Inkjet ink level sensing?

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?
Thanks.
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snipped-for-privacy@invalid.net says...

I believe HP just estimates based on the cartridge type it detects, and the amount of ink it uses.
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underneath :

Any inkjet that uses chipped cartridges guestimates the emptiness. Old Canon printers certainly used the reflective prism, IP4000 was about the last that had unchipped carts. and thus had an accurate end level warning. I think most printers now would guestimate on the safe side judging by the complaints of half full carts being classed as empty. I dont know if Canon still uses the prisms on their modern chipped carts but I would think probably not as the guestimate method gets you to buy more ink earlier! Someone with one of the more modern Canons will know Im sure! ... HP have had chips since very early days so levels will be guestimated by software. C+
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On 08/03/2015 1:22 AM, Charlie+ wrote:

My old(?) Canon MP 620 uses chipped cartridges as far as I know and when I get an empty signal -the cartridge is fully emptied.
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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.
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