Dual valve, single regulator

[diver-man]

Not all plugs have a way to depressurize them: DGX HP Sealing DIN Plug.

Well, as you pointed, this is a bad design.

There are other designs as well, e.g. a button.

But during the dive I would recommend the version with a hex bolt to protect the unused valve.

 

[KenE]

I have the plugs without pressure relief and it is not as dangerous as one might assume. I use mine for transport only, however, I have pressurized a plug due to travel. I turned off the valve, opened the plug one revolution and waited for the pressure to leak past the o-ring and threads. Then it came off by hand. The key is to break the o-ring seal and wait a bit for the pressure to leak out. Remember, as you rotate the plug you are increasing the volume of the space which in turn reduces the pressure of the gas contained in that space. So, you are not likely to get a projectile other than a foot or two, and only if you continue to open it before the gas has leaked past the threads.

 

[Zef]

 

[Crazy_Ivan]

If you put an unsealed plug in it (i.e. a dust cap) and accidentally turn on the valve then it might slow down how fast it drains but it is still going to drain. If you put a sealed plug in it and accidentally turn on the valve then the plug can become a projectile when you try to remove it. Neither option is good...

As you start to unscrew the plug the o-ring will lose it's ability to seal and vent the pressure, but there will still be plenty of thread engagement to hold the plug in place.

 

[halocline]

I wouldn’t dive with a tank valve that has no regulator attached to it. If you are diving in a scenario where you need the redundancy, then you need the 2nd regulator. The whole point of an H valve or Y valve is to have an alternate 1st stage. It’s not the valves that fail in these scenarios, it’s the regulator.

Anyhow, I sort-of get the idea that keeping an unused valve plugged is an additional protection, but this absolutely a solution-in-search-of-a problem. And, those plugs don’t protect the threads, meaning if you dive in salt water with them installed and don’t remove them after the dive, they can easily get frozen in place due to salt corrosion.

 

[Seaweed Doc]

I wouldn’t dive with a tank valve that has no regulator attached to it. If you are diving in a scenario where you need the redundancy, then you need the 2nd regulator. The whole point of an H valve or Y valve is to have an alternate 1st stage. It’s not the valves that fail in these scenarios, it’s the regulator.

Anyhow, I sort-of get the idea that keeping an unused valve plugged is an additional protection, but this absolutely a solution-in-search-of-a problem. And, those plugs don’t protect the threads, meaning if you dive in salt water with them installed and don’t remove them after the dive, they can easily get frozen in place due to salt corrosion.

I agree that this is a "wrong tool for the job" issue. If you're going to have two valves, take the second reg every time. If you don't have two regs, don't have two valves.

I do acknowledge Angelo's point that you can switch a reg from one valve to the other if the valve fails. That'd be a very specific and rare sort of failure with a complicated solution. It's also reminiscent of the good old days when you dove with just one second stage. If you ran out of air, you can always buddy breathe, right? Yet now we always have our "safe second" with us. (OK, I know there are still divers out there who don't have a second second stage, mostly cleaning boat hulls and diving solo in my experience.)