Another Burst Disc Question

[Quarrior]

As I understand it, cave divers like to have double discs to prevent failure.

What I don't understand is why? Is it because of transport or because of disc failure underwater?

I can almost see it being a problem on-land since most cavers like a "cave fill".

However, I don't understand it if the issue is underwater bursting of the disc. The reason I don't understand it is because from the time you go under, the pressure in the tank is being reduced continually, thus eliminating the potential of the disc letting go.

 

[Rick Inman]

My betters will add/correct me, but from what I've read, a burst disk can get old and fail at any point in the tank's pressure, not just when it's "full".

 

[dlndavid]

then if that is the case, why not change them out periodically?
Or just weld the thing shut.

 

[Mark Vlahos]

then if that is the case, why not change them out periodically?
Or just weld the thing shut.

I cave dive and I replace the disks every year with the inspection and O2 cleaning. I also have the properly rated disk on each of my cylinders.

Mark Vlahos

 

[Quarrior]

My betters will add/correct me, but from what I've read, a burst disk can get old and fail at any point in the tank's pressure, not just when it's "full".

I would think a cave/technical diver would have their act together better than to let their equipment get old and warn out than that.

 

[karstdvr]

Why double disk the burst disk. I had my 120 cuft sidemount tank blow a burst disk after being filled to 3600 psi. I happened to have my watch handy,and it was competely empty in 1 minute 26 seconds. How far could I go with that much time-not far!

 

[JustinW]

The whole cave fill idea confuses me. Why buy a low pressure tank and overfill it when they offer high pressure tanks of the same dimensions and weight. Guess I need to be schooled on this one.

 

[H2Andy]

basically, the double burst disk thing started as a way to pump the crap
out of a tank and get tons more air than normal in it.

as i understand it, you can't really overfill high pressure steel tanks because few shops have the required pressure (i believe 3500 is about max for most) and you quickly hit a diminishing return effect in pumping these tanks higher than rated.
also, partial pressure blending makes high pressure steel tanks undesirable.

i also know there's a 5,000 psi or so (or maybe 5,500 psi) burst disk now
available, which would do the same thing, thus no need to double-disk.
but back then, all they had was the regular burst disks, so they had to
double them up.

 

[in_cavediver]

The whole cave fill idea confuses me. Why buy a low pressure tank and overfill it when they offer high pressure tanks of the same dimensions and weight. Guess I need to be schooled on this one.

Simply put, in 2002, when I bought my tanks, the HP 130's did not exist, only LP 104's and HP series. (essentially, PST's E8 line was LP, E7 line was HP). Why don't I swap out for new ones, well for one, there expensive and I have other things to spend the money on. My tanks are bought and paid for and simply work. At some point I'll get another set and I cross that bridge when I get there.

 

[loosebits]

The whole cave fill idea confuses me. Why buy a low pressure tank and overfill it when they offer high pressure tanks of the same dimensions and weight. Guess I need to be schooled on this one.

I suspect both LP and HP tanks have close to the same strength (at least closer than the differences in their rated pressures would suggest); one just got marked differently. I am comfortable tanking a LP tank to 3600 but would not be comfortable overfilling a HP tank by the same percentage (that would be 4700). Even if the tank could take it repeatedly (as in every week for 5 years), it would be hell on my 1st stage.

Since people are willing to fill them to about the same pressure, it just becomes an issue of liquid volume, weight and buoyancy characteristics.

An LP104 is pretty much the same tank as an E8-130.. pretty much. And E8-130 at 2640 is about 100 cuft and an LP104 at 2640 is really 106 cuft (http://www.techdivinglimited.com/pub/tanks.html) so the 104 actually holds a bit more gas than the E8-130 at 3600/4200/whatever. They were just discontinuing the 104's when I got into doubles, I imagine a lot of cave divers cried that day.

Now the LP108, either Faber or Worthington, is a nice sized tank (my next set of tanks will be the galvanized Worthington 108's). 108's filled to 3600 and you've got 290 cubes. You'd need to fill a E8-130's to almost 3900 to get that and who wants to mess with thirds on 3900? 3900 seems like such a weird number; I prefer 3600.

Another reason is I love my 104's and I'll be damned if someone is going to tell me I should have gotten 130's