"Cave Fills"

[lerpy]

Just saw another post in another area of SB, person talking about tanks and getting "cave fills". I did not know what a cave fill was until I got my first one in Florida ( not in cave country ) a few years ago. Took my tanks to a shop, and just asked for a fill. Guy chatted to me and I told him I was vacationing and was going to be doing my cavern course. When he was done he said he had given me a little extra, a cave fill. Now I am used to getting a few hundred pounds extra, cool down and all that, but the fill was at 4000lbs, I might have come close to frying an egg on the tank it was so hot. I have to be honest, ( and this has happened again on my double 130's a 4000lb fill) it kind of gives me the heebee geebee's tossing a set of tanks with 4000lbs in them into my truck and driving with them. I know the burst disk should be what goes if the pressure gets to high, and I have been around for one of them going, but still.

Ironically I have never gotten a "cave fill" in cave country.

So what are the thoughts of the experienced folks, ( I am new to cave diving), is my concern unfounded, or am I making a good choice when I say, 3600-3700 in my 130's is plenty?

My experience filling in cave country is that I get the 3600-3700 at the most. My understanding as well is the high pressure can do damage to a first stage.

 

[PfcAJ]

For starts, there's been a grand total of ZERO issues from cave fills with in hydro/vip lp 2400psi(+) steel tanks. I challenge anyone to show me an incident involving a cave filled tank that had a current hydro and vip.

36, 37, 4000psi, whatever. At that point its all the same. Getting the tanks super hot doesn't seem good, but I'm not sure its actually 'bad' for them at the temps we're discussing. To me its just annoying because they cool down so much when I get in the water. My 3800psi fill is now like 3500 or less

High pressure fills aren't going to damage your regs, either.

Long story short: Its fine. Carry on.

 

[The Chairman]

First, you haven't told us if your tanks are LP, NP or HP. That would be 2400, 3000 or 3500 for the uninitiate. Taking a 3500 tank to 4000 is less than %15 over it's rating. However, filling a 2400 to 3600 is a %50 increase. Hey, I love the fact that my LP120s have 180 cf of air in them, but they've got to be work hardening as they are being pumped so high.

 

[mk706]

Is 3700 in a 3442 mid pressure tank overfill? Suppose yes. Worried no. 3700psi in a HP130 is about 140 cu ft. Whereas 3700 in an LP108 is 151cu ft.

Many cave divers double up burst discs for LP tanks, or use 5250 discs. A 4000psi burst disc with a cave fill is a bad idea.

To dovetail off PfcAJ, tanks should be filled slowly and kept cool. They should always be kept cool when filled.

 

[lerpy]

My

First, you haven't told us if your tanks are LP, NP or HP. That would be 2400, 3000 or 3500 for the uninitiate. Taking a 3500 tank to 4000 is less than %15 over it's rating. However, filling a 2400 to 3600 is a %50 increase. Hey, I love the fact that my LP120s have 180 cf of air in them, but they've got to be work hardening as they are being pumped so high.

My bad HP 3440. Good to know, and I appreciate it. Will not worry about it if I get such fills in future.

 

[Coztick]

I think the higher the pressure, the greater the effect temp. has on that pressure due to the volume affected if that makes sense. I know, as a fabricator of pressure vessels and piping, they have a safe operating temp. As well as pressure. SOP goes down as SOT goes up. Filling that hot to get max. operating pressure when cold doesn't sound safe to me but l've got no degree or specs to qualify my theory. Good on you to question it, got me curious. At min. It may affect longevity. I'm sure an engineer, tec. or doctor of physics will set us straight.

 

[The Chairman]

My bad HP 3440. Good to know, and I appreciate it. Will not worry about it if I get such fills in future.

No worries. You don't know, 'till you ask. While I always appreciate getting extra in an HP tank, a cave fill for my LPs allow me to go a lot further than a normal fill. A lot further.

 

[Littlerayray]

My thoughts are that a tank when hydro'd gets filled to a crazy amount so in theory a cave fill isn't going to hurt your tank and the burst disc will protect the valve

 

[The Chairman]

I think the higher the pressure, the greater the effect temp. has on that pressure due to the volume affected if that makes sense.

Remember that the correlation is between absolute temps and absolute pressures. To arrive at absolute pressure, we only have to add 14.7 psi. However, to get to absolute temps, you have to add 460 F or 273 C. That means that the tank in @PfcAJ's post was only probably overheated to about 115 F. It would feel hot to our hand, but it's really not close to affecting any steel metallurgy. Here's the formula I used. It's derived from STP or the Ideal Gas Laws.

(Pi/Pf)Tf=Ti

 

[The Chairman]

My thoughts are that a tank when hydro'd gets filled to a crazy amount so in theory a cave fill isn't going to hurt your tank and the burst disc will protect the valve

The normal protocol for hydro is 5/3s of a tanks pressure.

2,400 is pushed to 4,000
3,000 is pushed to 5,000
3,440 is pushed to 5,750

Obviously, an LP tank given a cave fill is close to being hydroed on every fill.