Double LP85's or HP100 - Diving Wet-Steel Backplate

[2airishuman]

Getting ready to move to doubles. Torn between the two above. I dont plan on a dry suit so will be diving 7 mill in cold water. OMS Steel backplate. Any input?

I dive in 7mm in cold water with a steel twinset and will try to answer your question. I recently switched my twinset from HP120s to HP100s to make the weight more manageable, and I have an LP85 that I dive as a single. I dive a DSS steel backplate. My wetsuit is a two-piece speardiving suit.

The short answer to your question is that these two cylinder choices (LP85 and HP100) are really more alike than they are different so it simply doesn't matter much. Therefore price and availability may influence your decision more than anything. Most LP85s are near neutral when empty, as are HP100s. All you're trading off then is total weight vs. rated capacity, and the usual tradeoffs of LP tanks vs. HP tanks apply regarding fills, cave fills, transfills, etc etc.

A fact to consider is that bands for LP85s don't fit larger cylinders, so if you anticipate moving to a larger size at some point you may be better off with HP100s.

Yes you need to be sure you can get to the surface in the event of a buoyancy failure. I use my twinset for open water, non-technical dives, and can swim up my kit with an empty wing if I ditch weights. You'll have to do your own math based on your body's inherent buoyancy and the flotation your wetsuit provides at depth.

 

[Basking Ridge Diver]

we will actually compare that to the low pressure equivalent, which are the LP85's and hold 83.5cf at working pressure.

http://www.indianvalleyscuba.com/se...ction/information/CYLINDER SPECIFICATIONS.pdf

LP 85 - holds 85 according to Indian Valley - where are you getting 83.5 or am I just missing your point?

Edit - ok found an old scubaboard thread LP 85 this is plus 10 %... I never noticed before...

 

[IDS_Bill]

ok Bill, someone has clearly told you some very wrong stuff, it isn't necessarily your fault, but you have to come around to this, especially as a technical instructor because this is literally the basis of the ideal gas law where PV=nRT

We will use Faber tanks because they are the only new ones available, and we will assume a linear capacity increase regardless of fill pressure.

You mention HP100's with 25cf taken out of them, so that means they are at 3030 PSI, and now hold 193cf. Crossing over from an AL80 will do nothing which was your point.

Unlike you, we will actually compare that to the low pressure equivalent, which are the LP85's and hold 83.5cf at working pressure. If we fill those tanks to 3030 psi we get 83.5*2*3030/2640= roughly 215cf. Some rounding errors converting from imperial to metric, but they are the same nominal capacity of 108cf, and they have to be because they hold the same water volume. Please also remember that when below the working pressure of that AL80, because the LP95's are bigger tanks volumetrically, they will have a lower increase in pressure when transfilling.

You are also carrying a tank that is 3lbs heavier than the HP100, and if you are having to wear lead, you have an extra 2lb buoyancy disadvantage which means your total rig weight is 5lbs heavier for the same nominal gas volume at working pressure. If you compare the LP85 to the HP100 which is the fair comparison, you realize that while the HP100 is 3lbs heavier on land, it is also 3lbs heavier in the water so the total rig weight is identical for both of the tanks. The advantage you have of the HP100 is never having to fight a dive shop to fill over the working pressure to get some extra volume. Same math applies for the LP95 vs the FX117 which are nominally identical tanks where the FX117 is 1.5lbs heavier on land, but is also 1.5lbs more negative in the water so net weight change is 0.

To follow your example set above. A set of lp85's filled to 3442, with 25cf removed will be at the exact same pressure as the FX100's. Thus, the only reason to ever choose the LP85's over the FX100's is if the 3lbs/tank more negative of each FX100 is problematic either on land, or in the water. Many cave divers will continue to choose the LP85's because of their buoyancy characteristics, but there is also no problem getting 3600psi cave fills. If I was diving in cold water where I needed that extra ballast, or diving in a location where cave fills weren't possible, then there would be absolutely nothing that would convince me to ever purchase a LP tank if I had the HP equivalent available. The problem is instructors and divers will often talk about the same math that you just did, but since you aren't comparing equivalent tanks, the argument doesn't matter. Tanks of the same volume will hold the same amount of gas at the same pressure regardless of what their nominal capacity is at whatever working pressure the manufacturer determines. That is the absolute basis of the ideal gas law. You are not comparing vessels of the same volume, and because of that the argument is irrelevant.

In all of your chest pumping, you have missed my very simple point.

I sometimes make my choice between LP or HP based upon the efficiency of crossing over from AL80s if that is the situation in a remote area (no compressor on the boat). We can go round and round about numbers all day but my point was simple.

 

[tbone1004]

@IDS_Bill that's because your point is completely wrong. Your transfilling efficiency has NOTHING to do with the working pressure. It has EVERYTHING to do with the fact that you are using a tank that inherently holds 25% more gas *you stated 100's as being 100's so a 12 liter vs a 15 liter tank*.

