There is all kinds of stuff going on here.. Skinnee nailed one by mentioning that the radiator would have to be 100% efficient, which isnt going to happen. Also mentioned above was using average temperatures, which actually changes the problem around. If you get super technical and were able to take temperature readings from one side of the tube to the other (like the diameter of a circle) you would find that you have a temperature distribution. The temperature of the fluid right next to the wall of the tube is going to be different from the temperature at the center line of the flow. Though the temperature difference might be small, when you start crunching out numbers it will make a difference. Also mentioned above was 'error'.. There is an actual system for calculating the error of experiments; Kline-McClintock. That system goes through all the partial derivatives, resolution error, deviation, and mean to come up with an error. It is really tedious, but will give you a range of where your final result lies. BUT, most modern stuff that we use tells you what the error is (+/- .05% of the reading or whatever). And, then there is the almighty calibration - which I didn't really see mentioned. Calibrating thermocouples is easy..stick it in boiling water and it should read 100C, then make an ice-bath, stick it in that and it should read 0C. When calibrating our thermocouples we just put them in boiling water and found out how much off each one was...which was usually within the error given in the thermocouples description. All equipment should be calibrated before every test..pumps wear out, thermocouples degrade, etc etc.