The HP No.81 Ink Cartridges for the HP DesignJet 5000/5500 series deliver 680 cc of ink. Residual ink in the Ink Cartridge is required to avoid starvation in the Ink System, that could cause damage to components. Ink Cartridges are designed with:

-Smart chip storing Ink Cartridge information.
-Ink connector to deliver ink to the Ink Tubes.
-Air connector that forces ink out of the Ink Cartridge.
-Ink Type lockouts that avoid Ink Cartridges with the wrong type of ink being installed in the Ink Supply Station.
-Color lockouts allowing insertion of only one color in a given slot.