Double valve on a single tank

[USCScubaboy]

Was browsing through craigslist when I found this (Scuba Tank Aluminum 80 with Double DIN valve).

Really curious about why someone would want a double valve on a single tank.

 

[stairman]

Its called an h-valve and is used for redundancy. That way you have 2 1st stages, as well as 2 2nd stages. If one of your regs freeflows etc, you can isolate it [shut it down] and still have another reg to safely get you to the surface. This is required equipment for single tank intro to cave training till one get used to or can purchase doubles.

 

[Tortuga68]

The ad is for a Y-valve:



not an H-valve:

 

[DevonDiver]

Yep, it's a good idea for recreational redundancy. Allows you to shut off a free-flowing regulator, yet still access all your gas with the other regulator. You'd need a separate first stage though... two 1st stages, one with your primary second stage, the other with your octo. LPI on the right (as worn). SPG on the left (or both on the left).

 

[Tortuga68]

LPI on the left. SPG on the right (or both on the right).

Why not both on the left. Or LPI or the right and SPG on the left

 

[Scared Silly]

The other reason is laziness. If one dives doubles most of the time one has their regs set up accordingly for that rig. If you switch to a singles rig there is no need to re-rig your regs. As for rigging them same as for doubles, the SPG is on the left post along with the backup second stage. Right post has primary reg and LP inflator.

Images of the values from a recent classified ad

 

[drbill]

Never quite understood the thinking that H- or Y- valves give you real redundancy. I use neither but I've been told that the H valve has a single air pathway and dip tube from the tank, as do some of the Y valves. The worst incident I've ever had occurred when the dip tube in my K-valve clogged inside the tank and cut off all air flow. My understanding is that an H-valve has a single dip tube and air pathway so it would fail to deliver under that scenario as well. I have been told that some (not all?) Y-valves use dual air pathways with separate dip tubes in which case they would provide true redundancy. Please correct me if I'm wrong on that.

 

[DevonDiver]

Why not both on the left. Or LPI or the right and SPG on the left

Because the right post is where they have their primary... and which they'll shut-down if that primary free-flows... meaning they'll also lose their LPI and SPG.

Following the free-flow, they'll be unable to confirm their gas. Naturally, they should be on their way up anyway... but not knowing adds stress.

They'll also have to orally inflate the BCD, which adds task-loading to an already stressed diver.

Never quite understood the thinking that H- or Y- valves give you real redundancy.

I agree. It's limited redundancy - specifically, it is a redundant regulator, not a redundant air source. Any issue with the cylinder, cylinder valve or gas contents (contaminated air) will still cause loss.

That mitigates against more prevalent failures (free-flow, burst hose, second stage failure), but not against more rare cylinder/valve failures. Personally, I don't agree with it for deco/overhead (even limited) for that reason. For recreational, non-overhead, divers it presents a relatively cost effective method of mitigating specific risks.

 

[Tortuga68]

I think you're reading what I wrote backwards or something... if LPI & SPG are both on the left, and they shut down the right, nothing changes

I have been told that some (not all?) Y-valves use dual air pathways with separate dip tubes in which case they would provide true redundancy. Please correct me if I'm wrong on that.

Some Y-valves, but not all, have dual dip tubes. That makes the valve redundant, but you still have the tank neck o-ring as a common failure point

 

[DevonDiver]

I think you're reading what I wrote backwards or something... if LPI & SPG are both on the left, and they shut down the right, nothing changes

My bad... was my initial typo that caused the confusion. Have edited/corrected it now. Serves me right for posting on threads whilst attempting to cook (burn) steaks... newbie task-loading! LOL

Same as you would on doubles...I did wonder why you had asked that!