolate one tank from another in the unlikely event of a tank or manifold o-ring failure. But virtually everybody uses a manifold that has two first stages on it. Very old manifolds may have one outlet, but they were designed for much smaller tanks.

So, inherent in the concept of redundancy is the need for two single stages. Believe me, if you are pursuing cave diving, the cost of two regulators will be only one of MANY expensive outlays.

And regarding your last question, most people do connect everything up and use gas from both tanks at the same time. It has advantages, because you have one SPG that monitors your total gas supply, and you don't have to worry about switching from one tank to another, or about getting off balance because one is empty and the other is full. People who dive independent doubles or sidemount watch their gas consumption, and switch from one tank to another at certain predetermined pressures, to even themselves out from side to side, and to make sure either tank is capable of getting them out, should one fail.

You don't have to take a class to dive doubles, but I would highly recommend doing some reading, and perhaps purchasing something like the Intro to Tech DVD from 5thD-x. You should know how a manifold works, and how to manage valve failures, before you undergo any technical training in doubles. The absolute BEST strategy is to hook up with somebody who already dives them, who can mentor you a bit and help you with adjusting your harness and your gear to help with your trim, as well as introduce you to the proper use of a manifold and the management of failures.

Welcome to the world of technical diving -- It can use more women!

[cummings66]

Originally Posted by LCF

Most sane people don't dive doubles for shallow, recreational dives. You don't need the gas, you don't need the redundancy, you don't need the weight, and you don't need the expense and complication of having a manifold and two first stages. So why DO people dive doubles? They dive them when they need a much larger gas supply, or when they feel they can't afford the risk of depending on a single regulator providing gas. So you see doubles in use for deeper dives, and for overhead (real or virtual) diving.

I think this part is in error. Many of my buddies and myself included dive doubles on recreational dive profiles. We do it to stay in practice and in shape to carry them. They are heavy and if a person doesn't keep in shape you can find that when you do finally need them it's heavier than you remember. Keep using them and you won't have to worry about your muscles forgetting the load. It keeps you fit.

Doubles are also nice if the dive site is such that you really don't want to trudge back up that hill again for another cylinder, it minimizes the time spent getting ready for the second dive.

At any rate, once a person dives doubles and likes it IME most of them keep diving doubles for everything because it's more stable and it's no big deal.

Again, there I go with the generalizations, "most", that really applies to areas. You're area is apparently different than my area, here we use them for rec dives, there you use them only for tec dives. Most does not fit across the US.

[MSilvia]

Originally Posted by LCF

Most sane people don't dive doubles for shallow, recreational dives.

A lot of folks here use doubles on shore dives too, myself included. It's a great way to maximize range and bottom time, helps maintain proficiency, and as far as expense and complexity... I already own them, so expense isn't a factor, and I believe that you should be practiced enough with your isolator manifold that the complexity is a complete non-issue, or else you shouldn't be using doubles on dives where your life may depend on your proficiency with them.

Originally Posted by aprilgoddess1987

1.) When do you swap from one tank to another?
2.) Are there setups for using one first stage or do you have to have 2
3.) just because you have two tanks do use use both tanks

1. I dive with the isolator open, which connects the air space in the two tanks, so either regulator breathes out of both tanks at the same time. It's effectively one big tank, unless I close the isolator. It helps keep the load balanced, and saves you having to worry about swapping regs all the time.
2. There are setups that use one first stage, but you don't have any redundancy if you set up that way. It really defeats the purpose of using doubles, or at least eliminates one of their major benefits.
3. Yes... see #1.

[CompuDude]

I agree with the resident Matts (LOL) that people DO use doubles for recreational dives, but also with LCF. I think "most" DOES apply. MOST divers do NOT use doubles, let alone doubles for recreational shore dives. MOST implies a majority... and if you dispute that a majority of people do NOT use doubles for shallow recreational dives, well, I'd recheck my figures. That's the nicest way I can think to put that.

She didn't say ALL. She said MOST, and was correct to use that term.