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Tall tank liquid level measurement

Question asked by artemr on May 6, 2017
Latest reply on Jul 18, 2017 by Wa$iM

Hi, I have several questions about liquid level sensing applications using AD7745/7746/7747 capacitance-do-digital converters.


1) For my application I have to measure liquid level in relatively large tank with height from 1.5 and up to 3 meters. It looks like in most cases capacitive approach is used for relatively small tanks with height in range 0.1 - 0.5 meters. Could you give any recommendations for my case - is it possible to achieve at least 1 cm (worst-case, 1-2mm is goal) precision  for 1.5 - 3m height tank?


2) AD7746 uses 2-capacitors measurement scheme. In my case, empty space may be filled with vapor under pressure with temperature varying in wide range, which means I have to measure empty space reference capacitance, too. So I'll need either 3 x AD7745 or AD7745 + AD7746 ICs. In both cases,sensing capacitors will be connected to exciation sources from different ICs. Won't that reduce sensivity significantly? AD7147 have more channels, but it have significantly lower resolution and I'm afraid I won't meet precision requirements.


3) Liquid may be corrosive chemical, which can destroy shielding in direct contact with liquid. I saw similar question here, and answer was that if shielding is not connected to liquid directly, system is getting much more succeptible to noise. Won't AD7747 based system perform better in case if I cant connect ground terminal to liquid?


Thank you.