I am told that if I use a 100 Cubic Foot tank I can get more down time.

Does anyone use a 100 Cubic Foot tank and if so, does it really give you more down time?

I find it impossible to rent and therefore I would have to buy 2 tanks and this is expensive. Before I do this I need to know if it's worth the investment.

Should I buy aluminum or steal?

Komodo wrote:

The larger the tank the more gas you have, so technically, the longer you can stay down. It depends on what depth you are at. If you are at 15m or below you may be into decompression before you get low on gas, depending on your gas usage and whether you are using nitrox.

A normal Ali cylinder would be 80cu ft, so technically you would have 25% more air by having a 100 cu ft and 25% more time for the same depth dive.

Ali or steel, depends where you are and what they cost. In the UK I would say steel as they are much cheaper. Ali 100's are very rare here, Steel 15L's are common. Steels hold slightly more air than an equivalent sized Ali cylinder.

You'll have to make a decision based on your dive buddies and your dive times.

I use Steel 15L's for Scallop Diving as they give me more bottom time in the shallows. Beyond 13m 15L's are wasted as I end up in deco in the 2nd dive before I use half a tank. If I use 12L's (Ali 80 = 11.1L) I use both cylinders at a similar time to getting close to deco.

as another gas guzzler I have used 15L (nitrox) a few times for dives in Egypt. If I use the 15L it just gives me more contingency and I usually end with more in the tank, but it allows me to stay down as long as the mrs who seems to have mini lungs lol.

If you are buying tanks though you may want to think where you will be in future. I know it hasn't taken me that long to cut my breathing down to closer the mrs and I'll probably be sticking with the 12l next time. If you do get 15ls I'd definitely recommend getting nitrox tanks and training if you haven't already. Will make them last longer, but you'll still be restricted to how long your buddy can last

One thing to think about. A steel 100 CF high pressure can be filled to 2800 psi (low pressure) to give you the same fill as an 80 CF but with a ton less strain on the equipment (reg). It can also be filled to 3000 psi and you get about 90 CF. If you fill to 3445 psi or so you really need a DIN reg. At the lower pressures you can use a yoke. Just passing on what I read and what my instructor told me.

I think I need to read my diver's manual again

I tried last weekend 100 Cubic Foot tanks in my Nitrox course. I didn't see any difference as to my down time underwater.

Charon wrote:


In the UK a Yoke reg is tested and required to reach 232B not 200B as in the US. Most cylinders in the UK are steel and have a working pressure of 232B (3364psi), we also have Ali cylinders that are 232B as well as 200B. DIN regs can be 232B or 300B, nothing to do with the reg just the fitting. Most regs can be used up to 300B (4350psi), they are designed to, the fitting that they have is what is most convenient to the user.

Komodo - My last dive on a 15L(100cf) was for 128 minutes whereas on as 12L it would have been closer to 100 minutes.

I guess I'm doing something wrong.

are you sure you weren't deeper for longer?

The 128m dive was only to 8m max. I commercially dive for scallops and have to be as efficient on air as possible to maximise my income. I dive an average of 10 dives a week in the summer and 6 dives a week in the winter. I also try to keep fit, as I find it does help underwater. Some fit people find themsleves guzzling air as they are used to that when they exercise, it took me a while and a lot of diving to be able to stop doing that

I recently did a 47m dive on a single 15L, 25 minutes at the bottom with 80B left after switching to a deco gas.

I only have 24 dives, so I have not had the chance to do the same dive with different size tanks. So I guess I need to count on you guys for the real difference between the 2 tanks.