Abstract: A color ink jet printer having a replaceable printhead assembly with a plurality of replaceable ink tanks, one ink tank for each color of ink, senses a patch on each ink tank. The presence of the patch indicates the ink tank is new. In one embodiment, the patch is adhered over a recess in each of the tanks and is punctured by a pin which is triggered as soon as the patch is detected, so that the patch cannot be detected again. The energization of each group of heating elements associated with a respective ink tank is counted and stored in the printer memory and, as each energization results in the ejection of an ink droplet of known volume, a predetermined number of energizations, which represent the volume of usable ink in each ink tank, is stored in the memory. When this predetermined number of energizations is reached, an out-of-ink display is shown to the user.