Detecting ink states for printers based on monitored differential pressures

An apparatus comprising:

• a print cartridge;
• a primary ink supply to provide ink for the print cartridge at an ink outlet of the primary ink supply, the primary ink supply having an associated first air pressure;
• an auxiliary ink supply to provide ink for the print cartridge at an ink outlet of the auxiliary ink supply, the auxiliary ink supply having an associated second air pressure different than the first air pressure;
• a sensor to sense a differential between the first air pressure and an ink pressure associated with the primary ink supply;
• and
• a controller to detect a state of the primary ink supply based on the sensed differential.

An apparatus comprising:

• a printhead;
• a first ink reservoir to provide ink for the printhead at an ink outlet of the first ink reservoir, the first ink reservoir having an associated first air pressure;
• a second ink reservoir to provide ink for the printhead at an ink outlet of the second ink reservoir, the second ink reservoir having an associated second air pressure different than the first air pressure;
• a sensor to sense a differential between the second air pressure and ink pressure associated with the second ink reservoir;
• and
• a controller to detect an ink state for the first ink reservoir based on the sensed differential.