Tank sizing, HP & LP?

[M_Bipartitus]

Huh?

Why would partial pressure nitrox fill be an issue? Even if you do transfill from larger O2 tank you mostly start with 0psi (unless you have a booster). (At least my LDS does that, they ask you to drain the tank and first fill with O2 and tap with air.)

If the tank is not drained, the concern should be obvious. Even if the tank is drained the O2 pressure required is higher in a HP vs a LP for the same mix and the same capacity tank. If, for example, the shop's supply tank is down to 700psi you won't be able to fill a HP tank with 40% nitrox (needs over 800psi), but you may be able to get a 40% nitrox on a LP tank as it only requires around 600psi. If the shop has a boster, it shouldn't matter, but may take longer.

 

[Deep South Divers]

If my stated goal is to replace lead with tank weight, would I be better off with a LP 95 (which is also easier on the rest of my rig)?

Thanks.

You're getting great information here.

Aside from the bouyancy differences that have been pointed out (and thus differences in the amount of lead on your belt), don't forget that the weight of the tank is in a different place on your body when you remove lead from your belt and trade for a heavier tank. This will change your trim - that can be good, and that can be bad - it depends on your specific situation. For me, diving LP steel tanks means going to an aluminum plate (with aluminum tanks I dive a steel plate) to keep my trim right, especially in doubles. Steel HPs aren't as bad for me - but both put my CofG further away from my back, which is to say that trim becomes more "finicky" than when I'm using aluminum tanks.

Also, +1 on the fact that some shops fills ALL tanks to 3000 or 3300 psi as a standard procedure. Thus, in those shops, if you're having them fill an HP tank, you'll be getting a short fill and your 100 is now closer to an 80. A really expensive 80.

For those that point out that, for example, an HP100 = 20 cuft of gas more than an AL80, I like to point out that for the price of one HP100, I can buy two AL80's, and 160 cuft of gas is definitely more than 100 cuft of gas. Of course, that usually gets their panties in a wad...

Given that most of my diving is in salt water, I've also had some corrosion issues with ALL of my tanks... But then, I dive just about every day, and for more "normal" divers, it'd probably be a long time before any issues arose. When there is corrosion, however, aluminum tanks turn hazy, then dark. The dark is aluminum oxide, which is very hard, and actually will help prevent further corrosion. Steel tanks turn an ugly brown (which always manages to get everywhere) which is iron oxide... And iron oxide is very soft, so the rate of corrosion increases.

Someone else mentioned that some galvanizing is better than others - I've found this to be true, too, and am interested in some of the epoxy coatings that some tanks are coming with today... But I've found a good solution is to dive exclusively aluminum tanks in salt water. Once a year, when I VIP my tanks, I clean them up and make them shiny again with a stainless wire brush, stainless wool, or Scotch Brite pad (depends on the level of corrosion) - simple. All of my tanks are stripped, spun finish to promote this. I have found that painted aluminum tanks suffer wherever there's a scratch (it WILL happen) and salt water leaks beneath the paint... Regardless of how well the tank is rinsed after the dive.

All of the regs I own are DIN regs - but every valve on every tank that I own is a convertible valve so that anyone can use my tanks at any time. I like Thermo's version of the convertible valve.

If you choose an HP tank, Thermo's convertible valve may not be appropriate pressure-wise - you may need a full DIN valve for that (the orifice on a full DIN valve is slightly deeper than the orifice on a convertible valve, and therefore rated to handle higher pressures).

If it were me, I'd dive AL80's exclusively in salt water and consider others only if I was diving dry in fresh water. Even then, AL80's make great stages, so you can't go wrong owning them. Being set up for AL80's also makes vacation diving easy, as they seem somewhat universal and easy to get - at least on this side of the world. If I were doing long penetrations in freshwater (i.e. caves) in a drysuit, I'd dive LP95's (or LP104's) doubled... With AL40's and AL80's as stages. If I were diving a "light" backplate, transpac, or "regular" BC that needed the trim raised onto my back, I'd consider a set of HP steels - but for the price may still opt for aluminum and just use some trim weights.

Good luck with your decision!

