Conversions Factors for Thermal Conductivity Unit
In physics, thermal conductivity, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per kelvin-meter (W·K-1 ·m-1 , i.e. W/(K·m) or in IP units (Btu·hr−-1 ·ft−-1 ·F−-1 , i.e. Btu/(hr·ft⋅F). Multiplied by a temperature difference (in kelvins, K) and an area (in square meters, m2 ), and divided by a thickness (in meters, m), the thermal conductivity predicts the rate of energy loss (in watts, W) through a piece of material. In the window building industry "thermal conductivity" is expressed as the U-Factor, which measures the rate of heat transfer and tells you how well the window insulates. U-factor values are generally recorded in IP units (Btu/(hr·ft⋅F)) and usually range from 0.15 to 1.25. The lower the U-factor, the better the window insulates. The reciprocal of thermal conductivity is thermal resistivity.
There are a number of ways to measure thermal conductivity. Each of these is suitable for a limited range of materials, depending on the thermal properties and the medium temperature. There is a distinction between steady-state and transient techniques. In general, steady-state techniques are useful when the temperature of the material does not change with time. This makes the signal analysis straightforward (steady state implies constant signals). The disadvantage is that a well-engineered experimental setup is usually needed. The Divided Bar (various types) is the most common device used for consolidated rock samples.
The transient techniques perform a measurement during the process of heating up. Their advantage is quicker measurements. Transient methods are usually carried out by needle probes.
CRC Handbook of Chemistry and Physics, NIST, ASME