Here's a formula for you.

http://www.tlv.com/global/TI/calculator/air-pressure-loss-through-piping.html
1 bar is 14.5038 psi



DN15 = 1/2: ID
DN20 = 3/4" ID
DN25 = 1" ID


www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).