hp vs lp

[elan]

Excuse my ignorance on this but why go with lp tanks over a hp tank? Is there an advantage to using LP tanks? better for doubles?

If you compare by physical dimentions and given you can overfill the LP to the HP pressure I see no advantage of using one over the other. They often come in pairs, Worth LP85 - HP100, LP108 - HP130, LP95 - HP 118 . although they are almost the same tank they are not the same. LP108 is longer than HP130 , LP85 longer than HP100. So that might be better might be not depending on your body. I went with LP108 as I feel it works better for me than HP130.

If you compare the tanks by the volume instead of the dimension the HP will be smaller and lighter, but more difficult to pp blend and in general get a good fill.

You can always under fill an HP tank but it is not true the other way.

 

[elan]

For arguments' sake a LP109 is roughly the same dimension as a HP130. I believe that the HP130 weighs more empty because te walls of the tank are more robust. I've heard of folks replacing (doubling-up) the burst discs on the LP109 and cramming 4000+psi into it - at which point it carries more air than the HP130. Probably not applicable unless you're expedition diving, and certainly not the safest method - but it's one reason I've heard folks picking LP tanks.

I personally own 8 HP tanks because we do lots of shore diving and the smaller format and lighter weight are beneficial (also diving in cold water with a drysuit the negative properties get lead off the waist).

Which LP109 are you referring to ?

Worth LP108 is bigger than HP130 and heavier by 3 pounds. The internal volume of the cylinder is also bigger. If I remember correctly the LP108 doubles factor is 8.2 while 130 is around 7.6

 

[mathauck0814]

Right - so at its working pressure it holds less than the HP130, but it's still bigger and heavier (+1 for HP130). That said, if you bolster the burst valve the internal volume can be pressurized to hold more cf of gas than the HP130 (+1 for the LP108).

Where I do a lot of shore diving, things like size and weight are of importance to me so I like HP tanks. For folks that do a lot of expedition diving off boats, an overstuffed LP might be a better choice.

 

[elan]

Right. So for me the bottom line is its not really an hp vs lp but rather the properties of each tank needs to be considered.
The only HP tank that worked for me was 130. I think it would be great if worthington came up with a 7.25 by 26 " hp tank.
Untill then Faber LP 85 is my favorite.

 

[OldNSalty]

Which LP109 are you referring to ?

Worth LP108 is bigger than HP130 and heavier by 3 pounds. The internal volume of the cylinder is also bigger. If I remember correctly the LP108 doubles factor is 8.2 while 130 is around 7.6

What's this double factor?

 

[elan]

What's this double factor?

cuf per 100psi

 

[nielsent]

One difference that i have noticed with some, but not all, HP tanks is that they are made under a special permit, so they don't have 3AA in their stampings. As a result, if the special permit were to end up going away, then the tanks wouldn't be allowed to be filled. All LP tanks are 3AA steel, and this is the standard, and does not expire because the permit is up.

-- nielsent

 

[elan]

Good point about the permit. Although so far even PST permits seems to be renewed

 

[Slamfire]

I recently started researching on this subject because I just got a couple of LP108's, so please correct me if I state something wrong.

It seems the 3AA certification is not exclusive to just scuba cylinders. Pressurized cylinders used in other capacities also use that certification. Those other capacities require some extra conservatism in terms of the pressure each cylinder is supposed to handle safely. Because of this, scuba cylinders can get exemption certifications that are not as conservative as the 3AA.

It is common to find 3AA certified LP scuba cylinders. HP scuba cylinders usually have an exemption certification. In other words, 3AA LP's meet higher standards. Therefore it is logical to see that their certified rated pressure is lower than HP's which use looser standards.

I got this chart from the faber website. I added the last column myself.


You can see that all cylinders in the chart have the same diameter and internal volume. Unless they have shoulders/bottoms of different thicknesses, we can assume that the all 5 cylinders also have the same external dimensions. The cylinders marked in pink are US only versions since they have no TC certification. Only the cylinder marked in yellow is 3AA certified. This one weighs 41 lbs. The HP version of this cylinder weighs almost 43 lbs. If both of them have the same dimensions, the difference in weight would have to be explained by the use of a denser steel alloy.

Like wise the exemption certified LP versions weigh only 34 lbs. Following the same logic we can conclude that they use a less dense alloy for these tanks.

In light of this, I would feel fairly safe routinely filling the 3AA certified cylinder to 3000 psi and would not expect the tank to fail hydros or show any other signs of metal fatigue. I would even feel ok occasionally filling it to 3442 psi for dives where I'll need 140 cu ft per tank.

 

[Leadking]

you are correct in the material spec. 3AA is a material spec. not a scuba spec.

You are wrong about lp having greater safety built into the cylinder. All cylinders go thru the same safety protocols when designed and tested. It is this fallacy (that lp's are under rated) and some how the safety factor on Exempt cylinders is less.