High Pressure versus Low Pressure

[Seaslave]

I'm in the market for a new set of tanks. I have searched the forums and haven't found an answer for this question. Besides size and weight why would I want one or the other. What are the reasons I would use to decide one veresus the other? Size and weight are not an issue, I just want more gas. For now they will be used as single tanks for air and possibly nitrox. I may double them up someday. I am looking at 121cf tanks but can't decide between LP or HP? Any advice is welcome. Thanks.


Mathue

[Dmitchell]

I would recommend LP. If you ever might want to blend in them it's much easier.

I haven't ever dove LP121's but the HP 120 IMO sucks mostly because I'm only 5'8" and with valves, they are almost half that.

Dave

[Sounder]

The Faber HP120s suck... they're really heavy EMPTY (let alone full!) and LONG!!!

On the other hand, I LOVE LOVE LOVE my Worthington HP130's. Short and squatty but not much more to haul around than more traditional sizes. If you'd like to dive a HP130 just let me know!

Other considerations is buoyancy when empty... i.e. AL80's float when empty, which can cause a problem at the end of a dive if you're fighting to stay down. Proper weighting can fix this problem though.

[Seaslave]

I'm 6'2" and dive 99.9% from a boat so lugging them around isn't an issue. The blending thing could be an issue. Any other things I should consider. I am struggling to see why, other than size or weight of the tank, why anyone would want anything other than a LP tank. The ones I buy will of course be steel. Are all steel tanks negatively or neutrally buoyant when empty? Thanks for the input so far.


Mathue

[dsteding]

Here's the LP/HP thing in a nutshell:

LP 95s are the same size as HP 119s, 104s are the same size as 130s, and 85s are the same size as 100s.

So, think of an HP 119 as a 95 when filled to 2640, and a 119 when filled to 3500, it is the same basic tank. Given that, why not get the HP tanks? True, blending may be an issue some day, but most places have boosters now, and if you go the home blending route, you'll probably end up with one as well. The HP Worthingtons are particularly nice, I like the galvanized finish because it is much more durable than the painted fabers.

[Cera]

The HP tanks are smaller (as stated above). One of the things that I heard (just rumors) was that alot of the new (cheeper) steel HP tanks were failing thier second vis because of rust (the first vis AFTER use). They survive after tumbling them, and don't fail hydro. I have only had one 'new' tank fail out of 8 and it had water in it! (Meaning the person who had it did something serious to get that much water in it....not just normal fill/use issues)
The LP tanks generally have a bigger swing in weight full vs. empty. I assume you have seen the charts... (I can try to find them if you haven't seen them)


Maybe this site can help
http://www.diveriteexpress.com/library/ ... l#pressure

[Dmitchell]

Because even with a booster, and filling slow, you have to deal with the heat of compression and while it's not all that difficult to carefully blend to 3500, it's a lot easier to blend to 2640.

I get what you are saying about getting the bigger tank and being able to fill to what you need "legally".

I don't know if it matters but the suggested retail on all of these varies as well.

The Suggested retail for Worthington's is:
95- $285
108 -$299
121 $315
HP
119 $329
130 $389

Here's a spec sheet on the tanks so you can see the differences.

http://www.seapearls.com/tanks-steel.html

Dave

[RSdancey]

Besides size and weight why would I want one or the other.

Some dive shops will not fill a tank over 3000psi, regardless of its markings. Some stores limit because they don't want to stress their compressor above 3000, and some stores due it out of ignorance (or because they don't want to try and get tank monkeys trained up to be able to read DOT markings on tanks and worry about someone overfilling a tank that shouldn't be overfilled.)

So if you have a HP tank, and you find yourself in a situation where you can't get it filled to working pressure, you'll end up with less gas than you thought you'd have. That could ruin a whole dive plan.

Ryan

[Dmitchell]

Besides size and weight why would I want one or the other.

Some dive shops will not fill a tank over 3000psi, regardless of its markings. Some stores limit because they don't want to stress their compressor above 3000, and some stores due it out of ignorance (or because they don't want to try and get tank monkeys trained up to be able to read DOT markings on tanks and worry about someone overfilling a tank that shouldn't be overfilled.)

So if you have a HP tank, and you find yourself in a situation where you can't get it filled to working pressure, you'll end up with less gas than you thought you'd have. That could ruin a whole dive plan.

Ryan

I'd find a new shop, and being a shop owner I don't say that often. I can't say that I enjoy running my compressor to 3500 but it's a necessary evil. Besides, most modern compressors are setup to run to 5 or 6,000 psi now adays.

