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FDC1004: Container material selection for capacitive liquid level sensing

Part Number: FDC1004
Other Parts Discussed in Thread: TIDA-00317,


We are successfully detecting liquid levels using the FDC1004 and the TIDA-00317 reference cable. However, we ran into an interesting issue while testing different container materials. In all test cases, the capacitive readings were consistent when filling up with water the first time. But when we'd remove water from the container, some materials would perform better than others. For example, when using a high density polyethylene container, the capacitive values at 4cm of water were vastly different when filling up and emptying. Testing with an acrylic container, the values when emptying the container were much more predictable.

The conclusion we came to was softer plastics can be more porous, causing it to retain liquid or more quickly build up film causing water to stick to the sides throwing off the capacitive readings. I haven't seen any documentation on material selection in the application notes, but the material under test always appears to be acrylic. Has anyone done any analysis on the effect the container material can have on the long term capacitive readings? Are there materials that are better suited for this application than others? Has anyone implemented the FDC1004 in their product and found a solution to working around build up?

Any insight or resources would be greatly appreciated!