.....actually i think i am not good enough with decoupling and star grounding techniques. if you know suggest me any links where i can find such study material.

let me tell you the main reason for using separate 12V is, the 4-20mA output. [I just use the same supply for 4-20mA and relay].

When the conductivity sensor is connected to analogue circuit, with same power supply of MCU circuit. the conductivity sensor has exited with electrical signal and hence due to its metal body the same signal is connected to the liquid media / metal pipelines. In other word we can say it gets connected to mains earthing.

On the other side, When they run the 4-20 mA to outer systems, and 'by chance' the outer system is also sharing its 'power supply ground' with mains earthing, Then this all will distrurb all the measurement, if no isolation is used in the measuring istrument. they sometimes call this as 'grounding loop problem'.

Thats why they want electrical isolation between input circuitary and analogue output circuitary of the instrument.