I want to control a motor (pump) speed via vfd in such a way that when temperature of the water increases the pump speed also increased and when temperature of the water decreases the speed of the motor (pump) also decreased ..... can we used external PID control with a VFD OR use a VFD having a PID control option and then tune up that PID control ?

It has to implement to maintain the temperature of water to a certain range

To control the cooling system as i stated above , what things / parameter should i calculate and measure first ?

i have ABB ACS 550 VFD which has a in built PID function , so how to tune up its PID ?
Do i need of a sensor i.e Pt 100 and a current signal converter for this setup ?