Let's say you want to replace a broken O-ring from some obscure product for which no data or manuals exist. How do you determine the O-ring diameter and thickness?
The O-ring may have been stretched, so surely measuring the old one won't do?
You could measure the O-ring groove, but how much wider than the gap do you want the O-ring for efficient sealing? How much smaller than the shaft do you want the O-ring for a good seal?