 

[CamG]

Greetings sabbath999 and this is a really good thread with lots of links to the valuable information to research and decide which way to roll.
Currently I have both LP 95's and HP 100's I like them both each have different characteristics but equally awesome.
On the topic of HP fills, unless you have your own compressor it can be really tough to get a real HP fill. This is just my experience but others have shared the same experiences.
You can always get a good LP fill and my 95's at 3000psi are holding 119 cubic feet of gas. I do not push them to often only when I need the extra gas. Cave / wreck dives.
I rolled with the LP's first for this reason alone and fell into the HP's for a awesome deal. Never turn down HP 100's that are like new for a really great $$$$$!
If you are ordering new I like the Worthingtons for the hot dipped galvanized finnish.
My 95's are Worthingtons and the HP 100's are PST's both are doubled up.
I just purchased LP 77's for sidemount independent doubles and looking to try some Faber 85's to experiment. The fun never ends as long as their is $$$$ in your pocket or close friends with your LDS!
Keep your eyes open for some good used tanks and better yet try them out before you buy them if possible. I have had the opportunity to dive many different tank sizes and configurations. Great experiences not just for me but those trying my tanks and gear. We try to have a pool party when we can try new gear and break in new stuff. Local dive clubs rock people and a great LDS does not hurt.
CamG Keep diving....keep training....keep learning!

 

[Beecher]

definitely do it

 

[Splitlip]

more complete comparision here

Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

 

[Splitlip]

I think very few people with HP100's would want to sell. They are nice tanks, maybe not the perfect solution for all, but very nice. Congrats on the hockey game, you guys were awesome!

IMO, there is NO comparison between an al 80 and an HP (3442 worthington) steel for singles.

The HP weighs ~3# more dry, but in the water is ~6# more negative. Net a dry rig about 3# less.

Full, the hp provides about 30% more gas. Even with a short (3000 PSI) it still has about 12% more gas than an al 80. The steel is smaller yet provides superior trim IMO. Less likely to need to juggle trim weights around the body too.

 

[Mike Boswell]

My wife and I use steel HP-100's and love them - much better than AL80s for all the reasons given above.

Yes they are more expensive than an AL80 to purchase, but consider the fact that you are getting 25% more air with each fill, for free. That extra air means longer dives or safer dives (more gas in reserve) or both.

Here's a little ROI analysis: If an air fill costs $5.00, the extra 20 cu. ft. of air in an HP100 is worth $1.25 for every fill. You will thus recover the cost difference between the AL80 and HP100 tanks in a little over 100 dives.

 

[Deep South Divers]

Yes they are more expensive than an AL80 to purchase, but consider the fact that you are getting 25% more air with each fill, for free. That extra air means longer dives or safer dives (more gas in reserve) or both.

Here's a little ROI analysis: If an air fill costs $5.00, the extra 20 cu. ft. of air in an HP100 is worth $1.25 for every fill. You will thus recover the cost difference between the AL80 and HP100 tanks in a little over 100 dives.

Interesting thought process.

All of the above depends on your limiting factor... If you're diving "no-deco" dives and your NDL is your limiting factor, then no amount of "free gas" is going to even come into play.

If you're running out of gas before you're running out of NDL, or if you're doing staged decompression diving, then the amount of gas that you have available to you WILL become a factor - but if you're doing staged decompression diving then you'll be diving doubles anyway, and what you'd have spent on a single HP steel tank will go a long way towards the purchase of a set of doubles. Why buy a system that holds 100 cuft of gas when, for the same price, you could have bought a system that holds 160 cuft of gas?

What's probably more important is the way that the tanks affect a diver's trim - and steel tanks can do that positvely or negatively. Good trim = a whole lot lower SAC rate, which equals a whole lot less gas consumed. Therefore it's possible for a smaller tank to last longer than a larger tank if the smaller tank trims the diver better.

...Which means that this is more about trim and bouyancy characteristics than capacity. Manufacturers make many capacities in both steel and aluminum, so if it's a larger-capacity tank that you want, that doesn't mean that a HP steel 100 is the tank you should get.

 

[GrumpyOldGuy]

Interesting thought process.

