Wrap a piece of paper tightly around the culm and mark where it overlaps.
(To be fussy, push a pin through both bits of paper and measure dot-to-dot.
Measure the length of the piece of paper (circumference).
Diameter = circumference divided by Pi (3.14)
It's probably easier to measure and divide in metric (millimeters),
then convert to inches (one inch = 25.4 mm).
This has the advantage that any error in meaurement is divided by three, so it's as if your ruler had three times the resoluton it really does.
Use a Xerox machine to copy a ruler onto some paper at 100% size. Now you can read the circumference right off the paper (you still have to divide that by 3.14.
Or use a fancy Zerox machine to blow that ruler up by 314% and read the diameter right off the paper.
Or use any crescent wrench on the culm, then slide it off and hold it on top of a ruler to read the diameter directly.
If you're doing a LOT of these measurments, make some "GO - NOGO gauges" from cardboard or sheet plastic.
Cut them to the sizes you're interested in, say every 1/8th inch from 3/4" to 2". The smallest one that slips over the culm is the size.
I got my digital calipers from Harbor Freight, but the Google search "amazon micrometer" gave several options.
Does anyone know if quoted diamters tend to reflect the thckest bulging part of a joint, or the narrowest part in the center of a section?