Thermal Mass
In packaging, thermal mass describes the thermal inertia of a load. It refers not only to its weight, but to its ability to absorb or release heat. Outside a phase change, C = m · cp applies to a uniform material, with the unit J/K. For several materials, their heat capacities are added together.
Why do loads of equal weight respond differently?
- Materials have different specific heat capacities. One kilogram of aqueous solution and one kilogram of dry solid are not thermally equivalent.
- With similar heat input, the temperature of a larger heat capacity changes more slowly. In simplified terms, ΔT = Q/C.
- Initial temperature, shape, distribution and heat transfer also influence the actual temperature history during shipping.
Is an empty shipper always the worst test case?
No. A small load may respond quickly to ambient conditions, while a large or excessively warm load may place a greater demand on the cooling system. Suitable test cases are defined with a rationale for the system. A test dummy should represent the relevant thermal properties of the real product.
Related terms: Volumetric Heat Capacity · Climate Chamber Test
Technical references:
OpenStax: Temperature change and heat capacity
ISTA: Overview of Procedure 7D