Absolute vs Direct Thermal Conductivity
Absolute thermal conductivity measurements
Absolute thermal conductivity measurements are performed independent of thermal conductivity calibration materials. An absolute measurement method is considered non-comparative, which is accepted as the highest accuracy possible.
Direct or calibrated thermal conductivity
Direct or calibrated thermal conductivity measurements is also referred to as a comparative method. The comparative method uses a single or in some cases multiple calibration reference materials to cover a range of thermal conductivity. With a comparative method, the accuracy of a result is a composite value based on the accuracy of the absolute method to measure the reference materials as well as the comparative device itself.
The Transient Plane Source TPS is an absolute method often relied upon to test calibration materials that are used in calibrating comparative methods.
Transient Plane Source TPS and Comparative Methods
Transient Line Source (TLS): The TLS is an ASTM (D5334) method for testing soils and similar materials. This method requires a single or multi-point calibration with reference materials of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.
Transient Hot Wire (THW): The THW is an ASTM (D-7896-14) method for testing liquids. This method requires a single point calibration with a reference material of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.
Transient Hot Strip (THS): The THS method for testing solids. This method requires a single point calibration with a reference material of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.
Modified Transient Plane Source (MTPS): The MTPS method for testing solids and small particle powders. This method requires a multi-point calibration with reference materials of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.
Heat Flow Meter (HFM): The HFM is an ASTM method (C518) for testing low thermal conductivity material. This method requires a single-point calibration with reference materials of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS or Guarded Hot Plate (GHP).
Guarded Comparative Longitudinal Heat Flow (GCLHF): The GCLHF is an ASTM (E1225) method for testing ceramics, polymers and metals. This method requires a sinle-point calibration with reference materials of similar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.
Guarded Heat Flow Meter (GHFM): The GHP is an ASTM (E1530-11) for testing solids. This method requires a single point calibration with a reference material of similiar thermal conductivity tested by an absolute method like the Transient Plane Source TPS.