Are you responsible for improving the thermal transport properties of thermal insulation? Are you working in R&D of polymers? Are you working in QA of insualtion materials and you collect thermal conductivity data all day long for CE-compliance? Are you working for a building material institute or certified laboratory – and are you dealing a different kinds of materials? Are you dealing with one kind of material all day long? Are you dealing with thermal conductivity for specifiying the specsheet of your material? Then maybe the measurement range of a heat flow meter is very important for you? The questions is:
There are many applications for a Heat Flow Meters – but is it the right measurment method for thermal conductivity for my material or product?
The simple answer is:
The measurement range of heat flow meters is not defined in “thermal conductivity” – it is defined in “thermal resistance”!
The heat flow meter method standard ISO 8301 has a clear imagination of the thermal resistance ranges, that can be measured. And it is quite easy to understand! To get a stable heat flow, which is the basis for a heat flow meter result, you need a certain kind of thermal resistance to maintain a temperature gradient between the hot and the cold plate! This is the main reason why heat flow meters are used for INSULATING MATERIALS. To calculate the thermal resistance, use this equation: This is, how the measurment ranges are defined in ISO 8301:
Thermal restistance of 0,5 (m²K)/W or higher (up to 8 (m²K)/W): Those are mostly low conductive, soft and foamy material. Those can be measured without any problems.
Thermal resistances between 0,3 and 0,5 (m²K)/W: Those are slightly harder materials with low thermal condictivity. This limit requires, that also those samples must be measured with the help of additional thermo-couples on the surfaces.
Thermal resistances between 0,1 and 0,3 (m²K)/W: Those are materials with a higher thermal conductivity and mostly harder surfaces. Also on those samples additional thermocouples are required.
Thermal restistances between 0,02 and 0,1 (m²K)/W: Materials can only be measured with additional thermocouples and results are determined with reduced accuracy.
Thermal resistances lower than 0,02 (m²K/W) are out of measurment range.
Here you see an example calulation based on glasswool with a thermal conductivity of 0,035 W/m/K – you can see, that not all samples from glasswool are suitable for a proper measurement:
But what can I do, if my sample has a lower thermal resistance than 0,5 (m²K)/W ?
For low conductive insulating materials:
Increase the thickness of your sample by stacking your samples! (e.g. for thin XPS-Sheets from 2mm thickness: Put 10-15 Sheets above each other. This makes thermal resistance 10 to 15 times higher and delivers reliable results)
Use the so called “blind specimen method”: Put your “unknown” sample to be measured between two samples with known thermal resistance. From the total thermal resistance minus the thermal resistance of the known samples, you can calculate the “unknown” thermal resistance. By knowing the thickness of the sample, you can calculate the thermal conductivity.
For higher conductive insulating materials:
Use additional thermocouples (= instrumentation kit) to optimize the temperature measurement on the contact surfaces.
Conclusion Heat Flow Meters are a perfect instrument to measure thermal conductivity on a wide range of materials. If you are not sure, if you can measure your samples with a heat flow meter, you need to get a feeling for your thermal resistance.