One-hand or two-hand valve drills (back mount doubles)

[Beau Holden]

While this is definitely true (happened to me), it could be a beneficial experience if you know ahead of time what to do if that happens.

Happened to me. Tanks already low and burning gas in the pool on drills. Isolator was closed, not checking SPG and reg was getting hard to breathe. Like this just ain't happening, check SPG and I have 900 PSI. OK, now what the heck could be going on? Went to necklace and went through valves and I will never forget that sound of tanks equalizing under the water. Taught me a good lesson about checking and rechecking valve position before the dive, during drills and checking SPG for dropping or not dropping even in the pool.

 

[NothingClever]

Happened to me. Tanks already low and burning gas in the pool on drills. Isolator was closed, not checking SPG and reg was getting hard to breathe. Like this just ain't happening, check SPG and I have 900 PSI. OK, now what the heck could be going on? Went to necklace and went through valve and I will never forget that sound of tanks equalizing under the water. Taught me a good lesson about checking and checking valve position before the dive, during drills and checking SPG for dropping or not dropping even in the pool.

Also happened to me. Dive shop closed my isolator (I have no idea why). I was on a solo dive in the Red Sea and became perplexed I hadn’t budged my SPG needle in the first 15 minutes. I checked my isolator and suddenly heard the tanks equalizing. Although I had gas for days, it was still unsettling. I always tell shops to keep my isolator open regardless of how hip they think they are.

 

[boriss]

Also happened to me. Dive shop closed my isolator (I have no idea why). I was on a solo dive in the Red Sea and became perplexed I hadn’t budged my SPG needle in the first 15 minutes. I checked my isolator and suddenly heard the tanks equalizing. Although I had gas for days, it was still unsettling. I always tell shops to keep my isolator open regardless of how hip they think they are.

Did you ask them? There should be no reason to ever close that valve, especially as all the shop does is fill the tanks and for that the valve needs to be open.

Having said that, I always check that the isolator is open when I pick up the tanks and test their contents as well as prior to every dive.

 

[boulderjohn]

I was on a solo dive in the Red Sea and became perplexed I hadn’t budged my SPG needle in the first 15 minutes.

When I took my TDI advanced nitrox exam quite a few years ago, I had a problem with one of the questions. It said something like "You have been diving for a while, and when you check your SPG for the first time, you see that the pressure is dropping about twice as fast as expected. What is the likely reason?"

The instructor told me that the answer was that the isolator was closed. I said that would only be true if the diver had a nonstandard configuration, with the spg coming off the right post. With a standard configuration, there should be no drop in pressure. He thought about it for a minute and then said I was right. The exam was about 10 years old at that point, and he said none of his students had ever pointed that out before.

 

[Nick_Radov]

Also happened to me. Dive shop closed my isolator (I have no idea why). I was on a solo dive in the Red Sea and became perplexed I hadn’t budged my SPG needle in the first 15 minutes. I checked my isolator and suddenly heard the tanks equalizing. Although I had gas for days, it was still unsettling. I always tell shops to keep my isolator open regardless of how hip they think they are.

The really dangerous part of this is that you were possibly breathing the wrong mix. Depending on when the dive shop closed the isolator during the gas mixing procedure (either before or after adding oxygen and topping off with air), you could have ended up with the wrong amount of oxygen in one tank or the other. When we analyze a mix after filling, we usually only put the analyzer on one valve. It's a good reminder to always verify the isolator is open before analyzing, and then conduct a thorough flow check as part of the pre-dive checklist.

 

[Nick_Radov]

The pool is a good place to practice. Hopefully, you have a buddy to work with. I recall an adage about doing valve drills along the lines of: "It's only a matter of time before you mess up the sequence and find that you have turned off all your air." Practicing valve drills with a buddy who is there at the ready to shove a working regulator in your face in case you mess up is a good idea. Ask me how I know.

Yep. I've seen video from a tech class where student #1 had a simulated right post failure so he switched to his backup regulator and shut down his right valve. Then student #2 came over and "helped" by closing student #1's left valve.
The instructor was right on top of them, so no harm done.

 

[boulderjohn]

Yep. I've seen video from a tech class where student #1 had a simulated right post failure so he switched to his backup regulator and shut down his right valve. Then student #2 came over and "helped" by closing student #1's left valve.
The instructor was right on top of them, so no harm done.

That is very possible, both with in-pool training and in a real life situation. The skill of helping a buddy with a valve problem needs to be more a part of training than it typically is. (Including me--I should be doing it more than I do. It is not a regular part of the class, so I have to add it. I do add it, but not enough.)

 

[PfcAJ]

The really dangerous part of this is that you were possibly breathing the wrong mix. Depending on when the dive shop closed the isolator during the gas mixing procedure (either before or after adding oxygen and topping off with air), you could have ended up with the wrong amount of oxygen in one tank or the other. When we analyze a mix after filling, we usually only put the analyzer on one valve. It's a good reminder to always verify the isolator is open before analyzing, and then conduct a thorough flow check as part of the pre-dive checklist.

Pretty sure that nearly killed someone at Eagles Nest in the early 2000s. IIRC one tank was full of helium.

 

[lostsheep]

The really dangerous part of this is that you were possibly breathing the wrong mix. Depending on when the dive shop closed the isolator during the gas mixing procedure (either before or after adding oxygen and topping off with air), you could have ended up with the wrong amount of oxygen in one tank or the other. When we analyze a mix after filling, we usually only put the analyzer on one valve. It's a good reminder to always verify the isolator is open before analyzing, and then conduct a thorough flow check as part of the pre-dive checklist.

Wouldn't this be assuming a) that nothing clever was breathing nitrox and b) that the shop was doing partial pressure blending

I've always been leery of partial pressure blending using one valve and assuming mixing through the manifold. I'm sure there is gas exchange through the manifold but how long does that take realistically? I've never experimented with mixing via one valve vs two so I really don't know.

 

[boulderjohn]

Pretty sure that nearly killed someone at Eagles Nest in the early 2000s. IIRC one tank was full of helium.

I have heard such stories, but it is hard for me to figure out how one tank can be filled with helium and the other with something else. I have done a ton of partial blending in my life, and I can't figure out what kind of a sequence would be required for me to make such a mistake. I have certainly made mistakes that were caught when we checked the blend, but I am having trouble figuring out how I could make that one.

BTW, if you do mistakenly get different mixes in each tank, don's assume they will blend when you open the isolator.