Understanding Infrared Thermography Reading 3

Thermal Inertia
Thermal inertia is a term commonly used by scientists and engineers modelling heat transfers
and is a bulk material property related to thermal conductivity and volumetric heat capacity. For
example, this material has a high thermal inertia, or thermal inertia plays an important role in this
system, which means that dynamic effects are prevalent in a model, so that a steady-state
calculation will yield inaccurate results. The term is a scientific analogy, and is not directly related
to the mass-and-velocity term used in mechanics, where inertia is that which limits the
acceleration of an object. In a similar way, thermal inertia is a measure of the thermal mass and
the velocity of the thermal wave which controls the surface temperature of a material. In heat
transfer, a higher value of the volumetric heat capacity means a longer time for the system to
reach equilibrium.
The thermal inertia of a material is defined as the square root of the product of the material's bulk
thermal conductivity and volumetric heat capacity, where the latter is the product of density and
specific heat capacity:
e = I = √(kρC p ) See also Thermal effusivity
k = is thermal conductivity, with unit [W m −1 K −1 ]
ρ = is density, with unit [kg m −3 ]
C p
= is specific heat capacity, with unit [J kg −1 K −1 ]
e, I = has SI units of thermal inertia of [J m −2 K −1 s −1/2 ].
Charlie Chong/ Fion Zhang
http://en.wikipedia.org/wiki/Volumetric_heat_capacity#Thermal_inertia