Help purchasing tanks.

[CuracaoJ]

Okay, I've read through a lot of old posts on tanks and buoyancy. I'm thinking of purchasing 6 tanks, and need some help making calculations on the buoyancy issue, because, even after reading, I'm not sure that I understand the math.

First of all I live in Curacao, and do almost 100% recreational diving, between 150-300 dives per year usually. I dive a back inflate BCD, needing 16# of weight with my usually 5mm full. The reasons for wanting to buy tanks are for the convenience. I don't like having to skip work on Monday to return Sunday's tanks, and also because I have recently starting spearing lionfish to sell in my restaurant and I would like to extend my dive times past the usual hour or so that I normally get.

So my requirements are, more Cu ft of air, better buoyancy (defined as me carrying less lead), and Yoke, not DIN (as I don't want to switch out all my regs).

I'm considering either steel LP 108s or HP 120s (under-filled with Yokes).

Stats:

LP 108: 45.9 real weight, -10.7 full, -2.6 empty, 108.6 CuFt air.
HP 120: 38.0 real weight, -11.0 full, -2.0 empty, 104 +/- CuFt air (under-filled to 3000 PSI)
AL 80: 31.4 real weight, -1.4 full, +3.4 empty, 77-78 CuFt air

Now on buoyancy I'm confused after reading some posts as to when/if "real weight" comes into play. I've read a lot of posts where people are talking about dropping 8-10# by switching to steel. But, the way I understand it is that I need to look at the difference in the empty buoyancy column (+3.4 vs -2.0). So by switching from an AL 80 to an HP 120 I get to ditch 5.4#s of lead, however my total rig weighs 1.2#s heavier for the walk to the water (38 real weight - 31.4 real weight adds 6.6#s to my rig, but I drop 5.4#s of lead, for a total gain of 1.2#s?), although heavier, this gives me a much better buoyancy characteristic in the water.

Can someone please check my math. If it's correct it would seem the best purchase for me would be the HP 120s, which would also give me the future chance to change to DIN and get substantially more air if I ever wanted to change all my regulators over.

Thanks,
Jeff

 

[rob1967]

Get the HP 100s and fill them to 3500. The 120s are big and heavy, so if you are an average size guy, you will find they are a pain to lug around. If you don't believe me, try before you buy and see for yourself. I love my HP 100s.
I had one guy tell me you can't use yoke for HP, but my LDS says it's not true; I use yoke, no problem. You will like the buoyancy characteristics of the HP100. I was able to drop about 7 lbs compared to aluminum. The exact amount of weight you drop will be determined by trial and error. I thought the weight drop would be more determined by the buoyancy of a full tank, I figure if I have enough weight to sink from the surface, I should have enough weight to be neutral on my safety stop on a near empty tank, but this may vary.

 

[CuracaoJ]

Yeah, I realize they are gonna be a pain to lug. Even at 6'3", 220# I'm not looking forward to carrying tanks that weigh 7-10#s more. However, HP is out for me for two reasons. The first is economic. I have a dive shop that is spitting distance to my house, but they aren't really that keen on filling me to 3400+. I don't know if it is an equipment issue on their end or what, but it's their equipment, so their rules. The other place that I have to go is 10+ miles away one way trip, the overfill costs and extra dollar, and at $5 a gallon for gas, in my truck, it would cost me $6-7 just to drop them off, and double that if I didn't wait to pick them up. This would make the whole process actually cost more than a rental fee. The other issue is that while I've heard that I can put a yoke on a 3400+ tank, I'm not sure that over the course of the years this would be worth the effect it would have on my gear.

I'm not specifically wanting my own tanks to save money, but I certainly don't want it to cost me money, which is what would happen if I go with anything HP and have it topped off.

 

[Scared Silly]

You are looking at just Worthingtons might also look at Faber:

Cylinder Specifications

Their LP 108 is 41lbs dry versus their HP 120 which is 39lbs. I believe their measures are sans valve. Me I got an LP108 and fill it to 3000psi and call it good. My wife does the same but with lp98.

