Sample thickness in ASTM C518 thermal conductivity testing

In thermal conductivity testing according to ASTM C518, a test sample is clamped between two plates and compressed to certain thickness.

Some customers are concerned if thermal conductivity results are affected by the compression. This article aims to provide some explanations to this concern.

3 thicknesses

Conceptually, there are 3 thicknesses for a sample:

  • Uncompressed thickness: the thickness of a sample in the free state without compression;
  • Installation thickness: the thickness of a sample in the intended installation. In an installation, the sample may or may not be compressed;
  • Testing thickness: the thickness of a sample during thermal conductivity testing. During testing, a sample is always compressed for good thermal contact.

Among the 3 thicknesses:

  • The installation thickness could be the same as the uncompressed thickness (if the sample is not compressed in an installation), or smaller than the uncompressed thickness (if the sample is compressed in an installation).
  • The testing thickness is always smaller than the uncompressed thickness, as a sample is always compressed during testing.
  • The testing thickness should be as close to the installation thickness as possible, for fair product performance rating.

Testing thickness and thermal conductivity

When a sample is compressed to a smaller thickness, its density increases and the increased density affects the thermal conductivity measurement result.

A sensitivity study was performed by OTM in 2020. In the study, when the sample was compressed by 10%, the result variation was less than 1.7%. The thermal conductivity measurement result is not so sensitive to the testing thickness variation. If the compression is small (e.g. less than 5%), the result variation is negligible for general engineering applications.

Determination of testing thickness

The heat flow meter used by OTM supports two thickness control modes:

  • Automatic thickness: a sample is compressed by the instrument with a constant pressure of approximately 2.5 kPa and the sample thickness under compression is automatically measured as the testing thickness.
  • Manual thickness: a thickness is input manually and the sample is compressed to the manually input thickness as the testing thickness (provided that the sample can be compressed to this thickness with less than 2.5 kPa of pressure).

In practice, there are two scenarios:

  • Rigid or firm materials
  • Soft materials

Testing thickness of rigid or firm materials

For rigid materials (e.g. polystyrene foam or polyurethane foam) or firm materials (e.g. high-density rockwool), they cannot be compressed significantly in nomral installations (e.g. more than 5% of compression).

The testing thickness of a rigid or firm material is determined with the automatic thickness mode mentioned above.

Testing thickness of soft materials

For soft materials (e.g. low-density rockwool or glasswool), they can be compressed significantly in normal installations (e.g. more than 10% of compresssion).

The testing thickness of a soft material is determined with the manual thickness mode mentioned above.

The customer needs to declare the installation thickness of a soft material sample. The declared installation thickness will be used as the testing thicknes.

If an installation thickness is not declared, we will assume that the installation thickness is the same as the sample nominal thickness. If the installation thickness is the same as the uncompressed thickness, the sample may be compressed by up to 5% for good thermal contact.

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