The generally accepted methods, described in standard EN12667 to measure the thermal conductivity are:
- The guarded hot plate method is an absolute method which does not need samples for calibration.
- The fluxmeter method on the other side needs a calibration with samples, measured on a guarded hot plate system.
The hot wire method is not accepted as a method to measure thermal insulation, while it is in fact a two dimensional method, measuring the thermal conductivity in a cylinder around the wire. The EN12667 methods are measuring in one direction because they give the thermal conductivity through a board. On top of that, the hot wire method is a transient method, which measures the heat diffusivity and thermal conductivity.
Although less accurate, the method is fast and needs only a small sample. A cylinder of 20 cm with a certain radius is already large enough to do comparative measurement. This is very interesting when testing new formulations for a foam in a laboratory environment. The above was extensively calculated in a paper about the hot wire method for low density cellular materials. It can be concluded from the paper that the method is reliable for cellular glass, because cellular glass is opaque.
In case a unidirectional thermal conductivity is needed, we can replace the hot wire probe by a surface probe, like for example used in the ISOMET of Applied Precison Ltd. This method can be applied by for example fast non-destructive QC in the ware house by internal and external inspectors. Today, standard QC measurements according EN12667 take easily a few hours up to 24 hours after sample preparation while the surface probe transient method is ready in less than 30 minutes without sample preparation. In a comparative mode of working, the method is reliable.
I would never trust transient methods if not tested on samples, measured with the EN12667 methods. But after testing and checking, they can be very helpful on a comparative basis.
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