O rings are used to block a path which may otherwise allow a liquid or a gas to escape. The o-ring is placed into a groove to secure them in place, and then compressed between two surfaces.
HOW TO MEASURE AN O-RING
To measure the outside diameter, use a ruler to measure from one outer edge to the other of the o-ring. You can determine the third diameter of the O-ring with 2 diameters using the following equation: Outside Diameter (OD) = Inside Diameter + (2 x Cross Section). Inside Diameter (ID) = OD - (2 x Cross Section).