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mrmccoy
01-20-2010, 07:10
I have noticed there are a lot of Lp tanks for sale. I'm guessing they are cheaper then a Hp tank. Other then price what is the draw to Lp tanks? I know most are on doubles but why not us a Hp to get the most gas?

FishFood
01-20-2010, 07:53
I guess 'most gas' is determined by where you're located. Have one filled down here and you'll get more gas than an HP of comparable size.

Scuba Pete
01-20-2010, 08:05
HP fills are harder to come by. I did my Cave class with a borrowed set of the old hp 100s the 3500psi version. While i loved the way they trimed out, they just didnt have enough gas. My buddy was diving LP98's and our instuctor was lp104's. I was always calling the dive on thirds. My tanks had roughly 205cf, buddies where 267cf, and our instructor 289cf. The amount of gas is a huge criteria for me. LP tanks also have a 50+ years of history while the new hp 3442psi tanks only have 5-10 years. The LP tanks are proved while the HP tanks have yet to be. Also another point for the LP is that the HP tanks are on a DOT exemption. If the DOT does not renew the exemtion then you have a bunch of money in tanks that you can only use for a maximum of 5 more years as they will not pass the next hydro. There is now a shift in people thinking that if you get a HP tank then you dont have to overfill the tank to get the gas needed. There are also more people who would give you a full fill in a hp tank than people who would fill a lp tank to the same pressure as the hp.

CompuDude
01-20-2010, 12:20
LP vs. HP and which is "best" depends very much on your local fill stations.

LP tanks used to be cheaper than HP tanks, which were new at the time and higher priced, but the difference is not as great any more. Now it comes down to availability and fill stations.

Many fill stations and LDS's follow the rules. This means an Al.80 gets filled to 3000 psi, an LP tank gets filled to 2640 (see below for a caveat), and an HP tank gets filled to 3442/3500 psi.

When the stations follow the rules, you're going to get more gas in a lighter tank with HP, every time.

In certain areas, however, particularly Florida cave country, "overfills" are common with LP tanks. They pretty much ignore the rules and fill the tanks, rated to 2640, up as high as they want... 3000, 3500, or even higher. In this case, you're going to get a lot more gas in the LP tanks, compared to the HP tanks which people are generally more loathe to overfill. (Aluminum tanks are also not safe to overfill, but LP tanks seem to handle it better and have a good track record in this regard.)

On the other hand, in other areas, it's not uncommon for fill stations and LDS's to have old, small and/or underpowered compressors that really struggle to fill tanks to high pressures. Heck, sometimes they have a hard time filling an Al.80 to 3000 psi, not to mention an HP100 to 3442! In that case, you want to buy an LP tank based on the capacity you need: If you really need 100cf of gas, you'd better buy an LP104 just in case, because if they won't overfill (slightly) an LP95, you won't get 100cf, and if they can't fill your HP100 to 3442, you're not going to get 100cf, either. (You could, however, buy an HP130 and let them underfill that, and still end up with 100cf as long as you get at least 2640 psi.)

Important caveat with LP tanks: LP tanks are size-rated (from the factory) to include a 10% overfill. The tanks are actually only rated to 2400 psi, but with a "plus" rating (an actual Plus sign stamped into the metal next to the hydro year stamp), you're allowed a 10% overfill, or to 2640 psi. Only at 2640 psi do you actually have a "full" fill that gives you the rated capacity of the tank (85cf in LP85, 95cf in LP95, etc.).

The LP tanks require the Plus rating to be re-certified every 5 years to at the time of Hydro testing to maintain their "plus" rating. If you don't have the Plus testing done (by a hydro facility that's willing and able to do it), your LP85 (which is only 85cf when filled to 2640 psi) just became an LP77, since the tank is now only "allowed" to be filled to 2400 psi. Some areas pretty much ignore this rule and fill to whatever they want anyway, but if you're in an area that follows the rules, this can bite you if you don't know what you're doing.

Locally, for me, fill stations follow the rules and will not overfill LP tanks. As a result, it makes more sense to buy HP tanks. Some places with less-robust compressors can't fill them quite all the way every time, but I still get more than I would with an LP tank, in a much more compact and manageable package.

mrmccoy
01-22-2010, 08:04
Thanks guys! I just noticed and figured I would ask about it.
So it basicly depends on what your local fill stations will and won't do. Another thing I just thought about was size, the HP cylinders are smaller then the LP due to the pressure differences.

Scuba Pete
01-22-2010, 08:19
Yes that is true. A HP 100 is smaller than a LP100 would be. But HP's are comperable in size to their LP cousins ie. A lp85 is comperable in size to a HP 100.