I'm guessing this is some problem with the ultrasonic frequency making its way to the wires carrying the DS18B20 signal,
It could be in which case add decoupling to the supply:-
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
On the other hand it could be how you have wired the grounds together. Make sure that the ground to your sensor is not caring heavy current from your cooling device.