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Thread: HP vs LP

  1. #1
    TadPole DiverDan28's Avatar
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    HP vs LP

    Hey everyone. Can someone explain the diffrence between HP tanks and LP tanks. I have AL 80s and looking at steel to lose lead. I dive fresh water and am looking for 120s or 100s. Also if a HP 120 can hold 130 why don't we call them steel 130? Just curious? And lastly with the huge price difference are HP tanks really worth it.

  2. #2
    Grouper
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    My take is that (at least locally) dive shops don't want to run their compressors up to the pressure required for HP. I've heard them say that it puts strain on them and so they charge extra. Also a hot fill (which comes up short once cool) is more likely. Another thing to consider is that (I think) you need a din connecton for HP. I've also heard that LP puts less strain on your equipment. Don't know how much of any of that is true.

    As far as fill pressure. My Faber LP's can only be filled to 2640 psi here. I'm told that in Europe, with a higher rated burst disk and the din connection that they are filled to 5000 psi. Same caveat.

  3. #3
    Megalodon
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    The advantage to the HP is itís a smaller package for more air. A HP119x8 is 24Ē w/o valve and the HP121x8 is an insane 29Ē (want one?Iíll sell you mineÖ.). Getting a full fill at times is problematic and a hot fill is common, no matter how slow they fill.

    The advantage to a LP while bigger can get a full fill at lower psi and will also put less stress on you regulator when you consider 2650 vs. 3442. A lot of cave divers will overfill the LP steel tanks to get more air. If you went big with the LP121, you get 138cuft at 3000psi. The overfill debate is ongoing and interesting to say the least so Iíll leave it alone.

    My personal favorite is the Faber LP85: Itís fairly small, easy to carry at 35lbs full and if I get a bit of an overfill, Iím not worried.

    As far as value, Give the life expectancy of steel and the forgiveness with overpressure, I think they are.

    I once saved a man in Wichita just to watch him dive...(inventor)

  4. #4
    Quote Originally Posted by DiverDan28 View Post
    Hey everyone. Can someone explain the diffrence between HP tanks and LP tanks. I have AL 80s and looking at steel to lose lead. I dive fresh water and am looking for 120s or 100s. Also if a HP 120 can hold 130 why don't we call them steel 130? Just curious? And lastly with the huge price difference are HP tanks really worth it.
    There is HP then there is HP. The original HP tanks were 3500 PSI and only DIN valves. The idea of having 3500 PSI or higher and a Yoke valve was frowned upon. Someone came out with a 3442 PSI tank. They still called it HP but now you can use it with Yoke valves. So if your regulators are Yoke valves you have to go with 3442 PSI or lower.

    LP tanks are typically 2400 PSI. When you buy them they have a plus rating (+) and can be filled to 10% beyond 2400 PSI or 2640 PSI. During hydrostatic testing they determine if they can still have the (+) rating. If they can they stamp them with a (+) and you can fill them to 2640 PSI but there is always a chance they won't get the plus rating and you can only fill them to 2400 PSI. If the tank is say a LP121 then you can fill it to 121 cu.ft. @ 2640 PSI. This means if you don't get the plus rating an LP121 is really 110 cu.ft. It is rare to get an LP tank which does not get a plus rating but it can happen. More often I have seen shops fail to test the tank for the plus rating and it comes back without the (+). You then have to pay to get it re-tested, complain to the store and see if they'll re-test it properly, wait 5 years until you need to have it tested again.

    Why is an HP119 really 123 cu.ft.? Good question. A lot of people have asked this and I've never seen a good answer. I think you'd have to ask someone at Worthington or Faber. Divers, shops, distributors, etc. have never given me a good answer. Some guess it is marketing. They came out with an HP119 expecting it to be around 119 cu.ft. They market it as the HP119 and later find out it is really 123 cu.ft. Rather than change the name to HP123 they just stick with HP119.

    It used to be the case that finding a dive shop which could fill an HP tank was rare. So you buy an HP tank but the shop fills it to 3000 PSI because that is all their compressor can handle. I purchased steel tanks in Canada and all the shops around there have a lot of technical divers. Filling a tank to 3500 PSI is a common thing. So worrying about a short fill isn't as bad. Now I know when I worked at a shop which filled a LOT of tanks they would fill AL80s to 3300 PSI because they did a hot fill and knew it would cool down to 3000 PSI. To fill a 3442 PSI tank they would have to fill it to almost 3800 PSI. That is going to put a lot more stress on the compressor. The last shop I worked at that did mostly HP tanks would insist you leave the tank for a few hours or overnight. They would then slow fill the tank and not worry about giving you a short fill. They also had a computerized system and a 16 tank bank. They could fill 4 tanks at a time and unattended. It was more common to get short fills on HP because the staff didn't have time to give you a slow, cool fill. Bottom line, make sure the shop gives you a proper fill and expect to leave the tanks overnight. I usually drop the tanks off on a Monday and pick them up on a Thursday because I only dive weekends.

    Also, the buoyancy characteristics for LP versus HP will be different. As noted, an HP120 is going to be 24" long but an LP121 is going to be around 29" long. For really tall people having the long tank spreads the weight out nicely. For shorter people the long tank might mean you are hitting your thighs on the bottom of an LP121.

    Stress on the regulator is also a concern. If you use HP all the time you might find you need to service your regulator a little more frequently. When I had a lower cost regulator (not cheap but just not really expensive) it could go 2 years without a service on LP tanks. I switched to HP tanks and noticed it went out of tune in just over a year. Now my current regulators went 3 years without a service on HP tanks and are starting to show signs they need a service. So HP will put stress on your regulators but if you are using high-end regulators it won't matter too much.

    Finally, if you live in certain areas you can get what people call a cave-fill. The cave divers of northern Florida wanted to maximize the amount of air they had available. If you take an HP130 and fill it to 4000 PSI you might get 151 cu.ft. of air. However if I take an LP121 and over fill it to 4000 PSI it will hold 182 cu.ft. So they tend to get the LP tanks and over fill them. Some argue that you are shortening the life of the tank by overfilling it so much. Cave divers argue that they overfill the tank when doing hydrostatic testing and that you should be able to hydrostatically test the cylinder 10,000 times. I don't know where they came up with these numbers. I do however know that if a tank is going to explode it will explode when you are filling it. So if I ask a shop to overfill my tank the shop is putting themselves in danger and I, the diver, are fairly safe.

    Personally, I like to use HP tanks in Canada because I do wreck diving and dive thirds. One third to get down, one third to get back, one third for emergency. So if I have 123 cu.ft. of air in the tank I really only use 80 cu.ft. of air on each dive. However diving in Dallas it open water diving. In an emergency I can just swim to the surface. So I'll dive a tank down to 300 PSI left. If I used a HP tank I'd probably end up underwater for well over an hour. My throat gets really dry diving an AL80. I cannot imagine staying down long enough to breath down an HP119 or even an HP100. Maybe if I start diving deep again I'll being my HP tanks down to Dallas but if you are staying around 30 to 60 feet (where all the good marine life is) then an AL80 seems fine to me.
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  5. #5
    TadPole DiverDan28's Avatar
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    Wow that's a detailed explanation. I never thought about the impact on my regs another eye opener for me. I have the envoy regs with the yoke. So I may just run LP unless I find a steal on some HP tanks. Then save and buy better regs. I'm new to diving and am kicking myself as I should have got certified when I was 16 living in California. I'm sorta kicking myself about my Scout BC and the cost of a bigger bladder now but, I needed to start somewhere. Thanks everyone
    Walk a mile in my shoes and your feet will be tired too.

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