How long before you started diving doubles?

[GetNarc'd]

LP tanks are somewhat easier to fill with PP blending, the tradeoff as you say is tank size -- a LP120 is bigger physically than an HP120. Of course, an LP120 is also "equivalent" in theory to a HP-something --150 ?

After reading this and the other posts I did a quick search to learn more about lp and hp filling pressures and capacities. This link explained a lot, including what limey and Perroneford mentioned.
http://www.diveriteexpress.com/library/tanks.shtml#vollie

 

[JEScholz]

LP tanks are rated with a plus rating to 2640 psi. It is customary (not saying it's "right") to fill them to 3300-4000 psi here in N. FL.

Aluminum tanks should not be overfilled.

What is the blow out disc rated at in your steel tanks?

 

[in_cavediver]

What is the blow out disc rated at in your steel tanks?

Burst disc? We deenolt need no stinkin' burst disk!

Actually, there are several options used, SS discs, double disking and simply using an 'HP' burst disk in an LP tank. Of those, only the higher rated disk still acts as a burst disk. The other two options (SS and Double disking) place the burst disk pressure up in tank burst pressure range.

 

[Meng_Tze]

I think it is also known as a M5 bolt....

disclamer: this IS a joke and I do not condone nor advise anyone altering anything on the tank or valve as they have been supplied by the manufacturer.