Try this from McMaster-Carr. If the link below doesn't work search for 24125T22 on their web site. It's on the bottom of page 1162.
http://www.mcmaster.com/cgi/enter.cgi?pagenum=1162
From McMaster-Carr's catalog:
How do PSI (for Pipe Fittings) and Schedule (for Pipe) Relate?
PSI (pounds per square inch) is the amount of pressure a pipe fitting can endure. Schedule refers to the thickness of pipe.
While there are no steadfast rules relating psi to schedule, we...