Tank age and Max # of Hydros

[GoState!]

Thanks to all for your input. I did some of my own research after my initial post and confirmed many of the responses concerning some dive shops refusing to fill a tank because of its age. I called several dive shops along the East coast coast and found that for the most part as along as the tank passes hydro and vis within prescribed intervals there was no issue with filling it. However, one poster mentioned a Fla. shop who practiced refusing to fill old tanks. One gentleman I spoke to noted the 6531 tank alloy advisory as justification for pretty much refusing any tank older than 1988. I guess if there is a question, just ask! Thanks again!!

 

[duckbill]

One gentleman I spoke to noted the 6531 tank alloy advisory as justification for pretty much refusing any tank older than 1988.

Pure ignorance. 6061 is a completely different alloy, and steel is, well duh, a completely different material. He should be embarrassed for actually voicing his lame opinion.

 

[GoState!]

As you can probably tell, I have never dove with a steel tank nor been exposed to anyone using steel. So my question is...steel outlives aluminum??? Pros/cons of LP vs HP steel??

In any rate lets test some of everyone's math skills. I asked a prominent online scuba retailer if 2 aluminum 80's at 3000 psi had more or less usable air than one HP steel 130 cu (3442 psi) tank and got a simpleton response that said 80+80=160 which is more than 130 so the 2 80s win out. The methodology here is incorrect. Can anyone give me an answer????

 

[OldNSalty]

an AL 80 actually holds ~77 cubic feet I believe so I give them 10 points for deriving the correct answer but -50 for using the wrong values. So what exactly is your concern with their methodology and what do you mean by "usable air"?

 

[duckbill]

Usable air does not include what one would consider a reasonable amount of reserve.

Let's say for now that 80s are 80s (O.K. they aren't, but let's make this easy math), and you want to end up with a 13 CF reserve.

Twin 80s would give 80+80-13=147 usable CF. (Reserve hit at 244 psi.)

A single 130 will give 130-13=117 usable CF. (Reserve hit at 344 psi.)

 

[OldNSalty]

Thanks duckbill. I would suggest that factoring in usable air for these 2 examples is a bit pointless for all but the most extreme values of a reserve. Even then, given that the reserve is a constant between both can it not simply be tossed out?

Even if the reserve were 130 then all it does is eliminate one of the 2 possible answers and you still have the correct answer.

 

[duckbill]

Depends.

If you're diving a single 50, 13 CF reserve is over 1/4 of your air supply! Hardly insignificant.

But if you were diving twin 100s, 13 CF is 1/15th of your air supply. I never "toss" the reserve amount when comparing tanks for just this reason.

I use 13 CF as a standard reserve based on that I was taught to consider 500 psi the reserve in a standard aluminum 80. I soon learned to calculate the reserve pressure for each tank as it varies quite a bit. 500 psi in a Al80 is equivalent to 260 psi in my twin 50s, for instance.

I'm just wondering if I passed the quiz

 

[oxyhacker]

The problem with figuring reserves this way is that when your pressures get that low you are running perilously close to the margin of error for many older gauges. Hence the the use of 500 psi regardless of tank size - it allows a reasonable amount of air even if the gauge is significantly off.

Usable air does not include what one would consider a reasonable amount of reserve.

Let's say for now that 80s are 80s (O.K. they aren't, but let's make this easy math), and you want to end up with a 13 CF reserve.

Twin 80s would give 80+80-13=147 usable CF. (Reserve hit at 244 psi.)

A single 130 will give 130-13=117 usable CF. (Reserve hit at 344 psi.)

 

[duckbill]

The problem with figuring reserves this way is that when your pressures get that low you are running perilously close to the margin of error for many older gauges. Hence the the use of 500 psi regardless of tank size - it allows a reasonable amount of air even if the gauge is significantly off.

That's what snorkels are for!
Very good point, especially if you rent. I know my gauges and have never had a problem. Then again, I rarely use my air all the way to reserve, and I'm never far enough from the surface to worry about it anyway.