how is the relationship between the central angle theta and the depth of an amount of liquid in a cylinder tank on its side h= r(1-cos(theta/2)?
The picture might help: http://i6.photobucket.com/albums/y234/Agent_0_Smith/tankonitsside_zps904c9330.jpg
cos(α/2) = (r-h)/r
r*cos(α/2) = r-h
h = r - r*cos(α/2) = r(1 - cos(α/2))
h = r(1 - cos(α/2))
Hopefully that helps.