A funny thing happened when the "carefully chosen" o-rings arrived.
They are all the wrong size!

My wife smiled, gently chiding me, a bit incredulously ...
"After all that work, you ordered the wrong ones?"

Yup. Mea culpa. They're *all* the wrong size!
Apparently, I didn't double check the numbers!

It (apparently) turns out that the "O-number" system I've been cross
referencing is a proprietary standard, used *only* by the Alladin company.
http://www.aladdin1950.com

So, for example, a Sta-Rite pump basket lid O-ring U9-375 cross references
to an Alladin O-12, which is not even close to the AS568 O-Ring size O-12.

An O-12 actually is this (much smaller) AS568 o-ring:
O-12 nominal CS=1/16" ID=3/8" OD=1/2" actual ID=.364±.005" CS=.070±.003"

While, measurements of "my" pump basket lid O-ring show it's closer to:
O-439 nominal CS=1/4" ID=6-1/2" OD=7" actual ID=6.475±.040" CS=.275±.006"

Even I had to laugh at myself, thinking: Here's a classic case of what
happens when you accept the numbers at face value (and don't double check
all sizes with actual measurements!).

New mantra: Check thrice; order once!

In summary, it seems that one company, Alladin, dominates replacement
o-rings for pools, and that company uses a proprietary (unpublished?)
o-dash numbering system that mimics the standard o-dash numbering system
in both look and feel - yet the two numbering systems are (apparently)
wholly unrelated! (AFAICT)

Given that, I'll need to measure all my old (worn) o-rings, in order
to cross reference to the "real" AS568 o-ring O-dash size.

Serves me right for taking things at face value!

QUESTION:
What's the best tool to *measure* o-rings (enough to match to charts)?
http://www4.picturepush.com/photo/a/...g/13295487.jpg