Detection method of O-ring size

A standard O-ring can be measured with a conical die. If the O-ring is placed on the cone and falls naturally by gravity, the inner diameter of the O-ring can be measured. As for the line diameter, it can be measured by three coordinates. It turns out that the O-ring is elastic and its accuracy is not very high. The dimension changes greatly after stretching. If you think the vernier caliper is not accurate enough, you can use a three-coordinate projector, but I personally don't think it is necessary.