Tank sizing???

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Wiznutaggie

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Searched but haven't come to a great answer yet as to what size of tank to get. Have been putting together my first set of scuba gear these last couple weeks and a tank(s) is all I have left to pick out. For the background to the choice: 1) Have signed up for nitrox cert 2) live in Texas and plan to mainly dive offshore rigs/wrecks in 60-120' (I have my own offshore boat), 3) I've bought a DR transpac xt with voyager singles wing 4) 6', 175#, good shape. 5) diving with 5/3 wetsuit.

I see faber tanks from 80-133cu (all I've ever used is AL80's). I assume more air is better, but where is the line drawn? A little math shows the 133 has the best price/cu ft. and weight/cu ft. ratios, but would this just be overkilling it? Since I'm not shore diving and don't hike a long ways, is there any reason to not go big with a single tank?

And lastly, there doesn't seem to be much availability in the ways of buying used tanks around here. Any suggestions on the cheapest source?
 

Marie13

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HP80s are 20”-21” tall. They’ll look like a soup can on your back. Significantly shorter than AL80.

HP100 are the sweet spot for a lot of people
 

lowwall

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Bigger isn't necessarily better. The largest tanks are harder to handle both in and out of the water. Most people will run out of NDL time before they run out of air with the larger tanks, so there is no advantage to pair with the disadvantages.

For most average to large folks an HP100 is the sweet spot. The really petite may want something smaller. XL air hogs might want to go to an HP120.

Here's a good blog post on the subject: Rich asks…Why are you diving with that Tank?
 

stretchthepenn

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An LP 85 steel tank might be a good way to go, as well. It's about the size of an AL80, has the extra weight of steel, and with an overfill to 3000 psi, holds 100 cf.

Plus, since it's an LP tank, you will always be able to get a full standard fill to 85 cf, whereas some shops can't handle HP tanks and might short you.
 

lowwall

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An LP 85 steel tank might be a good way to go, as well. It's about the size of an AL80, has the extra weight of steel, and with an overfill to 3000 psi, holds 100 cf.

Plus, since it's an LP tank, you will always be able to get a full standard fill to 85 cf, whereas some shops can't handle HP tanks and might short you.
An LP85 is essentially the same internal volume as an HP100 (it may differ by .1 liter depending on manufacturer). Thus they will hold the same amount of gas if filled to the same pressure.

With either cylinder, at the LP85's rated + pressure of 2640psi, you get 81.1 cubic feet of gas. At the HP100's rated pressure of 3442psi, you get 101.3 cu ft.

Reference: Calculating SCUBA Cylinder Capacity | Dive Gear Express®

Outside of Florida cave country (or self-fills), you are unlikely to get an LP tank filled to anywhere near the 3442psi standard fill for an HP tank. Thus there's no good reason to buy an LP tank over the HP tank with the same internal volume unless you can get it cheap, you know someone who will fill it well above its rated capacity, and you are willing to take the risk of handling an overfilled tank. I will note that cave divers have been overfilling tanks for years without them blowing up.
 

DiveClimbRide

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An LP 85 steel tank might be a good way to go, as well. It's about the size of an AL80, has the extra weight of steel, and with an overfill to 3000 psi, holds 100 cf.

Plus, since it's an LP tank, you will always be able to get a full standard fill to 85 cf, whereas some shops can't handle HP tanks and might short you.
Orrrrrr buy a hp100 and if you can't get a proper fill you still have 80cf. PLUS you aren't overfilling a tank and shortening it's life and putting the fill employees at risk. Also it's heavier so less weight needed.
 

BoltSnap

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I use 18L tanks filled to around 230 Bar when diving from boats especially when going deeper than 20 meters and not using doubles. Make sure that you have proper length ladder that can handle the weight for you to climb back into the boat with the tank on your back. With your height, you will have no issue with the 133 and that's what I would go with if I were in your place.
 

AfterDark

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Breathing gas to a diver is like ammo no such thing as too much.
Depends on how much tank you can/want to tote and if what you want is a good fit for you.
You might try renting as many different tanks as possible to see what works, for you.
 

Rol diy

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If you are diving singles.
120 cuft or 130 cuft. Steel be my option....
Or something like a low pressure 104 which you can over pump sometimes....

Extra air is nice. Often my buddy will do 2 110 ft dives on his 130 hp tank.
 

tbone1004

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An LP 85 steel tank might be a good way to go, as well. It's about the size of an AL80, has the extra weight of steel, and with an overfill to 3000 psi, holds 100 cf.

Plus, since it's an LP tank, you will always be able to get a full standard fill to 85 cf, whereas some shops can't handle HP tanks and might short you.

don't do that. Unless you are cave diving and need very specific buoyancy or trim characteristics for sidemount there is no reason to choose LP over HP tanks.

Orrrrrr buy a hp100 and if you can't get a proper fill you still have 80cf. PLUS you aren't overfilling a tank and shortening it's life and putting the fill employees at risk. Also it's heavier so less weight needed.

sort of that, but a modern LP85 and FX100 will ALWAYS have the SAME amount of gas when filled to the same pressure.

If you are diving singles.
120 cuft or 130 cuft. Steel be my option....
Or something like a low pressure 104 which you can over pump sometimes....

Extra air is nice. Often my buddy will do 2 110 ft dives on his 130 hp tank.
He's not tall enough for a 120, maybe a 117, but not a 120.

For quick reference to cut this low pressure recommendation nonsense.
LP85=FX100
LP95=FX117. The FX120 holds the same amount of gas but is a different form factor.
LP104/108=FX133
LP120=FX149

With the exception of the FX120, these are all practically identical in terms of interior volume meaning that when filled to the same pressure they contain the same amount of gas, and they all have about the same exterior dimensions. The FX series are a bit heavier both on land and in the water which is not a bad thing. The reason that people think that the LP series holds more is a failure to use the Z-factor for nonlinear compressibility of gases which at a "cave fill" pressure of 3500-3600psi is 6% which is not insignificant.

@Mikkel Simoes if you are buying new, which you are likely restricted to if the market isn't great, buy hot dip galvanized tanks, and buy FX100's. It will give you a better sized tank for your height *yes even at 6' an AL80 is almost too long*, but gives you a much better weight:gas ratio while not having so much extra gas that you are going to be wasting it with your buddy if they are diving with AL80's. If you have something like a 130, then it's not quite enough to get 2 dives out of for most people and you'll be sitting on a lot of extra gas and carrying around a lot of extra weight for no reason. FX100's are the ticket for most people and what I would recommend in this case.
 
https://www.shearwater.com/products/swift/

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