All of the above depends on your limiting factor... If you're diving "no-deco" dives and your NDL is your limiting factor, then no amount of "free gas" is going to even come into play.

If you're running out of gas before you're running out of NDL, or if you're doing staged decompression diving, then the amount of gas that you have available to you WILL become a factor - but if you're doing staged decompression diving then you'll be diving doubles anyway, and what you'd have spent on a single HP steel tank will go a long way towards the purchase of a set of doubles. Why buy a system that holds 100 cuft of gas when, for the same price, you could have bought a system that holds 160 cuft of gas?

What's probably more important is the way that the tanks affect a diver's trim - and steel tanks can do that positvely or negatively. Good trim = a whole lot lower SAC rate, which equals a whole lot less gas consumed. Therefore it's possible for a smaller tank to last longer than a larger tank if the smaller tank trims the diver better.

...Which means that this is more about trim and bouyancy characteristics than capacity. Manufacturers make many capacities in both steel and aluminum, so if it's a larger-capacity tank that you want, that doesn't mean that a HP steel 100 is the tank you should get.

I actually like Mike's point of view, although his math needs a little tuning.

The old AL80 holds 77 cu ft, so it is actually close to 30% more air in a HP100, if you get a good fill. Next, the usable air is the total volume less the reserve (rock bottom), which is generally a constant value based on depth and air consumption. For example, if you need 27cu ft of reserve, the usable for for an AL80 would be 50 cu ft, using the same reserve on an HP100, you would have 73 cu ft usable, or 46% more. All in a tank the same size and weight as an AL80.

Lets be honest though. We tend to us the AL80 a lot because they are cheap and universally available. The are not outstanding tanks, they are just good enough. Maybe my dive with be limited by NDL and the extra air is not used, but not all my dives are and I will take all the time I can have submerged.

 

[Deep South Divers]

Lets be honest though. We tend to us the AL80 a lot because they are cheap and universally available. The are not outstanding tanks, they are just good enough.

The terms "outstanding" or "just good enough" are relative. It depends on what you want in a tank.

It's true that an HP steel 100 has more capacity than an AL80 - that's apparent even in their names. And these are the natural tanks to compare, since they are dimensionally close to the same size.

For what it's worth, most shops willing to fill a steel to 3442 psi are also willing to fill AL80's to 3300 psi, depending on some factors... Making the capacity difference less. But that's not really the point.

The point is that the tanks have other characteristics that are more important - like inherent bouyancy. Steel tanks are inherently more negatively bouyant than aluminum tanks, which can be a good thing or bad thing - it depends on it's application and how it changes the diver's trim.

Another important factor is the tank's resistance to corrosion. For the same reason that boat, dock, and boat trailer parts are better designed out of stainless steel or aluminum than galvanized steel, tanks exposed regularly to salt water tend to experience less corrosion issues if they're made of aluminum instead of galvanized steel (I've never seen a stainless steel tank and suspect that the material wouldn't do well with the pressure).

Maybe my dive with be limited by NDL and the extra air is not used, but not all my dives are and I will take all the time I can have submerged.

Agreed. But if you're running out of air, get a larger tank - or better yet, multiple tanks (or even better yet, reduce your SAC rate - it's usually free and has a lot of additional benefits). An aluminum 120's capacity exeeds that of an HP100, and has basically the same bouyancy characteristics and corrosion resistance of an AL80. For that matter, AL100's are also available, and exceed an HP100's capacity when filled to 3300 psi.

I'm not "against" HP steel tanks - I'm just saying that they should match the application and that their unique characteristics aren't a benefit in every situation. Sometimes they're great, and sometimes a different tank is a better idea.

By default, if you set up your rig to handle aluminum 80's (cam band adjustment and weighting), then you'll be most likely to be set up correctly at the dive site, assuming that you're renting tanks. Of course, that doesn't apply to every geographical area - some areas favor a certain "style" of tank... But in the Western hemisphere, the AL80 is the standard and therefore should be the default tank to set up your rig for.

...With regards to owning a tank (the OP's original question), the above doesn't apply - but if you're going anywhere and renting tanks, you'll generally have to modify your rig if you don't normally dive AL80's.