My Canon S450 gives me a low ink warning. How does it detect the ink level in the tanks?
New Canon ink jet printers use a optical sensor to keep track of the ink level.
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Other printers also have optical detectors, this is not so outsdanding, you have to love marketing people. 
I am have tried to find an optical sensor but cannot see any sign of one which could detect ink level in the tanks which are pretty much surrounded by the print head assembly. How can I find the optical sensor(s)?
You might have to take it apart to see the optical sensor. Keep in mind that an optical sensor may include infrared sensing, so you may not actually see light. IIRC, IANAEE, the cartridge could have a strip of resistor material that will change it’s resistance value base on the amount of infrared light that hits it. As more infrared light hits the surface, as more of the ink has left the cartridge, the resistance will decrease. The electronics will pick up a change in current or voltage and signal the printer’s processor that the ink is low.
Yes I think that is the case also, optical sensors are integrated inside the ink cartridge itself. So if you are really curious, it could get messy. Other printer makers use this system to discourage the use of clone cartridges, since the sensors usually are not present in the clone ones.
I think I might have located it. The printer head assembly has some small windows in the underside through which you could see the ink tanks. As the head moves to the extreme left it goes over a tiny thing which may be the optical sensor but I was expecting it would be by transparency but it seems there is nothing on the other side so it must be by reflection. At any rate, covering that window in the printer cartridge with tape does fool the printer and the alarm disappears so I guess that’s how it works. I refilled the black tank but I need to refill the colors also.
It asks if the ink has been eating and sleeping well.
[sub]'cause, y’know, feeling “low” is a sign of depression…[/sub]
On my old crappy Epson, it was calculated in software. So if you replace the cartridge before the other one is empty, it will continue to assume the new cartridge is running out. So when you put in a new cartridge you had to tell the software it was replaced so it could start its cald. over again. Didn’t work very well at all. Esp. with the color carts. running out at different times.
The easiest way to tell would be simply lifting the cover and looking at the cartridges but printer makers go to great lengths to discourage this. (Issues with people refilling carts. instead of buying $$$ replacements.)
I think there may be another answer
Q. How does the printer know the ink is running low?
A. It always happens when the store runs out of refills.
I went to the store and they told me they sold the last one some hours ago and it will take some days to get more. I think the ink level in the printer follows closely the stock level of ink refills in the store.
Epsonds have a chip in the cart. That’s why you have to buy the cart.