It's complicated, this is the short version, and you'll find lots more information in old threads using the search function

A service pressure of 3 tons per sq. in. = 8,938 psi service pressure (CIP)

European (CIP) proof pressure of 850 kg per sq. cm. (BAR) = 13,920 psi proof pressure = (about) 10,730 psi service pressure (SAAMI)