We have determined the dependence of the measured overall heat transfe
r coefficient, U, on the agitation speed, the volume of fluid inside t
he reactor vessel and the calibration time by designing 36 experiments
. The matrix of these experiments was prepared using software for the
design of the experiments. The overall heat transfer coefficient, U, f
or the above 36 experiments was calculated based on both the heat tran
sfer area at zero agitation and the observed area after the liquid lev
el climbed up the wall due to the fluid being stirred. The analysis of
the results shows the dependence of U, based on either initial area o
r observed area, on the agitation speed and fluid volume. Calibration
time was found not to influence the value of U.