With measurements it seems to me we use approx. far more often than ca.
ca., on the other hand, is commonly seen with historic dates, for instance:
This necklace dates to ca. 500 B.C.
In speech one would most likely use around, about or approximately with dates.
A museum guide or scientist would be more like to say circa.
It is sometimes the case that even if one reads ca. 500 B.C. in writing, he will read this aloud as approximately 500 B.C..
For temperature, weight, length and it seems most everything else, one would rarely use circa at all. It's rather technical sounding and pretty much reserved for historians, archeologists and the like who do things like carbon dating of objects and events. Even the measurement of time spans, except for in these fields, is usually stated with approx. or approximately. One might see ca. 0.5 milliseconds but more likely approx. 0.5 milliseconds.
I wouldn't say, for instance, "The race lasted circa 15 minutes."
"This piece of wood is circa two and a half inches long." ....Nope.
Approximately or about or sometimes around.
Remember that circa is still a foreign word to English speakers, so its literal meaning is obscure to many. For instance, few people would think to connect circa to circus or, for that matter, even circus to circle and eventually wind up with round and around. Same with circumstance and circumstantial. Few would see into this word as easily as Germans do with Umstand and umständlich.
There are many similar examples because so many English words use prefixes which aren't native to English and are never used as freestanding prepositions, so their meanings remain fairly obscure to probably most except for those who bother with language study.