Technically, acorrding to OSHA, anyone filling a tank is supposed to have specific Haz-Mat training before touching the compressor. If the shop isn't teaching them to read the markings, they aren't giving them the required training either so they are hanging in the wind.

[TCarter]

Size and weight are not an issue, I just want more gas.

Then just go with HP-130's, if you're okay with the cost. I own LP-95's and HP-130's. I like them both, although the 130's are noticeably heavier. I've never had a problem getting good fills (including nitrox) on either.

Terry

[dsteding]

All the specs you will ever need, but note that the Fabers on this page are listed WITHOUT a valve, so add 1.5 pounds to weight, and a bit for negative buoyancy:

http://tdl.divebiz.net/pub/tanks.html

If all you are concerned about is gas, then I agree, get a 130. It will be 110+ at most short fills (around 3000 psi) and will have tons of gas at rated pressure. The only way you'll get more gas in an LP is to dive a 121, which is a damn big tank (with crappy buoyancy characteristics, you now have 50 pounds of empty tank on your back that can be positive when empty like an AL 80).

Given your size, the 130 will probably trim out well on you as well. I dive them as singles and like them.

The link Cera posted also has a nice discussion on LP versus HP tanks. I guess the perspective is this:

You can buy tanks based on physical size, as in this is the size of tank that fits me. If physical size is the limiting factor, get an HP tank. It will allow you to have more air in less space, assuming both tanks are filled to rated capacity. If the same size LP and HP tank are filled to the same pressure, then they will naturally have the same amount of gas in them.

You can also go find the biggest, honkin' LP tank and toss it on your back. You can then maybe find someplace to overfill the heck out of that tank. As compared to its rated volume, you'll have more air . . . but probably not that much more than an HP 130 to begin with. And, if your LDS has trouble getting HP tanks to their rated pressure, this isn't going to happen anyways.

This all makes me very thankful for my LDS, between them and the occasional fills from Starfish, I always get good fills.

[Pinkpadigal]

Okay...I will show my ignorance...

Everyone talking about purchasing HUGE tanks, and overfilling them. As a recreational diver, I am confused. If you need more bottom time for deco stops, why not just get a set of HP80 or LP72 twins? It is a little more expensive, but you get as much bottom time as you need. The other advantage is that you would not need to overfill the tanks.

Doing a normal recreational dive, even going deep, I would think, for most people, diving with LP, HP or even an AL 100 would be a big enough tank. When I talked to Kal recently, he said he was seeing more and more people with big tanks getting bent because they were not watching their computer but figured they had "plenty of air" so they just stayed down longer. If it is a safety issue having a bigger tank, then why not dive a 100 and a pony?

With that said, the best selling tank in my store is Faber HP 117. I actually considered buying a HP100 but I dive safely with my HP 80 and I get cold or deco before I low on air. The only benefit for me would be the ability to do multiple dives on 1 tank which isn't a good idea for me. My husband actually likes his AL 80s. I asked him if he wanted to upgrade and his response was, why? He doesn't dive as much as I do, and he knows that he can always rent an AL 80 no matter where he goes.

So with this said, why are the big tanks so popular with recreational divers?

[boydski]

Because even with a booster, and filling slow, you have to deal with the heat of compression and while it's not all that difficult to carefully blend to 3500, it's a lot easier to blend to 2640.
Dave

Hi Dave,

I agree that its easier to mix to low pressure, but there's no reason you can't fill your HP tanks to a lower pressure. When filling for Trimix classes, we typically fill both the HP and LP cylinders to 2700, which is plenty of gas for the short training dives, and keeps the cost of the fills down for the students.

When diving deeper (like this Sunday's dive on the Al-Ind-Esk-A-Sea), I fill the HP tanks all of the way up so there's plenty of gas. One of the divers on the boat this weekend had to cut his planned bottom time as he was only able to get his LP tanks filled to 3000 psi.

If Mathue can afford it, I prefer the HP tanks, as they have the extra rated capacity that you can use if you need it. However, for most of my dives I treat them as LP tanks and am very happy with 3000 psi fills.

Of course, YMMV,

Scott (Boydski) Boyd

[dsteding]

Okay...I will show my ignorance...

Everyone talking about purchasing HUGE tanks, and overfilling them. As a recreational diver, I am confused. If you need more bottom time for deco stops, why not just get a set of HP80 or LP72 twins?

So with this said, why are the big tanks so popular with recreational divers?

