Scuba Tank Size

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Think about how many minutes the gas will last you at different depths. 1 cf per minute is a conservative estimate of surface breathing rate. That means 2 cf per minute at 33 ft, 3 cf / min at 66 ft, 4 cf / min at 99 ft.

2 85 cf steel tanks doubled together will give you roughly about 42.5 minutes of air at 100 ft. If you are practicing the rule of thirds, that means that you need to turn your dive at around 14 minutes if you are diving double 85s.

With double 133 tanks, you get about 66 minutes of air or turning the dive at 22 minutes instead.
 
2 85 cf steel tanks doubled together will give you roughly about 42.5 minutes of air at 100 ft. If you are practicing the rule of thirds, that means that you need to turn your dive at around 14 minutes if you are diving double 85s.
Don't use rule of thirds for no-stop open water diving. It's ridiculously conservative for that.

Min gas, dude. Min gas. My consumption is fairly middle of the road at 15-20 SLM (these days closer to the former than to the latter), and with a 10L my min gas pressure at 30m is around 110 bar. With 15-20 SLM, I need 120-160 bar for 20 minutes at 30m (that's NDL on air according to the PADI RDP), so if I descend with at least 230-270 bar in my 10L, I'm good.

Or to put it 'murrican units: as long as I stay within the air NDL, a single 100 carries enough gas for me.
 
Hello Fellow Divers. I am very new despite my 57 years of age. Have done 13 divers, 9 of which were in getting to the Advanced Diver (PADI). I am now convinced that the steel tanks are a much better long term use and cheaper to maintain than alloy but then I get told about standard 12 litre tanks and 12 litre 300 bar ones which hold more gas. Does anyone have a link so I can understand Bar - litres - cft. So I can understand how many litres is 100 cft etc.

Thanks in Advanced

DDD

12 litre tanks come in two capacities. The lower pressure 232 bar tank and the very heavy 300 bar tank.

Due to compressibility changes the 300 bar tank only contains 12 x 270 litres of air instead of 12 x 300.
Hence, it is not dramatically larger than the 232 bar tank. But it is dramatically heavier indeed.
Because of this, there is not a single diver in the whole country of Finland who would prefer 12/300.
Everybody has a 12/232 or two 12/232 tanks. We all have drysuits.
The weight of a 12/300 would be felt even worse if you dived wet.

Just my opinion.

Some people use 10/300 singles though, and they are ok. With a drysuit in cold water.

Litres vs. cuft:


A 12 litre tank can hold 12 litres of water. It is the absolute volume of the tank.
Cubic feet (cuft) refer to uncompressed surface air (once it is let out).

Hence, 12/232 will contain 12 x 232 = 2784 litres of "surface air" forcibly compressed 232 times down to 12 litres.
This 2783 litres equals to 98 cuft. This litres to cuft is a simple linear correspondence.

So, the cuft volume tells how much gas there is once it is let out.
The litres volume tells in what space the gas is constrained in. Multiply it by pressure to know how much it is once it is let out. Then convert this to cuft by a simple conversion (multiplication).
 
I dive a steel 120 cuf. It comes down to two issues: how much weight are you willing to carry and how much gas do you want. I am 6’7” (201 cm) and weigh 240 lbs. I burn though a standard Al80 like nobody’s business. A big tank makes sense for my type of diving because I am big enough to handle the size of the tank and I will use the gas. My daughter is fine with a similar dive profile using an 80. Putting a big tank on her would be pointless. Keep in mind that you are going have to enter and exit the water with a full rig.
 
12 litre tanks come in two capacities. The lower pressure 232 bar tank and the very heavy 300 bar tank.

Due to compressibility changes the 300 bar tank only contains 12 x 270 litres of air instead of 12 x 300.
Hence, it is not dramatically larger than the 232 bar tank. But it is dramatically heavier indeed.
Because of this, there is not a single diver in the whole country of Finland who would prefer 12/300.
Everybody has a 12/232 or two 12/232 tanks. We all have drysuits.
The weight of a 12/300 would be felt even worse if you dived wet.

Just my opinion.

Some people use 10/300 singles though, and they are ok. With a drysuit in cold water.

Litres vs. cuft:


A 12 litre tank can hold 12 litres of water. It is the absolute volume of the tank.
Cubic feet (cuft) refer to uncompressed surface air (once it is let out).

Hence, 12/232 will contain 12 x 232 = 2784 litres of "surface air" forcibly compressed 232 times down to 12 litres.
This 2783 litres equals to 98 cuft. This litres to cuft is a simple linear correspondence.

So, the cuft volume tells how much gas there is once it is let out.
The litres volume tells in what space the gas is constrained in. Multiply it by pressure to know how much it is once it is let out. Then convert this to cuft by a simple conversion (multiplication).
That’s very interesting, I didn’t know that you don’t get 300 bars on these cylinders.

Is there a Finnish joke about cylinders

Very Finnish Problems
 
That’s very interesting, I didn’t know that you don’t get 300 bars on these cylinders.

You get the pressure, you just don't get the "it's easier to figure the volume in metric" for a full tank that is touted as the ease of using metric".

I dive a steel 120 cuf. It comes down to two issues: how much weight are you willing to carry and how much gas do you want.

And it depends on the tank whether any of weight difference may be taken off the weightbelt due to the buoyancy difference. The 15# increase tank weight between my 120 and 100 is actually 8# on the walks, because I can take 7# off my weight belt. Needless to say, I don't take either of these tanks on my goat trail walks to the ocean.


Bob
 
I didn’t know that you don’t get 300 bars on these cylinders.
On which cylinders?

I've seen (and used) 10x300, 12x200, 12x232, 12x300, 15x200, 15x232, and 15x300.

I really won't recommend 15x300, though. They suck.
 
https://www.shearwater.com/products/peregrine/

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