high pressure vs. low pressure

[CD_in_Chitown]

It happens in the USA too but its certainly not routine, have you seen our litigious system?

 

[CompuDude]

In Europe LP steel tanks are routinely filled to 4000psi(the metric equivalent). Aside from DOT regulations, is there any reason why this should not be the case in the USA? Any Comments?

Because it's ever so slightly insane?

 

[FIXXERVI6]

of course, the real rule of 1/3's takes into account

1) amount of gas to get you to the surface once you get back to the entry point
2) is dependent on the heaviest breather breathing from the smallest tank for the return portion of the dive.

Right?

1. amount of gas to get you to the surface once you get back to the entry point, I'm not sure what you mean here, as long as your diving a spring 1/3 gas in 1/3 gas out, if your diving a strong syphon the rules of 3rds may not get you in on 1/3 and out on 1/3 but that is beyond the scope of this question I assume. Are you talking about enough gas to deco on? is this question in the context of cave diving? If it is the deco you refer to I don't deco on back gas, and that goes from rules of 3rds to planning deco gas/lost deco gas and saftey factors for deco gas, the rules of 3rds still apply even if you have to calculate a buffer for lost deco gas, sometimes I don't speak my mind well if this doesn't make sense I'll try to elaborate some more by posting again. There is also stages but 3rds also apply there as well, even if your doing the half plus 200 your still doing the rule of 3rds.

2. the rule of thirds is not dependent on the heaviest breather breathing from the smallest tank for the return portion of the dive. If you have a dive team with different volumes of gas you have to calcuate 3rds based off of the smallest gas supply regardless of who is breathing it, if you have one guy on 95's one on 108's and one on 121's the guys on 108's and 121's can only breath 1/3 of the volume in the 95's, most say it is ideal to have the heaviest breather on the smallest tanks but that is not considered the rule of 3rds by doing so.

 

[Packhorse]

I wish everything was liquid internal volume

Is that not normal in the USA?

In New Zealand all tanks I have seen are rated in WC ( water capcity).
This of course makes it very easy to calculate air. WC X Bar = air in litres.
Talking about tank size in cubic foot is confusing as you need to take into account the tanks working pressure.

 

[loosebits]

Thirds is simple. Everyone can use 1/3rd of the *volume* of the guy with the smallest tanks before they turn their dive. Everyone should be on the surface with no less than 1/3rd of that volume remaining.

 

[FIXXERVI6]

Is that not normal in the USA?

In New Zealand all tanks I have seen are rated in WC ( water capcity).
This of course makes it very easy to calculate air. WC X Bar = air in litres.
Talking about tank size in cubic foot is confusing as you need to take into account the tanks working pressure.

the raitings here are a pain, its given in cubic feet @ operating pressure

LP 108 is 108 cubic feet at 2640 (10% overfill with +)

E8 130 is 130 cubic feet at 3442

so on and so on

 

[limeyx]

1. amount of gas to get you to the surface once you get back to the entry point, I'm not sure what you mean here, as long as your diving a spring 1/3 gas in 1/3 gas out, if your diving a strong syphon the rules of 3rds may not get you in on 1/3 and out on 1/3 but that is beyond the scope of this question I assume. Are you talking about enough gas to deco on? is this question in the context of cave diving? If it is the deco you refer to I don't deco on back gas, and that goes from rules of 3rds to planning deco gas/lost deco gas and saftey factors for deco gas, the rules of 3rds still apply even if you have to calculate a buffer for lost deco gas, sometimes I don't speak my mind well if this doesn't make sense I'll try to elaborate some more by posting again. There is also stages but 3rds also apply there as well, even if your doing the half plus 200 your still doing the rule of 3rds.

2. the rule of thirds is not dependent on the heaviest breather breathing from the smallest tank for the return portion of the dive. If you have a dive team with different volumes of gas you have to calcuate 3rds based off of the smallest gas supply regardless of who is breathing it, if you have one guy on 95's one on 108's and one on 121's the guys on 108's and 121's can only breath 1/3 of the volume in the 95's, most say it is ideal to have the heaviest breather on the smallest tanks but that is not considered the rule of 3rds by doing so.

I think we agree, but just said it differently

If you use 1/3's for a shallow cave entrance, then you dont need to plan gas to get you out. But say you do a wreck dive at 100 feet deep where you want to use 1/3's.
You need to subtract out first, the amount of gas needed to get from 100 feet to the surface, and then 1/3 the rest -- or at least that's the way I'd do it

for #2, I dont mean the heaviest breather has to actually use the smallest tanks, but you have to plan as though he is going to be sharing gas with that person on the way out. As someone else put it (rather well) you calculate 1/3 of the usable cubic feet of the smallest tank and then convert that to psi on each persons tank.

I just wanted to make the point that it's a bit more than just dividing your psi by 3 !

 

[UP4AIR]

[
HP 100 pumped to 3600 has roughly 105 cubic feet of gas
LP 108 pumped to 3600 has roughly 147 cubic feet of gas


Ok....Am I missing something here? Obviously so....because I ain't found anyone jacking a LP tank up to 3600.

Keep apples to apples. LP tanks are rated at approx 2440 and with a 10% hydro can go up to approx 2640. These tanks are therefore carrying their cu/ft of gas at their rated pressure. The same stands true for HP tanks. 100 cu/ft of gas at approx 3440.

The advantages of LP tanks VS HP tanks are that they have less weight for the same volume of gas. HP tanks tend to be a smaller package for the same volume gas and the manufacturers accomplish this by making the tank walls thicker in the HP tanks thus added weight. The HP tanks will also require the use of a DIN fitting instead of the standard yoke fitting. Many shops will have difficulty filling HP tanks to their rated pressures, and therefor one will not get their rated 100 cu/ft of gas at a fill of only 300psi. Some shops,due to their filling methods, will have difficulty blending nitrox mixes in HP tanks.

So why dive HP tanks. Again, it's all about hte weight:size ratio. Have you ever seen anyone with double LP 120's strapped to their back. Not too too many I would guess...and if so....I would address that person as SIR(even if they were a female).

The bottom line is this : 100cu/ft of air is 100 cu/ft of air...and it don't mean squat at what pressure you get that 100 cu/ft. You are better off deciding which tanks you want to dive (i.e. HP VS LP) and go that route rather than buying Lp tanks and jacking them up above their rated pressure.

 

[CD_in_Chitown]

You're getting your fills in the wrong spot, I often am asked "How much you want" when I drop off my tanks. I don't exceed working pressure but I'm sure if I said 4K it wouldn't be an issue. Some guys go closer to the burst disk pressure than I'm comfortable with, some guys take the disc out and put in a bolt, effectively raising the PSI capacity. (I hear that's a cave diver trick *shrug*)

I think the quoted text was more of an illustration though.

 

[DepthCharge]

When I was diving OC I'd routinely fill LP tanks to 3500, more if it was a esp. deep/long dive. Tanks always qualified for a "+" rating when hydro'd. You're going to hear both agruments here..............OK to overfill, or you're crazy to. What it comes down to it what your comfortable doing. I dive RB's now.........some think I'm crazy for that too..........and other places is well accepted.