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Capacitive sensors can detect the presence of ink through a marker cap

QuikApps - Marker Ink Detection

In the production of markers (permanent, white board etc.) the assembled units are filled with ink on a production line. Various failure modes could cause the ink to not fill the marker or flow all the way to the tip. If not monitored, this results in the shipment of failed units and aggravated customers. If the cap is on the marker, visual inspection is not possible.

Capacitive sensors have a high sensitivity to most liquids compared to their sensitivity to plastic. A capacitive sensor can see "through" the cap and sense the liquid ink in the marker tip beneath the cap.

The production line can be designed to pass the capped tip of completed markers past a capacitive sensor. The presence of the ink in the tip will change the sensor output. If the output does not change, there is no ink in the marker and it can be sorted and the process examined for failure.

Because of the distance to the marker tip, a larger capacitive probe is required for the range. An A-Size sensing area is a good starting point for an experiment in your application.

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