2 questions on steel vs. aluminum

[Matteo]

had an opportunity this past weekend to dive w steel 80s for the first time and I really enjoyed the characteristics of them as opposed to their aluminum counterparts...so the first thing I am curious about is to get some feedback from those more experienced on the advantages and disadvantages of both steel and aluminum...the second question I have is about reserve...I've always been taught as most have to always keep 500 psi in reserve; if diving w/steel 80s, can you lower this in proportion, say keep a reserve of 350 psi?

 

[The Kraken]

Not significantly.
An aluminum 80 contains, if I remember correctly, 77.4 cubic feet of gas. A H/P 80 contains 80.
The nominal pressure ratings for a "full" tank for the steel and aluminum are 3442 and 3000 respectively.

You can do the numbers and see what ya come up with .

the K

 

[Mark Vlahos]

had an opportunity this past weekend to dive w steel 80s for the first time and I really enjoyed the characteristics of them as opposed to their aluminum counterparts...so the first thing I am curious about is to get some feedback from those more experienced on the advantages and disadvantages of both steel and aluminum...the second question I have is about reserve...I've always been taught as most have to always keep 500 psi in reserve; if diving w/steel 80s, can you lower this in proportion, say keep a reserve of 350 psi?

I agree the overall advantage of steel cylinders makes their purchase worth the additional cost, when used for "back gas" or the main cylinder on your back during a dive.

With regard to the reserve issue you have asked about I do not look at the pressure as the only criteria for a gas reserve. I look at the quantity of gas. If you consider 500 PSI in an aluminum 80 an adequate reserve you have 13 cubic feet. In a PST 3442 PSI steel 80 to get about the same 13 cubic feet of gas you will read about 550 PSI on the guage. It is reasonable to expect that if you have a problem this reserve is cutting it a bit thin. By this I mean that 13 cubic feet on the surface is plenty, but at depth when you dediced to head for the surface from lets say 50 feet the amount of gas in your cylinder may not be enough to get both you and your freaked out buddy safely to the surface with a safety stop. If both you and your buddy are freaked out you will burn through your gas pretty darn fast. Remember the reserve should be enough to get you and your buddy safely to the surface even if the unexpected happens. We all know that murphy will usually appear when you least expect it, why would you want to cut your reserve short and flirt with disaster.

I suggest that rather than looking for the right pressure on the guage, you determine how much gas you want and work that number back to what the pressure will be for that amount of gas. For me a better reserve is to make the surface with around 18 cubic feet of gas (about 700 PSI in an AL80). In the same steel 80 I used in my earlier example that would be a little less than 800 PSI. Remember, these numbers are for me at the surface NOT at any point still under water.

Sure, these numbers may cut a dive a little shorter, but they make sure I am still around for the next dive.

Mark Vlahos

 

[naiya]

I prefer steel tanks because they allow me to take more lead off my belt (a bigger issue to manage in my area). Steel does require a bit more maintenance to avoid rust, and if getting a high pressure tank (3442psi) a smaller compressor may not be able to fill to the rated pressure.

As far as an HP steel 80 vs. an Al-80, it's great to actually have the full 80cu with the steel.. but it is a lot shorter tank. It could actually be more awkward for a person with a normal torso length (the tank base won't rest). For that reason if going steel, I would consider the HP 100 or larger.

 

[Matteo]

ok, good stuff so far, this is what I was looking for....two things I should have mentioned in the original post;#1-physics is not my strong suit, but these responses are helping me understand the pressure vs volume thing a little more, #2-I should note that these particular tanks I was diving were not HP, the DM explained that a full tank was loaded to about 2400 to 2600 PSI, but obviously w/ the same volume of air, I had no difference in bottom time

 

[Charlie99]

had an opportunity this past weekend to dive w steel 80s for the first time and I really enjoyed the characteristics of them as opposed to their aluminum counterparts...

What characteristics did you enjoy the most?

What differences did you notice compared to aluminum?

The big advantage of some steel tanks is less total weight. With the PST LP80, however, there is only about a 3 pound advantage. (34# empty, 1 pound negative for a total of 33 pounds vs AL80 32 pound empty, 4 pound positive buoyant for a total of 36 pounds.) If you want to reduce weight, then tanks like the PST E7-80 or the older 3500psi PST 80 have a bigger advantage.

 

[Matteo]

Charlie, the 2 characteristics I enjoyed was the additional weight(less for me to lug around in BC and/or belt) and the fact that my hoses were much less rigid due to the LP...the DM said to expect 4 lbs negative and that was about right...a few on the boat did not heed this advice and were definitely struggling with being overweighted

 

[Mark Vlahos]

ok, good stuff so far, this is what I was looking for....two things I should have mentioned in the original post;#1-physics is not my strong suit, but these responses are helping me understand the pressure vs volume thing a little more, #2-I should note that these particular tanks I was diving were not HP, the DM explained that a full tank was loaded to about 2400 to 2600 PSI, but obviously w/ the same volume of air, I had no difference in bottom time

OK, a Low Pressure 80 will have 80 cubic feet when full at 2640 PSI if that cylinder was filled to 2400 PSI you would have about 73 cubic feet. 13 cubic feet would be a bit more than 400 PSI, and 18 cubic feet would be about 600 PSI. Sorry about the confusion on the pressure of the cylinder in question.

As mentioned by others a standard aluminum 80 actually holds about 77.4 cubic when it is full at 3000 PSI.

 

[blackclawdog]

Steel has a lot of advantages, but a serious drawback--cost. Frankly, I can't see why you would consider buying a steel 80 when you can get a steel 100 or a steel 120 with DIN valve. Diving with a steel 120, the only thing that should limit your dive (assuming you are doing a no-deco dive) is nitrogen loading, temperature, and your ability to hold in the pee-pee. I have logged 98 minute dives in a steel 120 (max depth 70 ft, multi-level dive), and still come up with over 800 psi--and I am pretty much an air hog. Problem is, the tanks cost over $500 a pop, and there are some corrosion issues that you don't have with aluminum 80s.

 

[Mark Vlahos]

Charlie, the 2 characteristics I enjoyed was the additional weight(less for me to lug around in BC and/or belt) and the fact that my hoses were much less rigid due to the LP...the DM said to expect 4 lbs negative and that was about right...a few on the boat did not heed this advice and were definitely struggling with being overweighted

The only hose that will show a difference in rigidity will be your hose to the pressure guage. All of your other hoses will be low pressure and that will not be any different regarless of the pressure in your cylinder since they are only given low pressure air and that is from your first stage and does not change between a high or low pressure supply.

Mark Vlahos