I dont know the exact definition of calibration but i know whats going on. You have two change the temp in the waterbath, measuring it with thermometer and then( i take it the bead thermistor is in the water bath and part of a circuit) measure the p.d. across the thermistor at given temps. eg, at 10 degrees the p.d is blah, at 20, it is blah, etc.you have to plot a graph. you should get a curve. then you can use this curve to measure temp without the thermometer. I think calibration is the part where you get the curve, coz you're, i dunno, calibrating the temperature or something.