Sample thickness in ASTM C518 thermal conductivity testing

Related services Thermal conductivity

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.

Proficiency testing on insulation material thermal conductivity test

Related services Thermal conductivity

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/(mK)
  • Result tested by OTM: 0.0344 W/(mK), with Z-score = 0

OTM is SAC-SINGLAS (ISO 17025) accredited for insulation material thermal conductivity testing. We perform both internal and external quality assurance exercises regularly for consistent and reliable measurement accuracy.

Agar gel thermal conductivity testing

Related services Thermal conductivity

Shown below are the photos of agar gel thermal conductivity testing using a thermal needle, according to ASTM D5334. The size of the thermal needle is 1.6 mm in diameter and 12 cm in length.

The thermal conductivity of agar gel (with 5-gram agar powder per liter of water) is tested, as part of the quality control measures when we test soil or similar gel-like materials.

Needle probe in agar gel
Standalone needle probe

Paint and coating thermal conductivity testing with ASTM D5930

Related services Thermal conductivity

We’ve helped a few customers in determining the thermal conductivity of thin materials, such as paint and coating, according to ASTM D5930.

In general, the thermal resistance of thin materials is negligible. In case it is necessary to determine the thermal conductivity of paint and coating. A pair of special samples, with thick paint or coating, need to be prepared, as illustrated below.

A sample with flat substrate and thick paint or coating
  • 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.

Arrangement of measurement probe and two test samples

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.

Thermal conductivity of thin materials (paint, coating, metal sheet)

Related services Thermal conductivity

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.

Thermal resistance of thin material

The reason is that thermal resistance is dependent on both thermal conductivity and thickness, with the following relationship:

Thermal resistance = Thickness / Thermal conductivity

In practice, due to the small thickness of thin materials (typically less than 1 mm), it is not practical to reduce the thermal conductivity of thin materials to achieve better insulation.

In wall/roof U-value calculations, the thin materials can be simply ignored. It is not meaningful to get the thermal conductivity of thin materials.

Thin material thermal conductivity testing

It may be still necessary to determine the thermal conductivity of thin materials. For example, the thin material is not used in a wall/roof system, but in a system with low thermal resistance.

For such scenarios, we can test the thermal conductivity of thin materials according to ASTM D5930, with the following practices:

  • For low thermal conductivity paint and coating, the paint/coating can be applied onto flat substrates for testing.
    • There is no specific requirement on the substrate type, as long as the substrate surface is flat. We recommend flat metal plates or glass plates.
    • The preferred substrate size is 50 mm x 50 mm (minimum size: 30 mm x 30 mm; maximum size: 100 mm x 100 mm). There is no requirement on the substrate thickness, as long as it is strong enough.
    • The thickness of paint or coating should be thick enough (3 mm or thicker preferred, 1 mm minimum)
    • 2 pieces of painted/coated samples are requried
  • We cannot test materials with large thermal conductivity [i.e. > 10 W/(m⋅K)], for example, aluminium or stainless steel.

Please refer to our thermal conductivity page for more details.

New release of lab profile 2020 edition

We are pleased to release our 2020 edition of laboratory profile, which is a 56-page comprehensive document, with rich information on our capabilities and many technical insights.

What are in the laboratory profile?

  • A comprehensive overview of the laboratory
  • Detailed introduction of 10 popular tests
  • 14 informative technical insights articles

You may download a soft copy of our lab profile.