Abstract: A capacitor is created in an ink tank supplying ink to an ink jet printhead by placing parallel capacitive plates on sides of the tank with the ink therebetween acting as the capacitor dielectric. An electrical AC potential is applied across the capacitor. Changes in current flow through the plates are detected in a phase sensitive bridge circuit and digital signals are generated and sent to a processor for conversion into low ink level signals. The output signal is also used to identify a particular type of ink by either comparing points on a characteristic curve of the ink volume verses capacitance or, in a second embodiment, by periodically sweeping an applied oscillator voltage over a selected frequency range and comparing measured electrical resonance parameters for the ink tank capacitor with stored information about resonance parameters for different types of ink.

Abstract: A detection circuit for detecting the existence of non-collapsing bubbles in the ink cells of a thermal inkjet printhead is connected to a heater element of an ink containing cell. The detection circuit has a sensing element of low resistance when compared to the resistance of the heater element so that printing and detecting operations can proceed simultaneously. Current in the heater element is proportional to the potential drop across the sensing element. An amplifier is used to measure the potential drop and is connected to a blocking capacitor. Non-collapsing bubbles are detected if the voltage drop across the sensing element varies from a reference level.