The heat transfer at an interface is considered a transient heat flow. To analyze this problem, the Biot number is important to understand how the system behaves. The Biot number is determined by:
The heat transfer coefficient , is introduced in this formula, and is measured in . If the system has a Biot number of less than 0.1, the material behaves according to Newtonian cooling, i.e. with negligible temperature gradient within the body. If the Biot number is greater than 0.1, the system behaves as a series solution. The temperature profile in terms of time can be derived from the equation
which becomes
The heat transfer coefficient, , is measured in , and represents the transfer of heat at an interface between two materials. This value is different at every interface and is an important concept in understanding heat flow at an interface.
The series solution can be analyzed with a nomogram. A nomogram has a relative temperature as the  coordinate and the Fourier number, which is calculated by
The Biot number increases as the Fourier number decreases. There are five steps to determine a temperature profile in terms of time.
# Calculate the Biot number
# Determine which relative depth matters, either x or L.
# Convert time to the Fourier number.
# Convert  to relative temperature with the boundary conditions.
# Compared required to point to trace specified Biot number on the nomogram.