Well done, Very well done. The construction of an accurate EC meter is a most complex subject for the reasons I mentioned... Unfortunately expensive to buy "Off The Shelf" however your solution is repeat-able, calibrate able and... Do-able The hardest thing here is separating what you can understand from what you just don't know yet... Electronics is a Very Complex Subject and a Little Knowledge... can be Most Dangerous... Remember Mickey in the "Sorcerers Apprentice"... He knew how to start the Brooms... IMO
Doc