Tank questions - LP, HP, + and advice for me.

[crpntr133]

What it really boils down to is whether you plan to dive mix in the future. If so, then the LP tanks are the way to go unless you're going to have a shop with a booster do all your fills, in which case you've got enough money to just buy one set of each and throw away the ones you don't like.

No mix, then the HP will serve just fine.

Never thought of this but a good point to bring up.

 

[Skeeter1097]

I have 108 lp's and thay usley feel them to 3200. that is about 140feet of gas per tank.

 

[BKP]

I have AL C80s which fill to 3300psi, and have never had a problem getting a full fill to that pressure.

I'm also looking at HP X7-100s right now. The 80s, at full psi give you 77 cuft. The steel 100s at full fill (3442psi) give you a full 100 cuft., at only a couple of extra lbs.(over the C80), and better negative buoyancy characteristics. If you don't want to stress your gear at full fill (3442 *will* stress your 1st stage more then 3000 over time), you can always get a 3000psi fill, which will yield a full 89 cuft.

The only advantage that I can think of for LP tanks is that they were considered (at one time) to be better suited to partial pressure blending, and they can be slightly overfilled with a hot fill, and then cool to full psi rating. However, the new 3442 cylinders have seem to become the favorites for the underfill capability, buoyancy, and weight characteristics. The LPs are heavier, larger, and while overfilling is a common practice (since the LPs were over-engineered), it's still an overfill.

Where's that James Bond pen-sized re-breather when you really need it?

The + designation allows you to do a 10% overfill on the so-designated LPs (3AA cylinders at 2400psi filled to 2640), . This is only good from the factory, until the next hydro, at which time, you have to do an extra step in the hydro testing, in order to maintain the + designation. However, most shops will apparently continue to overfill, even without the +.

 

[pescador775]

I scanned all these posts but I'm still not sure whether your questions were answered. High pressure cylinders have two important ratings, working pressure and test pressure. For the average user, the working pressure (WP) is all one needs to know. However, it gets a little tricky because some steel cylinders made of 3AA chrome molybdenum steel can be legally filled to 110% of WP. This practice goes back to WWII when it was decided that more gas could be transported this way with acceptable risk. The commercial interests adjusted their advertising such that the stated capacity of the steel cylinders factored in this "plus" fill. Thus, a steel "72" is actually a "65" unless filled to 2475 psi (2250 + 10%). This 10% overfill is guaranteed for 5 years only then becomes subject to retest. 3AA steel is a heat treatable alloy with high tensile strength and elasticity (ductility). However, there are more advanced materials, typically chrome manganese steels used in HP tanks. These tanks can accept higher pressures in thin walled pressure vessels. Examples are PST & Worthington HP 100's, etc. There are 3AA tanks which accept HP but the wall thickness is greater and the tanks typically weigh about 6 or more pounds more than a similar HP tank made of Cr-Mn high tensile steel. Examples are older Faber, Asahi, etc. Hence, the 3AA high pressure tanks are quite negatively buoyant and the "Special Permit", high tensile tanks are approx neutral buoyant. Regardless of alloy, all steel tanks require a protective finish. These vary from hot dip galvanizing, metallizing and paint. Of these, the hot dip galvo will last longest. Hope this helps.

 

[DirtRider]

Very informative and interesting. So the faber hp 120s I was considering above must have been the 3AA variety and hence why they are so much heavier for roughly the same external dimensions.
Any positives or negatives with these different alloys besides the weight?

 

[pescador775]

Advantages/disadvantages? Think of the Cr-Mn tanks as highly tuned sportscars. They have outstanding performance at a cost. You definitely don't want to get any thick rust within those thin (.187) walls. Special procedures are required for hydro testing and these high tensile tanks have had reported failures for no particular reason. Advantages are light weight and high capacity. Recently, it has become possible to buy these tanks with standard 3/4 threaded neck, a big plus. Lately, all USA made HP cylinders are hot dipped galvanized. The Faber 3AA, Cr-Mo, HP tanks are built like a "tank" (the other kind) and are internally anti rust coated. The exteriors are metallized and painted. However, as the Faber 3AA are plus rated to capacity it is encumbent to have the plus renewed. They should last forever if not abused, and provided the exterior is cared for. Recently, Faber has begun advertising a light weight 3442 psi cylinder. I don't know if the company still exports the 3AA HP type to the USA. It's days may be numbered.

 

[in_cavediver]

Very informative and interesting. So the faber hp 120s I was considering above must have been the 3AA variety and hence why they are so much heavier for roughly the same external dimensions.
Any positives or negatives with these different alloys besides the weight?

This is purely my opinion. I have dove (and own) PST LP 104's, PST HP-100's (borrowed), Faber LP 85's and Al 80's. If you are diving a wetsuit, its hard to beat the AL 80's. LP 85's/HP-100's are a close second and may be an excellent choice if you decide to goto a drysuit later. (AL tanks with a drysuit simply suck). If you want max air, LP 104's or the newer E-130's are simply put and excellent tank. I have dove them wet and they are very negative and not recommended without a redundant lift source.

Don't get overly concerned about gas capacity. Most every steel doubles set out there will give a VERY large amount of gas that will allow your to greatly overspend NDL times in rec limits. (tanks listed above start at 160cf and go up). I also wouldn't worry about the pressure rating to much. There are arguements for both sides of which to go and the good news, in the end, it doesn't make that much of a difference which you get. They both work fine for all types of diving.

Last bit, ditchable weight. With doubles, neither I nor my wife dive with ditchable weight. We try to dive balanced and trimmed rigs that will allow us to swim up the full negative weight. (up to 20lbs in my 104's with a cave fill). We also dive dry so we have some redundant lift there as well. We simply don't need to add lead.

If you can, before buying the tanks, try some out. See what you like and see if the weight really isn't an issue. There is a world of difference carrying the LP85's vs the LP 104's out of the water. (and silly me wants to try 121's)

 

[divinginn]

I have AL C80s which fill to 3300psi, and have never had a problem getting a full fill to that pressure.

I'm also looking at HP X7-100s right now. The 80s, at full psi give you 77 cuft. The steel 100s at full fill (3442psi) give you a full 100 cuft., at only a couple of extra lbs.(over the C80), and better negative buoyancy characteristics. If you don't want to stress your gear at full fill (3442 *will* stress your 1st stage more then 3000 over time), you can always get a 3000psi fill, which will yield a full 89 cuft.

The only advantage that I can think of for LP tanks is that they were considered (at one time) to be better suited to partial pressure blending, and they can be slightly overfilled with a hot fill, and then cool to full psi rating. However, the new 3442 cylinders have seem to become the favorites for the underfill capability, buoyancy, and weight characteristics. The LPs are heavier, larger, and while overfilling is a common practice (since the LPs were over-engineered), it's still an overfill.

Where's that James Bond pen-sized re-breather when you really need it?

The + designation allows you to do a 10% overfill on the so-designated LPs (3AA cylinders at 2400psi filled to 2640), . This is only good from the factory, until the next hydro, at which time, you have to do an extra step in the hydro testing, in order to maintain the + designation. However, most shops will apparently continue to overfill, even without the +.

where are you getting your fills in atlanta?I having probs getting a legal fill.

 

[SparticleBrane]

They've been doing cave fills (3600psi) on LP steel tanks for quite a while now and I don't know of any explosions/tank failures due to this high pressure. Hydro failures, perhaps, but I don't specifically know of any explosions due directly to the high pressures.

 

[BKP]

where are you getting your fills in atlanta?I having probs getting a legal fill.

PM'd you with the info...