Robust liquid level sensing with conductive fluids?

AR26

Other Parts Discussed in Thread: FDC1004, FDC2214

The FDC1004 allows use of the Out of Phase technique for robustness due to any hand interference. The example frequently given is a coffee maker. Suppose I now want to use salt water (conductive) in my coffee maker (yuck!). I think this suggests I should be using a part from the higher frequency FDC2x1x series, such as the FDC2214. Is there a similar technique as Out of Phase, but for the FDC2x1x series? It also looks like I'd lose the ability to shield electrodes, forcing use of an external driver.

Is FDC2x1x still the best choice for conductive fluids, despite these limitations?

over 9 years ago

0 David Wang11 over 9 years ago

TI__Expert 7400 points

Hi:

It really depends on how conductive the liquid is. If it is possible to measure the resistivity and calculate it's resistance, you can determine whether the FDC1004 can be used or not. The sampling period of the ADC is 10us. The liquid discharge time must be much larger so it does not degrade the signal.

Unfortunately the fdc2x1x does not have shield drivers or the OOP technique. For shielding I would rrecommend GND as a shield rather than creating one with a buffer. GND is more effective but it will have a much larger parasitic capacitance compared to the buffered signal.

Thanks

David Wang

Capacitive Sensing Applications

0 Evgeny Fomin over 9 years ago in reply to David Wang11

TI__Genius 9300 points

Let me just add that a conductive liquid in not a problem for FDC1004, it's conductive AND sticky liquid which presents a problem : if it sticks to the walls, it will throw off the results.

0 AR26 over 9 years ago in reply to David Wang11

Prodigy 20 points

Thanks. I'll go ahead and prototype it out and see what happens then.

0 Matt Paley20 over 9 years ago in reply to AR26

Prodigy 10 points

I had a similar problem. A mildly conductive liquid in a polystyrene tube produced terrible result but in polypropylene it was fine. The difference seems to be in the degree to which the liquid wetted the walls.

0 Yibo Yu over 9 years ago in reply to Matt Paley20

TI__Expert 8050 points

Hello Matt,

Thank you for sharing your testing results of different containers.

Regards,
Yibo

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Robust liquid level sensing with conductive fluids?