Please tell me What is characteristic time in [Fourier number](http://en.wikipedia.org/wiki/Fourier_number)? How can I calculate characteristic time? Suppose I started heating water in a closed container by immersion rod. And temperature increases continuously, Suppose at time, t=260 sec temp. at center is 40 deg C, and at t=480 sec, center temp is 42 deg C. Then to calculate Fourier number I need characteristic time. Then which time I should consider, 263 or 480 sec?