# Conductance-Definitions of thermal conductivity

- Jun 19, 2018-

For general scientific use, thermal conductance is the quantity of heat that passes in unit time through a plate of particular area and thickness when its opposite faces differ in temperature by one kelvin. For a plate of thermal conductivity k, area A and thickness L, the conductance calculated is kA/L, measured in W⋅K−1 (equivalent to: W/°C). ASTM C168-15, however, defines thermal conductance as "time rate of steady state heat flow through a unit area of a material or construction induced by a unit temperature difference between the body surfaces" and defines the units as W/(m2⋅K) (Btu/(h⋅ft2⋅°F))

The thermal conductance of that particular construction is the inverse of the thermal resistance. Thermal conductivity and conductance are analogous to electrical conductivity (A⋅m−1⋅V−1) and electrical conductance (A⋅V−1).

There is also a measure known as heat transfer coefficient: the quantity of heat that passes in unit time through a unit area of a plate of particular thickness when its opposite faces differ in temperature by one kelvin. The reciprocal is thermal insulance. In summary:

• thermal conductance = kA/L, measured in W⋅K−1 or in ASTM C168-15 as W/(m2⋅K)

• thermal resistance = L/(kA), measured in K⋅W−1 (equivalent to: °C/W)

• heat transfer coefficient = k/L, measured in W⋅K−1⋅m−2

• thermal insulance = L/k, measured in K⋅m2⋅W−1.

The heat transfer coefficient is also known as thermal admittance in the sense that the material may be seen as admitting heat to flow.