Please, humor me and try to make the same argument when you compare a LP95 against a FX117 or a LP85 against the FX100. The argument for low pressure vs high pressure is one that needs to die and is something that only exists in this country. If you go to Europe where tanks are measured by water volume the argument can't exist because you choose the size cylinder you want, and the working pressure you can achieve fills for, everything is compared by it's physical capacity not a made up capacity like we do in the US

 

[stuartv]

@tbone1004: IDS_Bill's point seems to be exactly what is relevant - at least, to me, if I wanted to dive with roughly 100 cu-ft of air, then I might be choosing between an LP95 and an HP100.

I know my local shops will not fill any tanks over their rated pressure.

If I know that I will sometimes fill my tanks somewhere that can't provide 3500psi (e.g. via transfill from an AL80), then it seems like the LP95 is the clear winner of which tank I should go with.

You're wanting to compare an LP85 against an FX100 and an LP95 against an FX117. You may be able to fill those LP tanks to the same pressure as the HP tanks, but a lot of people (e.g. me) cannot do that. So, those comparisons just don't work. If I need 100 cu-ft of air, an LP85 is not an option for me. If I need 120, then an LP95 is not an option. The water volume of the tank is irrelevant to me and my dive plannning - and the shops where I get fills. The amount of gas I can carry is what matters to me. The rated working pressure is what matters to the shops where I get fills. I suspect this is true for a lot of people, and I'm surprised that you would be giving out recommendations based on an expectation that someone could/would get their tanks filled beyond their rated working pressure.

 

[tbone1004]

I'm not making recommendations for LP tanks, that's the point. So to use your scenario, you need 100cf of gas to dive, and you see two tanks based on the stupid indirect measuring system we use here in the US which are LP95's and FX100's.

You try to weigh pros and cons and realize that there are two, and only two reasons to ever choose LP tanks. One: cost, quite often HP tanks can be more expensive. Right now at DRiS a FX117 is $340, and a LP95 is $300. The ONLY reason to choose the LP95 in that scenario is the second reason to consider LP tanks, which is buoyancy. This is really only a factor in deep warm water diving where you are unable to dive a balanced rig, i.e. Florida cave diving. In this scenario the LP95's are less negative in the water which can be advantageous. For you, since you are diving in Virginia and in a drysuit, then the FX117's will allow you to remove 3lbs of lead compared to the LP95's.

So, if you need 100cf, why would you buy a LP95 over a FX117 assuming you're buying new? The answer is, you won't unless you need the buoyancy characteristics of the LP tank, there is no reason not to purchase the HP equivalent, with the key point being equivalent. If you know that your local shops won't overfill, but are capable of HP fills, why would you buy just spend the extra $40 and buy the tank that will give you some bonus gas for everything over 2640psi?

 

[stuartv]

I see your point. It seems to be: Whatever tank you buy, if you're looking at an LP tank, buy the exact same tank with an HP rating for a few dollars more. If you're looking at an LP95, buy the FX117 for a few dollars more because it's the exact same tank, just with a higher rating for working pressure. (well, except that it's not the EXACT same tank because the 117 is 3 pounds more negative...)

That makes sense. Except, who buys new tanks?

My 4 x HP100s were easy enough to find. And from what I can tell, they have basically the same buoyancy characteristics as an LP95. So, that's a wash.

And they were all pretty darn cheap. Less than half the cost of new tanks, even after all necessary inspections.

I don't know what the going rate is on used LP95s, but if they are pretty close to the same price as used HP100s, it seems to me like an LP95 would actually be the smarter choice. Same buoyancy. Same amount of gas for normal fills. More gas if you're somewhere that can't give a 3500psi fill. And also more gas if you can get a cave fill.

OTOH, good luck finding used FX117s for a decent price. And if LP95s and HP100s are a big difference in price, then I guess it would just depend.

 

[tbone1004]

ding ding, that's the point. If you're buying new, and want the LP to get the physically larger tanks, just spend an extra $30-$50/tank and buy the HP equivalent. The 3lbs more negative is the thicker walls, but outside of warm water technical diving, you are quite often wanting that extra ballast *the 3lbs is in a set of doubles, the 117's are only 1.5lbs less positive* so you can inflate your drysuit more at the end of the dive to stay warm.

Yes, very few buy new tanks, and the E8, X8, and FX8 tanks are not common on the used market since they are relatively new which makes the argument for used tanks different, but Bill made a theoretical scenario where he would choose one tank over the other, which means we have to compare apples to apples and that is new tanks only.

 

[Scared Silly]

Let me give a real world example of what t-bone has been trying to say. When I was looking at a set of doubles I picked the Faber LP85s over the Faber HP100s for several reasons. The dry weight was lighter by 3 lbs. each, the buoyancy was 4.5 lbs lighter each (even in a dry suit in cold water I do not need much ballast), and though 15 cu.ft. less for my diving - both cold and warm waters it was not going to require the extra gas. And if really needed it I could cave fill them and get close enough. I should also note that at the time Worthington also sold an LP85 which compared to the Faber was heavier (dry and wet).

 

[Landlocked123]

I would like to thank everyone that replied to this thread. This confirms where i was headed with the move to doubles. It's going to wait 1 or 2 years and is going to go hand in hand with a dry suit. I feel I have a much more educated perspective on this move to doubles so again thank you to all for sharing.
George