DIR- Generic Learning Doubles (in Wetsuit?)

[Angelo Farina]

The routing doesn't necessarily remais the same. Nowadays, most people dive singles with only one first stage.

I know that this is the typical approach used by Padi and other commercial training agencies.
Here in Europe and UK, instead, other organizations favour the usage of larger tanks, typically 15 liters, and all have two valves and two posts (DIN compatible).
Some divers use both of them with two first stages (as me), some others rely on the safety of the DIN attachment and use only one first stage.
So for the number of divers who use already two first stages on their single tank, switching to a compact twin tank is a no-issue.
And typically the divers who want to switch to a twin tank are the same who were already using two first stages on a single tank of good size.
I see a progression here: one begins diving in a remote exotic location, in warm waters, where a single AL80 and a single reg are perfectly adeguate.
Then the diver comes back home, and continues diving in colder water, requiring a protective suit and more gas. So he switches to a single tank of more appropriate size, typically 15 liters, which invariantly is equipped with a double valve.
After a while, proceeding in his evolution, the diver buys a second first stage and a BP+wing.
And after a while in this setup, he is ready for the compact twin tank.
After this, a medium-sized twin tank is another step in this progression.
Avoiding big steps is more safe and satisfying, in my opinion.

 

[LanceRiley]

Many of us that had never heard that diving twin tanks in a wetsuit is dangerous or something
have been diving twin tanks in a wetsuit and in caves with the Faber 12.2L 232bar cyl the go to
forever.

Hi, im curious 3mm wetsuit? With steel doubles? I was told by my instructor never to dive steel doubles with wetsuit. Due to not being able to swim up when there is wing failure…

although… i want to because of its bouyancy properties

 

[Manatee Diver]

Hi, im curious 3mm wetsuit? With steel doubles? I was told by my instructor never to dive steel doubles with wetsuit. Due to not being able to swim up when there is wing failure…

although… i want to because of its bouyancy properties

He isn't DIR, so if you are wanting to dive DIR then your instructor's answer is correct.

 

[ginti]

Hi, im curious 3mm wetsuit? With steel doubles? I was told by my instructor never to dive steel doubles with wetsuit. Due to not being able to swim up when there is wing failure…

although… i want to because of its bouyancy properties

Trim properties, maybe... buoyancy properties don't change as long as you are correctly weighted.

Anyway, double bladder wings offer redundancy in case of failure. I don't like them, and if I have to use twin tanks with a wet suit, I go for aluminium ones with a standard wing. There are several reasons why I do not like double bladder wings, I share most of these reasons with the GUE philosophy, but in the end, it is personal, so choose whatever you prefer

 

[LanceRiley]

My b

Trim properties, maybe... buoyancy properties don't change as long as you are correctly weighted.

Anyway, double bladder wings offer redundancy in case of failure. I don't like them, and if I have to use twin tanks with a wet suit, I go for aluminium ones with a standard wing. There are several reasons why I do not like double bladder wings, I share most of these reasons with the GUE philosophy, but in the end, it is personal, so choose whatever you prefer

my bad. I meant trim properties

 

[boriss]

Hi, im curious 3mm wetsuit? With steel doubles? I was told by my instructor never to dive steel doubles with wetsuit. Due to not being able to swim up when there is wing failure…

although… i want to because of its bouyancy properties

There's a dilemma with answering this, since this is in the DIR forum. Keep in mind, this is NOT recommended as part of DIR but as a technical diver you can choose your configuration. As long as you understand the risks/rewards. The goal is to be neutrally buoyant when near empty.

With that disclosure is out of the way, here's how I dive (and I should mention this is the configuration for most divers I see on my local tech trips).

Aluminum doubles are about 8lbs positive when empty. LP85's are 4.6lbs positive when empty and HP100's are 1lb negative when empty (see table below).

Before taking into account the gas, that's a ~4lb and ~9lb difference in buoyancy, respectively. Now, to dive AL80's I'd have to have a SS plate and additional weight, which we'll exclude from the equation for now. For diving steels I use a CF plate. Assuming the same amount of gas, that means my true delta is ~0lb or ~4lbs more negative compared to aluminum doubles. So you can see, it's not a huge delta, but it needs to be understood and it adds up.

	Buoyancy empty (doubles)	Gas when I fill them locally
AL80	8.2lbs	~155cuft @3000psi
LP85	4.6lbs	~190cuft @3200psi
HP100	-1.2lbs	~210cuft @3600psi

Even though we aim for neutral buoyancy when empty, we don't dive empty. And in case of steels, we carry a whole lot more gas (additional weight). So everyone who dives in wetsuits here has redundancy in the form of dual bladder wings, lift bags, etc. We have two of everything.

Hope this helps!

 

[grantctobin]

	Buoyancy empty (doubles)	Gas when I fill them locally
AL80	8.2lbs	~155cuft
LP85	4.6lbs	~190cuft
HP100	-1.2lbs	~210cuft

Please show your math for these HP100 and LP85 calcs

 

[boriss]

Please show your math for these HP100 and LP85 calcs

Not sure which number you're questioning. I'm traveling right now, but took a screenshot of my spreadsheet, with the Z-factors listed. All specs and formulas from DGX.

 

[grantctobin]

Not sure which number you're questioning. I'm traveling right now, but took a screenshot of my spreadsheet, with the Z-factors listed.

Or list what pressure you’re filling each of the cylinders to. At the same psi there is not a 10cf/cylinder difference in gas volume between those two cylinder types.