HP or LP double

[limeyx]

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.

Exactly my point! to a +/- 2-3 cft or so, an LP95 is (generally) a complete subset of a HP-119, whereas an HP-100 is not a subset of an LP95 (or the other way around).

So wherever you see LP95 generally you could write HP-119. There may be some specfic cases (like a really negative LP95 say) for exceptions, but generally speaking.

 

[limeyx]

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.


Different in looks, very close in volume.

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?



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.


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

yeah, the numbers are close enough -- basically I'm not going to worry about +/- a couple of cft either way.

In some cases the tanks look identical (PST LP80/HP-100 are almost indistinguishable)

Rescue gas is calculated for "worst case"

In a cave dive, this (generally) comprises a far greater percentage of my total gas than an ocean dive on 1/2's.

For an Open water dive, I (generally) only need to account for my buddy in my rock bottom gas (to get us to the surface) unless I am penetrating a wreck where it looks more like a cave dive.

Price and availability were the two factors I mentioned in my post as points where the LP95 may be an advantage. I dont see the "gas volume" advantage, except that I guess some LP tanks like the 120 may not have an HP equiv.

However, the LP120 is a monster of a tank....

 

[lamont]

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?

I seriously doubt the manufacturers numbers are accurate to 4% -- those values look identical from where I'm sitting... Plus I thought the difference in cubic feet between overfilled LPs and E8/X8 tanks were from nonlinear incompressability...

If you need more gas, get LP104/LP108/HP130s or else go for jacked up LP120s...

 

[limeyx]

I seriously doubt the manufacturers numbers are accurate to 4% -- those values look identical from where I'm sitting... Plus I thought the difference in cubic feet between overfilled LPs and E8/X8 tanks were from nonlinear incompressability...

If you need more gas, get LP104/LP108/HP130s or else go for jacked up LP120s...

Yeah me too. My gas planning is not accurate enough to care anyway -- the TF is 3.5 for both tanks and they are treated essentially the same.

having said all this, I'm not saying I wouldn't necessarily overfill an LP tank (or that i would

BUT I hear "Oh yeah, LP tanks are great because you can get a full fill while you cant with an HP tank" all the time and it's a meaningless statement.

In my view LP80 is a complete subset of HP100, same for LP95/HP119, LP104/HP-130 excepting cost since it's harder to find the HP tanks used still probably.

 

[lamont]

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:

You mean 3500 vs. 3442? Yeah, but colloqually its easier to treat all ~3500 psi tanks as HP and 2640 tanks as LP when discussing scuba diving though... I consider 3300 psi Faber 100s as being "HP100s" as well... YMMV...

 

[cerich]

You mean 3500 vs. 3442? Yeah, but colloqually its easier to treat all ~3500 psi tanks as HP and 2640 tanks as LP when discussing scuba diving though... I consider 3300 psi Faber 100s as being "HP100s" as well... YMMV...

I follow your thought, just be aware you will run into different valve threading, so if you have a exemption tank and want a manifold and asked for a "HP Steel Tank Manifold", there is a very good chance you will get the wrong one.

 

[lamont]

BUT I hear "Oh yeah, LP tanks are great because you can get a full fill while you cant with an HP tank" all the time and it's a meaningless statement.

Yup, drives me nuts too.

If we used the metric system and tanks were rated on water volume and service pressure it would be so much easier to see that LP104s and HP130s were the same tank only with a higher service pressure.

All things being equal buy the HP version of the tank.

 

[limeyx]

Yup, drives me nuts too.

...

All things being equal buy the HP version of the tank.

But then I wont get a "full tank" ... oh wait

 

[lamont]

I follow your thought, just be aware you will run into different valve threading, so if you have a exemption tank and want a manifold and asked for a "HP Steel Tank Manifold", there is a very good chance you will get the wrong one.

Good point, the old PST HP tanks are not the same as the PST E-series in that respect...

 

[PerroneFord]

If you need more gas, get LP104/LP108/HP130s or else go for jacked up LP120s...

I thought that was clear from my post yesterday asking Phil about the 108 and 121s....

The other fly in the ointment here is the buoyancy characteristics of the HP tanks. They are made of a different metal and may have completely different trim than their LP volume bretheren. So yes, the 104/130 share similar volume at similar pressure, but they may trim very differently because of dissimilar construction methods and materials. Hence my question to XS Scuba.