HP or LP double

[limeyx]

Well, I've never heard math called misleading.. but ok. I selected tanks that at rated pressures have nearly the same capacity. For dive planning, in order to not do dissimilar cylnder calcs, it's helpful if everyone is working with the same VOULME of tank. Who cares what physical size the tank is?

But I'll humor you:

First example, I get 95cuft, you get 119.

Second example I get 108 you get 104

Third example I get 126 you get 121

Again, the math proves out that the ONLY instance you get more gas is when the shop CAN fill you to 3500 and they refuse to overfill me.

So what was your point?

The physical size of the tank matters (generall) because I could say "Hmmm, should I get an LP95" well, for maybe slightly more $ I can get an HP-119 and have the same form factor. Is that worth it? Maybe yes, maybe no.

From my POV (and for some of the reasons you state), I am not going to say either an LP95 or HP-100 because they are essentially different tanks.

And maybe you have a different LP95 concept, but, I calculate the tank factor for LP95 (where the tank is 95 cft at 2640) and of the HP-119 to be 3.5 in each case (the HP119 is actually more like 3.45 compared to 3.55 if we really want to get into it) so your numbers still dont look right.

And volume is definitely not the only consideration. Many people do many kinds of diving. Some of my buddies have higher breathing rates than I do, and so prefer larger tanks. Some people trim out horribly in certain kinds of doubles. While it may be normal in cave diving to have exactly the same tanks, that's not universally true.

The only reasons I could think to get an LP95 over an HP119 would be price, and I think the LP's can weigh somewhat less than the HP.

 

[PerroneFord]

The physical size of the tank matters (generall) because I could say "Hmmm, should I get an LP95" well, for maybe slightly more $ I can get an HP-119 and have the same form factor. Is that worth it? Maybe yes, maybe no.

Correct. And the general premise is that you can get more gas in a 119 than you can in a LP95. In your case, this might be true. In mine it is not, which is what I have attempted to demonstrate.

From my POV (and for some of the reasons you state), I am not going to say either an LP95 or HP-100 because they are essentially different tanks.

Different in looks, very close in volume.

And maybe you have a different LP95 concept, but, I calculate the tank factor for LP95 (where the tank is 95 cft at 2640) and of the HP-119 to be 3.5 in each case (the HP119 is actually more like 3.45 compared to 3.55 if we really want to get into it) so your numbers still dont look right.

Ok, well lets go through the math.

Example 1:

(95cuft * 2640) / 2640 = 95cuft
(119cuft * 3442) / 3442 = 119cuft

Example 2:

(95cuft * 3000) / 2640 = 107.95cuft
(119cuft * 3000) / 3442 = 103.71cuft

Example 3:

(95cuft * 3500) / 2640 = 125.94
(119cuft * 3500) / 3442 = 121.01

Good enough?

And volume is definitely not the only consideration. Many people do many kinds of diving. Some of my buddies have higher breathing rates than I do, and so prefer larger tanks. Some people trim out horribly in certain kinds of doubles. While it may be normal in cave diving to have exactly the same tanks, that's not universally true.

In your diving, do you calculate how much gas YOU need to carry to rescue your buddy with the larger tank? If so, then you are doing combined SAC rates on the smallest tanks. So we're back to smallest volume again.

You are right that some people trim out horribly in certain tanks. Hence my posts here with XS Scuba about the buoyancy characteristics of the HP tanks versus the LP tanks I am familiar and comfortable with.

The only reasons I could think to get an LP95 over an HP119 would be price, and I think the LP's can weigh somewhat less than the HP.

In my case, the reasons include more gas volume, cheaper price, and easier availability. No one near me sells HP tanks.

 

[gcbryan]

Well, I've never heard math called misleading.. but ok. I selected tanks that at rated pressures have nearly the same capacity. For dive planning, in order to not do dissimilar cylnder calcs, it's helpful if everyone is working with the same VOULME of tank. Who cares what physical size the tank is?

But I'll humor you:

First example, I get 95cuft, you get 119.

Second example I get 108 you get 104

Third example I get 126 you get 121

Again, the math proves out that the ONLY instance you get more gas is when the shop CAN fill you to 3500 and they refuse to overfill me.

So what was your point?

The point is if two tanks are physically the same and one is rated for a higher pressure then there is nothing to be gained by having the same tank that hasn't got the higher rating. Your numbers above show that.

Except in cave country most any shop will not overfill and most will be able to handles 3,500 psi.

Edit: I just saw Limeyx reply.

 

[PerroneFord]

Except in cave country most any shop will not overfill and most will be able to handles 3,500 psi.

Fair enough. But I live in cave country, and my questions to Phil and XS Scuba were based on my needs. If it doesn't make sense for others, then they need to make different choices.

 

[gcbryan]

Drop?

If the room held together, I'd think compression of gas in an enclosed space would raise the temp...

Unless the room in sealed there is no compression going on. Suddenly decompression would lower the immediate temp.

 

[PerroneFord]

Unless the room in sealed there is no compression going on. Suddenly decompression would lower the immediate temp.

Hmmm

So we have a scuba cylinder in a fixed space with compressed gas in it. We puncture the tank. The Tank gets depressurized, but the room gets pressurized by exactly the same volume. No?

 

[CompuDude]

Hi Lamont,

Normally HP refers to cylinders that are 2500 psi like the Genesis, the new exemption tanks are very close in pressure but not really "HP" tanks, they still use standard valves and yoke fittings are common.

The 130 Exempt are similar to 108 LP but not 130's HP, follow?:huh:

Did you mean 3500 psi Genesis?

I've never considered 2500 to be high pressure, as far as scuba tanks is concerned...

 

[CompuDude]

Hmmm

So we have a scuba cylinder in a fixed space with compressed gas in it. We puncture the tank. The Tank gets depressurized, but the room gets pressurized by exactly the same volume. No?

As the gas rapidly expands, it absorbs energy from the surrounding area. That's why your valve gets cold when you let the air out fast.

The room is not nearly as sealed as the tank was. Air will simply be pushed out every little door and window crack. Nor is the air as compressed in the room as it was in the tank. You won't gain any significant pressure in the room, certainly not enough to raise the temp enough to compensate for the huge energy absorption of the expanding air coming from the tank.

 

[gcbryan]

Hmmm

So we have a scuba cylinder in a fixed space with compressed gas in it. We puncture the tank. The Tank gets depressurized, but the room gets pressurized by exactly the same volume. No?

No. The room isn't sealed.

 

[PhilEllis]

As the gas rapidly expands, it absorbs energy from the surrounding area. That's why your valve gets cold when you let the air out fast.

The room is not nearly as sealed as the tank was. Air will simply be pushed out every little door and window crack. Nor is the air as compressed in the room as it was in the tank. You won't gain any significant pressure in the room, certainly not enough to raise the temp enough to compensate for the huge energy absorption of the expanding air coming from the tank.

What he said!

Phil Ellis