How do Xerox Workcentre printers detect ink levels?

Archived from groups: comp.periphs.printers,comp.sys.xerox (More info?)

Fortunately, I am able to view the ink levels for my Xerox M940. The
big drawback, however, is that the "Out of ink" message appears way
too early, especially with the Black ink cartridge. The only
workaround is to swap in/out a new cartridge. When I reinstall the
old cartridge, it will allow me to print again until it is *really*
out of ink.

How does the printer know when I install a new cartridge as opposed to
just temporarily removing the old one? Is there some kind of sensor?