The hose or tube diameter in a system may not necessarily represent the exact size of the o-ring required to seal the connection. To confirm the correct dimensional size of a replacement o-ring, the measurements of the inside diameter and the cross-sectional diameter of the seal need to be taken with an accurate measurement instrument such as a caliper or a micrometer.

O-rings known technically as static seals are rubber seals shaped like a torus (donut), which means they are circular in shape and also have a circular cross-section.

O-rings are manufactured from many different types of rubber and the grade or hardness of which can be selected to suit certain applications. Care needs to be taken with fluid compatibility against the seal material to prevent degradation and to ensure long service life.

Commonly available sizes range from 0.5mm to 10mm cross-section diameter and up to 2900mm inside diameter. Larger sizes may be carefully constructed by using a strong adhesive to join the squarely cut ends of a straight length of rubber of circular cross-section together.

O-rings sit in a grooved channel of specific dimensions and as mating components are located, the o-ring is deformed and compressed slightly into the channel and against the mating component. This action creates an interface obstruction seal and a resultant force that prevents the escape of fluid up to a point.

Installation should be done with care to avoid scraping or slightly cutting into the o-ring which may result in fluid leakage. All surfaces must be clean and an appropriate lubricant can be applied to the seal to aid the assembly process.