it is a home made sensor to calculate the onductivity of water
when you have a disolution of water and for example salt (NaCl), and you transmit to it DC, the water suffer a process called desionization, because of that i must use AC in order that water do not deionizat and with AC water can transmit electricity, that is what i need to make the sensor works.
I think this is the best explanation i can tell you about this part of my proyect
thank you for your help