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It is common for printers that use challenging agent or inks to include a Recirculation System through the

printhead. This allows for the agent/ink to mix both in the Agent Delivery System and the Printhead.

This method is especially useful in 3D-metals printers, where printhead reliability is crucial and refreshing

the printhead with new agent will increase the printhead life and achieve a better isothermal control in the

firing chamber.

One risk of recirculating through the printhead is that particles in suspension in the air of the print zone

might be ingested through the nozzles and end up in the agent recirculation system. As this recirculation is

performed frequently, the quality of the agent could diminish and lead to problems both in printed part

quality and reliability for the hardware of the printer.

This invention presents a new method to estimate the number of particles suspended in the agent by taking

advantage of the conductivity of the agent. As more metal particles are present in the agent, its conductivity

will increase. The method takes advantage of the conductivity sensors commonly used in agent/ink tanks

to estimate their level. By checking its conductivity (based on a transmitter and a receptor), its value will be

estimated and compared with the expected value of conductivity for a pure agent. If the value deviates from

the nominal value, the system will report it so proper maintenance can occur (e.g., filter replacement).

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