I totally agree with this, I dive a set of 100s or 119s as doubles when I need more gas. Heck, I dive doubles almost all the time because I eventually want to do dives that I'll need doubles for. That being said, if the OP wants mad gas in a single tank, the 130 is the way to go . . .

For me, the reason I own my single 130s is because I can transfill into my doubles for a second dive off of one of them. They aren't my ideal tanks from a trim standpoint as single tanks (but are getting better as I get more dives on them), but serve good dual purposes (as singles and as transfill donors). With my doubles, a longish (hour) deepish (100 foot) dive at, say Cove 2, has about a 100-120 cubic foot gas need (my SAC is around 0.65). With the 130s, I can transfill into those doubles and do the same dive again, holding sufficient reserve from a rock bottom standpoint back for emergencies.

Personally, I think the perfect single for most people is actually a HP 100 or a 119 (both the same length, with one being 7.25 and the other being 8 inches in diameter). So, I'm not surprised that Faber 117 is very popular at your shop.

[Dmitchell]

Amy,

I think big tanks are popular for a few reasons:

1. heavy consumption rates = shorter dives This can be related to physical size, comfort in the water, being cold, over weighting, physicla fitness, etc.

2. We all want longer dives. Face it, we spend thousands on gear, it stinks to drive all the way to the dive site, spend 20 minutes gearing up, dive for 30 minutes and go home to rinse your gear.

3. We want to be able to keep up with our buddies consumption rates. I know 2 ladies that will stretch an 80 much farther than some of the guys they dive with could ever take a 100. In fact, both ladies have 100's and if they are diving with anyone else, they leave them at home and bring thier 80's.

As for diving doubles, They're big and scary so alot of folks are turned off by them. Not to mention that you need a 2nd regulator and at least a new wing to dive them.

As far as the deco issue, You shouldn't be doing Deco unless you are trained to do deco and if you're trained to do deco, you wouldn't choose to do it on air or a single tank. So that's kind of a non-issue.

Personally, I have 2 old PST 3000psi steel 92's from the early 70's that I dive for singles (H-valves) and then I have a set of double 95's and double HP100's. Those are "my" tanks. I have about 80 others "at my disposal".

Of course the issue of tanks will all become mute in a few months when I get my Reabreather and Mel teaches me to use it!

Scott, I agree with you fully.

Dave

[Cera]

Any shop that doesn't fill the tank to working pressure, is not a shop I would frequent! I know some places are just lazy and give you a 'hot' fill, I have never had a shop flat out refuse to fill to the working pressure. And Nitrox should be no different.
My husband and I have a set of Steel HP 100's and S HP 80's and 2 sets of Steel LP 72's.
We hardly ever use the 100's because we will get cold or run out of Deco time before our air is gone. My husband also complains that the weight of the tank hurts his back (he is 6'6'' 185lbs). We prefer the 80's because of the size of the tank and its nice weight. We would prolly use the 72's but they are yoke valves and our regs are din.
Anyway, when I bought the 80's I bought a mirrored set of valves incase I wanted to make doubles(something the vendor recommended). I have never found the need to do so, but the option is there I suppose. I also have a set-up for the 72's to be doubles, but again, found no point for doubles.
I personally think that anythink over 100 is overkill, but some larger guys have such huge lungs that they require that much... So the question really comes down to weight and size.
I think that this is one of those questions that you have to research and answer for yourself....
I feel like perhaps I have gotten off the subject, so I'm sorry if that is true. Good Luck.

[Dogboy]

I don't know what will work for you, but I am really pleased with the HP (really medium pressure with a "+" rating) 120's I just bought. I know a lot of people post that they are too big and too heavy, but I figure every pound I put on my back is a pound I take off my weightbelt. They are something like 16 lbs neg full, and 9 neg empty. I was able to lose my 22 lb. weightbelt, add 4 lbs to my integrated BC, and it worked out great! I will still be doing some final adjustment on weight and trim, but overall, was very happy. As an added bonus, the heavier, longer tank took care of the tendancy for my back inflate BC (DUI Delta) to push me over face down, without having to use a tank weight. I like the added dive time, but really like the fact that after a 56 min dive at Redondo, max depth 87 ft., I came up with 1100 psi reserve. And this was with a 3180 fill (max fill with + rating is 3498). So at 3180, I basically had a 108. I like the longer bottom time, but really like the added safety of extra gas if there is an emergency. I haven't taken one of Grateful Diver's gas management seminars yet (hope to do one soon), but isn't that what it is all about? Having enough air for you and your buddy to make a safe ascent? I got a screaming deal (I think) on these tanks from Tech Diving Limited ($199 each plus $35 apiece shipping) so I bought 4. Unfortunatley, their web site now says they are sold out! I know that it really depends on your size (I am 6 ft, somewhere north of 200 lbs, my dive buddy son is 6'02, 190), but these fit us well. Of course, I wouldn't strap one on my 5'4 inch wife

[RSdancey]

If you need more bottom time for deco stops, why not just get a set of HP80 or LP72 twins?