 

[CuracaoJ]

All things being equal I'll probably go with XS (Worthington?). If my guesses are right about the buoyancy I like the dimensions on the XS HP120 its 7.25 x 28, with a 38# weight empty. Although I'm not that far yet, and which brand do go with is going to be dictated by what I can get from my favorite LDS. I haven't checked with them yet.

I'm still trying to figure out how much weight I'm going to get to drop. I understand rob1967s point about the full weight being what you look at, but for me my weight is always determined by how I am at my safety stop. That is why I'm looking at the empty weight. I like to be about 1# heavy at a safety stop with 500-700 PSI left in the tank.

 

[rob1967]

Hey, since you are a big and tall guy, maybe you will like the 120s. I'm 5'6", and I tried my buddy's 120s, and they were heavy. Maybe ok for a boat dive, but would be a pain for shore dive. Mind you, this is cold water with dry suits and the extra weight that entails.

If you can't get HP fills, you could also consider Al 100s. I think they exist.

 

[scubadiver888]

When I look at tanks the buoyancy at the end of the dive matters, the weight of the tank out of water and how it trims out for me.

The change in buoyancy is the weight of the air. This looks different because the tanks are different sizes but 100 cu.ft. of air is the same weight regardless of the tank you stick it in. So if you are getting the LP108 or the HP120 and filling them both with around the same amount of air, the buoyancy change will be the same.

When looking at going from AL80 to steel there is a big change in the final buoyancy of the tank. An HP120 is going to be around 3.3 pounds negative at the end of the dive (assuming 500 PSI). The AL80 is going to be 3.4 pounds positive at the end of the dive (AL80 is +3.4 @ 500 PSI, not empty). So you will need 6.7 pounds less lead with the HP120 at the end of the dive. The HP120 is around 6.6 pounds heavier than an AL80. So outside the water you would be +6.6 pounds for the tank but -6.7 pounds in lead. Walking into a shore dive or walking across the boat on a boat dive, you are going to be exerting around the same amount of effort. If you fill the HP120 to 104 cu.ft. you will have around 2 pounds more air.

On the other hand, the LP108 is a LOT heavier out of the water. So out of the water you will be exerting more effort. You'll have 8 pounds more weight to deal with, compared to the HP120. The buoyancy change will be around the same because they will have the same amount of air.

Bottom line, I agree that the HP120 is a better choice going by the numbers. It will have the same weight change as the LP108 in water (buoyancy shift). It will let you carry less lead than the AL80 requires. It has the potential of even more air if filled to 3442 PSI.

I would get it with Pro Valves. They have a DIN insert so you can use them as Yoke or take the insert out and use them as DIN.

The one thing that has not been talked about here is trim. Proper trim is about weight distribution. If I dive with an HP120 its length changes my centre to a lower point on my body. I find my feet down and head up. Not a good position for me. To fix this I have to move the tank further up on my back but then I hit my head on the valves. For this reason I use an HP119 (8 inch diameter but 4 inches shorter than the HP120). It is heavier out of the water but I trim out nicer with the HP119 than I do with the HP120.

The only way you'll know for sure which tank trims out well for you is to try them. It is possible that the LP108 gives you better trim than the HP120. Then you have to decide if you want better trim or less weight out of water.

 

[CuracaoJ]

Scubadiver888,

Thank you! That was exactly what I needed to hear. I was concerned that the buoyancy had something to do with the actual weight of the tank, as opposed to the empty buoyancy rating.

As for trim I think I'll be okay with the XS tank. It's at most 2.1 inches taller than the shortest rental AL80 I get here on the island. I have a transmitter for my D9 sticking right out of the top of my first stage now, if I need to I can move it to another port to lift the tank up (it's a good 3" tall). Plus I have plenty of trim pockets that I can move around as well if I need. Unfortunately being on a small island I won't be able to try the tanks out, they will have to be ordered blind as no one has HP 120s on the island that I am aware of.

Thanks for all the help.
Jeff