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I have an aluminum 80 and a steel 72. I was just thinking about the sizes of them and had some thoughts that I wanted confirmed. When I started out with the different tanks, I always assumed the steel was much smaller than the aluminum- since it was a smaller size to begin with, plus it was filled to 2000, rather than 3000. After thinking about it, it occured to me that the aluminum is 80 cubic feet of air when full, at 3000 psi, and the steel is 72 cubic feet of air when full, at 2000 psi. Is this correct?

Also, since the steel can be overfilled by 10% with the plus on the hydro, the steel then holds 79.2 cubic feet of air, which is about the same as an aluminum. Is this also correct (I think this might be a wrong assumption, since it is overfilled based on pressure, not volume).

I remember something about only getting a plus on the tank for the first 5 years, but my tank was hydroed this year and there is a plus on it- the tank is at least 30 years old. When bringing it in to shops, do you have to request that it be overfilled, or do they just do it automatically? If they do it automatically, should I request that my tank only be filled to 2000 psi out of safety reasons?

Just out of curiousity, does anyone know what the volume of each tank is? (i'm sure i could find it online, but does anyone know off hand?)

Thanks
Scott

Hi Scott,

My understanding is that most steel tanks need to be filled to the 10% overfill to reach their full capacity. I have several very old steel 72 tanks as well. Their service pressure is 2250 psi and with the 10% overfill they can go to 2475 psi. However, they only contain the full 72 cu ft of air at 2475.

I've heard that steel tanks used to only get the plus rating when they were new, but I don't think that is the case anymore. My old steel 72s all got the plus rating at their last hydro. All of the shops that I use automatically overfill the tanks by 10% if they have the plus rating. And assuming you've got the plus rating, I wouldn't be at all hesitant about using them at that pressure.