On Fri, 6 Mar 2015 23:03:13 -0800, DaveC wrote:

One source I found says that Canon uses light from an LED--bounced from a
prism at the bottom of the cartridge after ink is depleted--to trigger the
"Empty" signal. Epson apparently counts the spurts of ink and calculates the
arrival of "empty".

I couldn't find how HP do this. Anybody know?

HP has two methods. One is to just guess based on usage:
http://support.hp.com/us-en/document/c00216780
The other adds an expiration date to the cartridge:
http://support.hp.com/us-en/document/c01764161
Both suck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558