Brother MFC990CW Service Manual page 95

Ink near-empty/ink empty detection
The ink refill assembly has four ink empty sensors (photosensors of transparent type) that
monitor the ink levels with sensor actuators inside the ink cartridges.
Air intake port
Air
Ink empty
sensor
Ink
Ink supply port
Ink empty sensor
Attached to one end of the sensor actuator is a float. When there is ink in the cartridge,
buoyancy lifts the float, rotating the sensor actuator about a pivot near the center of the actuator
to block the light beam to the ink empty sensor, indicating that there is ink.
As the ink level in the ink cartridge drops, however, the float falls, eventually moving the sensor
actuator out of the beam.
Light hitting the sensor outputs the "ink near-empty" signal to the controller that shows the "Ink
low" message and activates a firmware counter tracking ink usage during ink-jet printing,
purges, and other operations. When this counter reaches a predetermined limit, the firmware
regards it as "ink empty" and shows the "Cannot Print" message to prompts the user to replace
it.
Disc valve S
Sensor actuator
Disc valve D
Sensor actuator
3-18
(InkBackflowPrevention)
Float (InkEmptySensor_2)
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