OTM participated in a proficiency testing (PT) program recently, with satisfactory results. A PT program is for test quality evaluation of laboratories. Typically, samples with known results are distributed by the PT organizer to a group of participating laboratories. The results tested by the laboratories are compared by the PT organizer to evaluate the test quality of each laboratory.
Below is a summary of the PT program on thermal conductivity testing and the performance of OTM in this program:
Number of participating laboratories: 84
Test type: insulation material thermal conductivity test
Sample: polystyrene board, at 25 °C mean temperature
Average results from all laboratories: 0.0344 W/(m⋅K)
Result tested by OTM: 0.0344 W/(m⋅K), with Z-score = 0
Substrate (the blue color part): the substrate needs to be flat and rigid. The substrate type does not affect the measurement result. Typical substrate types include glass, metal plate, and wood plate.
The preferred substrate size is 50 mm x 50 mm; The minimum size is 30 mm x 30 mm and the maximum size is 100 mm x 100 mm.
Thick paint or coating (the yellow color part): the paint or coating needs to be applied onto one side of the substrate, with large enough thickness
The preferred thickness is 3 mm or larger; the minimum thickness is 1 mm.
Sample quantity: two samples in a pair are needed.
Shown below is the arrangement during testing.
The measurement probe is a thin film (less than 0.1 mm in thickness, the red color part) with a tiny wire inside (refer to our thermal conductivity test page for details). The probe is sandwiched between the two test samples, next to the paint or coating material.
Because the probe is in contact with the paint or coating material and the measurement duration is very short, it is equivalent to insert a tiny wire into a bulk material block made of the paint or coating. Only the thermal conductivity of the paint or coating is measured and the result is not affected by the substrate material.
We are sometimes requested to test the thermal conductivity of thin materials, such as paint, coating and metal sheet.
Negligible thermal resistance of thin materials
For a wall (or a roof) system, the influence of such thin materials to the overall wall system U-value is negligible.
Shown below is an example calculated with our online ETTV U-value calculator. it is obvious that the thermal resistance of a 0.2 mm thick paint layer with 0.2 W/(m⋅K) thermal conductivity is only 0.001 (m2K)/W, which is negligible comparing to the thermal resistances of other layers (e.g. concrete, plaster, or insulation wool).
For thin metal sheets, e.g. 0.7 mm thick aluminium plates, the thermal resistance is further smaller, as the thermal conductivity of metal is much larger.
The reason is that thermal resistance is dependent on both thermal conductivity and thickness, with the following relationship: