Tank differences while diving

[Cleardiver]

Hi,
I just got certified for diving with Nitrox and am in the market for a couple of tanks. Can someone tell me what the real world (dive time) difference is between using a 77cu.ft Lower Pressure [ 2400+ psi ] steel scuba cylinder vs. a steel 100 cu. ft. tank would be?

I can buy the 77 cu. ft. tanks for $175 less (each) so it sounds appealing but I don't want to buy them and find I'm getting much less bottom time.

Thanks!

 

[The Kraken]

Depends upon the depth, the O2 content and the diver's SAC rate:

For an "average diver" you might find the following comparisons (these calculations are approximate and are based on a SAC rate of 0.60 CFM, a HP 100 cf tank, a square dive profile and 500 psi residual gas)

50'
77 cf = 40 min, 100 cf = 57 min

60'
77 cf = 35 min, 100 cf = 50 min

80'
77 cf = 29 min, 100 cf = 41 min

100'
77 cf = 25 min, 100 cf = 35 min

Hope this helps.

the K

 

[DA Aquamaster]

Take a look at this post.

NItrox and tank size

Nitrox adds another element to the depth versus time versus tank size issue as it moves the NDL's far enough back that a larger tank begins to come into its own and become much more usable than is the case with air dives within the NDL's.

So, while an AL 80 is more or less standard in recreational diving with air, it is actually a bit too short on capacity to allow a diver to fully benefit from nitrox. A 100 cu ft tank is much better suited to the greater bottom times possible with nitrox and in the case of tanks like the PST E7-100 or Worthington X7-100, the tank is only about a pound heavier and is a few inches shorter than an AL 80 in overall lenght.

 

[IceBergSlim]

Hi,
I just got certified for diving with Nitrox and am in the market for a couple of tanks. Can someone tell me what the real world (dive time) difference is between using a 77cu.ft Lower Pressure [ 2400+ psi ] steel scuba cylinder vs. a steel 100 cu. ft. tank would be?

I can buy the 77 cu. ft. tanks for $175 less (each) so it sounds appealing but I don't want to buy them and find I'm getting much less bottom time.

Thanks!

What part of that question was not covered in your courses?

You might want to ask your insturctors why they did not prepare you for these issues.

 

[bamamedic]

Well, a steel 100 holds 100 cubic feet of gas at it's rated pressure (which is something like 3400psi, I think).

A 77 is a low pressure cylinder, so it holds 77 cubic feet of gas at it's rated pressure of 2640psi (assuming the + stamp)

You've definitely got more gas with the HP100 (assuming all cylinders are filled to their rated pressure)...as to how much more dive time, well, that depends on your individual SAC rate, and how deep you intend to go.

 

[rstofer]

You don't have a profile so it is hard to guess if you are diving cold or warm. In nice warm water, in my opinion (and it is not universally shared), highly negative steel tanks are not a good idea. For warm water diving, I see no reason not to use an aluminum tank. At a minimum, it will require some ditchable weight and that's a good thing (again, in my opinion).

Another thing to consider is where you will get the tanks filled. There are MANY dive boats that will short fill the 3442 psi tanks and an HP 100 winds up with < 90 cf of air. There are some LDSs at dive spots that will also short fill the tank.

Even for cold water diving, if you use a wetsuit, you need to pay heed to how negative the tank is. Some tanks are so negative that you can't swim up from depth even if you ditch your weights should your wing happen to fail.

So, capacity is only a minor issue compared to type, pressure and buoyancy. But, for Nitrox, bigger is better because you have more bottom time.

I chose HP 100s.

Richard

 

[DA Aquamaster]

Steel or aluminum is not really the issue. An AL"80" only holds 77 cu ft.

Depending on the AL80 used, it is either 4.4 pounds positive when empty to about neutral when empty. In contrast 3442 psi steel tanks range from 1.5 to 2.5 pounds negative when empty, so they do not fall into the overly negative steel tank range.

Special permit steel tanks in the 100-130 cu ft range are all pretty much in the 1-3 pound negative range when empty so the "excessively negative" steel tank argument does not really apply although it is potentially treu with some larger low and mid pressure tanks and is endemic to the really large high pressure tanks Heiser made.

Where larger tanks are much more negative is when they are full, but that has more to do with the larger gas capacity and swing weight than anything else and even then it is not that much of an issue with a single tank.

Rstofer is right that you may not get a full 3442 psi fill with a 100 cu ft tank. At 3000 psi you'd get 87 cu ft and at 3200 you'd get 92 cu ft. But an AL80 is also commonly under filled by a couple hundred psi to only 2800 psi after it cools, and at 2800 psi it only hold 72 cu ft, so with the same 200 psi underfill the difference between an AL80 and an HP 100 is still a solid 20 cu ft.

 

[halocline]

What 77 cf LP steel tanks are you talking about? Faber LP80?

 

[wxboy911]

Don't forget to factor in your buddies tank...If you are out with guys that dive 100s then the 100 may be a better choice. I have a 100 and most of the guys I go with dive 80's so I always have extra air. Also, keep in mind if you were to buy an lp85 and get it filled to 3000psi you would be carrying close to 100cf. Not than I am telling you an overfill is a good idea or that you can get an overfill just anywhere or even that the lp85 was the best choice-but given the price difference I may lean toward the lp tank.

 

[divinginn]

I have both tanks,a Worthington 77lp and a 100 hp,I get about 10-15 more minutes out of the 100 if both are filled to their rated pressures. What happens a lot is the low pressure tank gets over filled to 2800-3000 and the high pressure tank gets under filled to about 3200 psi. In that case there is less difference in how much they hold. If the 77s are more in your budget,I would get those,they are better than the alum 80s. I prefer the 100 hp but they are pricey. I paid 215 dollars for the 77 and 350 dollars for the 100. What works best for me is to use the 77 on shallower dives under 70-80 ft and the 100 on deeper dives.