Putting one larger tank on a standard rig means buying a tank.

Going to doubles means thousands of dollars in extra expense for most divers.

A lot of people don't intend to to deco diving with their larger tanks. They just want to maximize their NDL time. An AL80 isn't enough gas to safely dive the I-Beams, for example, for most divers, for much more than a quick look around; regardless of NDL.

So people talk about getting a bigger single tank to extend their profiles without getting into "tec diving" and its related expenses.

[Sounder]

There's also the consideration of getting you and your buddy to the surface... when Erica and I were in Maui, we all jumped in and "met on the bottom." Well, the bottom was 94 feet and we were on Al80's.

Seems fine... until you pay attention during one of Bob or Brian's gas management classes. Then you realize that after a few minutes on the bottom, neither Erica or I had enough gas to fix a problem and then make a proper ascent to the surface with the other sharing. I'm sure glad nothing went wrong.

In watching your computer and accidently getting into deco on a larger tank without noticing it and getting bent... they still teach tables in OW. Owning a bend-o-matic should not (yeah, I know... SHOULD NOT, but that doesn't always matter) become a substitute for dive planning and execution. If you don't want to go into deco, plan your dive to stay out of deco.

My 6-8 psi.

[lamont]

The people posting about underfills on HP tanks and gas blending issues are missing the point.

Don't compare an LP104 with an HP100, they're two very differently sized tanks and the LP104s will be much heavier on your back. If you're thinking the LP104 looks like a good idea, you could also buy an HP130/X8-130 which is identically sized, identially weighted, but which carries another 30 cu ft of gas @ 3500 psi. You can also underfill an HP130 to 2640 and it behaves exactly like an LP104, so gas blending is not any more of an issue with HP tanks than it is with LP tanks.

The real question is cost, and what your and your dive shops opinion is on cave fills.

[Rob Holman]

And that is the crux of the issue. LP/HP is an arbitrary distinction for the most part. It's a rating on the tank. So get the tank size you want, and get it with the highest pressure rating you can. Heck, if my HP 130's (AKA LP 102's) could be rated to 4500, I would get the ones with the 4500 stamp.

If you get an HP 130, and you get a hot fill to 3000, who cares? you still have more than a LP 102 which is plenty for most recreation dives around here. If you get a hot fill on the same tank with a LP rating, you come out with less than 102 CF, a situation that form me will cut my dive short.

My dive buddy dives the skinny HP 100's. he was getting flack for diving HP tanks from some folks who dislike the HP tanks until he pointed out he was just diving LP 80's. Go figure.

Since moving to the E-8 130's, I return to the boat with over 1000 PSI after nearly every dive. I like the extra margin for error. And as someone else pointed out, you have to carry the weight somewhere, either on the tank or on your belt.

I do admit that smaller framed folks have a harder time with the large tanks.

One more consideration for those who have GREAT consumption rates and dive with small tanks. Make sure you have enough gas for you and your gas hog buddy (like me) to make it to the surface in an emergency. you may only have a .4 sac, but I have a .6 SAC that will probably jump to 1.0 under duress!

[lamont]

I do admit that smaller framed folks have a harder time with the large tanks.

right, but then do you want to be using an LP80 or an HP100? they're both going to cause the same amount of backstrain, but you get another 20 cu ft on a fill to rated pressure with the HP100...

the one exception to this rule is LP120s which have no corresponding HP149 tank, so if you really want gas and want to carry around a monster of a tank, get an LP120 and cave fill it to 3500 psi...

[Rob Holman]

I was thinking more that a smaller framed person might choose an HP 100 over an HP 130.

That is a big difference for someone who only weighs in at 110 pounds unlike my 200 pound frame.

[Sounder]

Like Mrs. Sounder? 5'8" and 120lbs - loves her HP100's.

She loved the HP80's when we borrowed them, but we stayed shallow (I had a HP100)... if she needed to share with someone like herself, there is (pun intended) "breathing room" with the 80. If she had to share with me it'd be a very different story.