Slumpert:
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I agree both levels will stabilize over time if A is closed, but if A is big enough it will drain faster than the constant level can flow through the filter media so there will be a drop between values.
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If it’s a waterworks project a manometer is a proven, cheap, technology that if you needed automate readings on just stick a ultrasonic rangefinder firing down the tube..
I'll buy your answer if 'A' is open. I also agree keeping the manometers is a good idea as it gives a manual backup to any electronic system. Removing them does not reduce the number ofholes you need in the vessel. Keeping them you could use ultrasonics as you suggest.