Hardness
Hardness is a physical measure, which is compression deformation degree. Shores hardness is tested by shores durometer, unit " degree"; Two discription: A&D, under 90 degree showed by shores A, over 90 degree showed by shores D. Normal rubber hardness unit is HS and IRHD, value of shores A is same with IRHD.
Temperature
According to working condition, choose suitable material.
Permanent Compression Deformation
One of index for o ring performace is permanent compression deformation of material. Under pressure, o ring elasticity deformed, pressure bigger, will cause permanent compression deformation. Its formula as follows,
Note: b0-oringinal thickness( section dia.) b1-thick under pressure b2-thickness after release
Normally, in order to avoid permanent compression deformation, max pressure under static seals is about 30%, under dynamic seal is about 20%.