Pony vs. Doubles -- Philosophical Difference?

[DaleC]

I discovered intentionally breathing off a free flow by accident recently (besides the brief overview during training). I was draining down a tank (that was going in for nitrox conversion) in the pool and began purging the reg. I was pleasantly surprised that I could just keep breathing away without rushing to remove the reg and trying to stop it. I discovered I could dive a whole dive that way if I wanted to (it would be kind of short though). Practicing the skill certainly takes the "stress" away from free flow concerns.

 

[divengolf]

I want to thank all the folks who have contributed to this thread. I've been following it for some days. I've just completed my IANTD Adv. Nitrox on my way to Deco. This has been a most enlightening discussion.

In Adv. Nitrox, I dove a 100 plus I slung a 40 for gas switching/deco training. Now I better undersdtand the value of doubles, which is the route I had planned to go. This discussion has reinforced that path. I've got 2 HP 100s that I plan to start diving routinely to get used to the rig. Then I'll add a 40 a a deco bottle when needed.

Despite all the bantering, this has been a real useful thread for us tech beginners.

Long live SB.

 

[OH-JJ]

Here's another option. hp80 and worthington 40.

Micklock,

Please tell me that is not your rig....

If it is, at the very least you really should switch the tanks around so that the valve handles are on the out side where you could reach them...

 

[Nemrod]

Why would he need to be able to turn the air off?

N

 

[mempilot]

Why would he need to be able to turn the air off?

N

Maybe not off while submerged, but most defineately on.

 

[DA Aquamaster]

I would like to make a comment here. There is no history of a burst disc (essentially an OPRV) failing below the surface. The bust disc tends to let loose during an overfill or during storage in elevated temperatures. Once the tank is in the water, cooling and breathing lower the pressure at a rate that bust disc failure is almost unlikely.

An extruded neck o-ring on the other hand can fail for a few different reasons. One, it may be old and vulnerable. However, if it was recently replaced, it may have been contaminated prior to installation. This contamination can cause breakdown and failure. Also, like you mentioned, extrusion, or not seated properly, can cause failure as well, but usually rears it's head during filling or shorthly there after like the burst disc.

The other less looked at failure of the o-ring is improper material exposed to high concentrations of O2. This can and does cause premature failure of o-rings, and I've seen it happen quite a bit.

So, the burst disc isn't much of a potential failure point once the tank is dived, but the o-ring is.



Absolutely wrong. Sorry, but degredation of the material happens due to contamination and high O2. I've seen o-rings blow out on people gear, my own included. And it wasn't from blatent improper handling.

We will have to agree to disagree on this one. Old burst discs will fail at far below the specified pressure 90-100% of test pressure.) It is most common for them to fail as pressure rises during a fill or in storage in a hot vehicle etc, but I have seem them fail just standing in a corner and I have seen them begin to leak in the water. A tank taken from a hot boat deco to a 35 degree bottom temp in 2-3 minutes undergoes a great deal of contraction at unequal rates and it can stress the disc. Saying there is "no history" of failure in the water is very presumptuous.

My neck o-ring experience has also been vastly different. In over 20 years of diving, owning 10 plus tanks for the vast majority of those years I have never had one fail. Not one. With annual replacement I have not had one fail on air or nitrox tanks and using viton, I have never had one fail on an O2 tank.

I suppose there are some solvents that would contaminate one and cause it to fail, but letting them near your tanks or o-rings is just not prudent. The only tank neck o-rings I have ever seen extrude have been on tanks that had valve that were either loose or in some way damaged.

In short, I worry a lot more about burst discs than tank neck o-rings.

 

[mempilot]

We will have to agree to disagree on this one.

No problem. I don't mind debate!

Old burst discs will fail at far below the specified pressure 90-100% of test pressure.)

This is/should be replaced at every VIP along with the tank neck o-ring. If it isn't, I can see your point, so I won't argue with you based on the point you bring up.

It is most common for them to fail as pressure rises during a fill or in storage in a hot vehicle etc, but I have seem them fail just standing in a corner...

This, we agree on 100%. But, this isn't a failure that is dealt with in the water.

...and I have seen them begin to leak in the water. A tank taken from a hot boat deco to a 35 degree bottom temp in 2-3 minutes undergoes a great deal of contraction at unequal rates and it can stress the disc. Saying there is "no history" of failure in the water is very presumptuous.

I guess this can happen, especially with an older burst disc. See my above point about replacing them every year.

My neck o-ring experience has also been vastly different. In over 20 years of diving, owning 10 plus tanks for the vast majority of those years I have never had one fail. Not one. With annual replacement I have not had one fail on air or nitrox tanks and using viton, I have never had one fail on an O2 tank.

You are lucky here, as I have been with the burst discs. When you replace your neck o-rings, look at the shape of the material. This o-ring is consistantly put to the test unlike any other o-ring in a scuba system. The shape becomes flattened from extraordiary pressuization and depressurization. It is very noticeable in tanks that get used weekly, or even daily. ie rental tanks or those owned by avid divers. They are also placed under long term stress when tanks are stored fully pressurized for long periods of time. This is why I don't refill my tanks immediately after diving unless I'm planning to use them with a few days. The constant pressure on the o-ring and bust disc is what ultimately wears them out prematurely.

I suppose there are some solvents that would contaminate one and cause it to fail, but letting them near your tanks or o-rings is just not prudent.

It's more a factor of whether the installers fingers and hands were free of contaminents or the o-ring was properly cleaned prior to installation.

The only tank neck o-rings I have ever seen extrude have been on tanks that had valve that were either loose or in some way damaged.

Agreed. This usually happens from improper handling or carelessness during installation. I do my own now, since I've been surprised in the past shortly after VIP.

In short, I worry a lot more about burst discs than tank neck o-rings.

I have no problem with this. At least you are congnicent of potential failure points, unlike the majority of divers.

 

[MedicNREMT]

(Sorry - forgot my username / pw and now I have to get 5 posts in before I can send a URL thru).

 

[Blackwood]

(Sorry - forgot my username / pw and now I have to get 5 posts in before I can send a URL thru).

Something tells me the point of the restriction isn't to get people to open up 5 random old threads and post in them to raise their count...