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.

Solar spectra in ASTM E903 solar reflectance calculations

Various solar spectra (i.e. solar spectral irradiance distributions) could be used in solar reflectance (of course, solar transmittance and absorptance too) calculations according to ASTM E903, as listed below:

  • ASTM E891:
    • Default solar spectrum used by the building industry and by OTM in SRI testing.
    • It is the default solar spectrum due to historical reasons and for compatibility with existing practices, data and instruments.
    • ASTM E891 is withdrawn and the same data set is available in ISO 9845-1.
  • ASTM E892:
    • Default solar spectrum for solar reflectance testing of non-roofing materials, according to LEED (note: solar reflectance only, not SRI)
    • Similarly, ASTM E892 is withdrawn and the data set is availble in ISO 9845-1.
    • ASTM E891 and E892 are different. ASTM E891 is in terms of direct normal solar radiation (no diffuse radiation); ASTM E892 is in terms of hemispherical solar radiation (with diffuse radiation).
  • ASTM G173:
    • Default solar spectrum included in ASTM E903, including both direct normal solar spectrum (equivalent to ASTM E891) and hemispherical solar spectrum (equivalent to ASTM E892)
    • It is not often used in the buidling industry, due to the historical reasons stated under ASTM E891.
  • ASTM E490:
    • Used in extraterrestrial applications, with AM0.
    • All other spectra mentioned above are with AM1.5 (refer to this article on air mass).

Surface temperature calculation model in SRI calculation (ASTM E1980)

As discussed in What is SRI, solar reflectance index (SRI) can be understood as the surface temperature on a 0 – 100 scale. We have an online SRI calculator, which calculates the surface temperatures and SRIs under 3 standard conditions. This post aims to explain the surface temperature calculation model in SRI calculation according to ASTM E1980.

Surface temperature calculation model

Below is the surface temperature calculation model in ASTM E1980.

The absorbed solar radiation (the term on the left hand side) is splitted to 3 components:

  • Low-wave radiation to sky (the first term on the right hand side)
  • Convection to air (the second term on the right hand side)
  • Conduction to materials beneath the surface (ignored)

For the quantities in the equation below:

  • Solar absorptance (α) and emissivity (ε) are material properties (tested in the laboratory)
  • Solar flux (I), sky temperature (Tsky), convective coefficient (hc), air temperature (Ta) are the standard conditions defined in ASTM E1980
  • Stefan Boltzman constant (σ) is a constant [5.66961 × 10-8 W/(m2K4)]
  • Surface temperature (Ts) is the unknown to be solved

The equation can be solved iteratively to get the surface temperature. ASTM E1980 also provides an alternative solution to the equation.

Standard conditions

For the standard conditions, the following are defined in ASTM E1980:

  • Solar flux (I): 1000 W/m2
  • Sky temperature (Tsky): 300 K
  • Air temperature (Ta): 310 K
  • Convective coefficient (hc): 5, 12, or 30 W/(m2K), corresponding to low-, medium- and high-wind conditions

Delta Ohm LR35 LoRaWan wireless data logging system

A LoRaWan based wireless data logging system was recently deployed by OTM. The system consists of:

The system was fully integrated and configured by OTM. OTM will also provide 1.5 years of maintenance and data collection services.

The instruments are distributed in various locations of two buildings. Shown below are two important instruments of the system (left: a LoRaWan data logger; right: the LoRaWan gateway).

LR35WH (outdoor type general-purpose data logger)
LoRaWan Gateway (indoor type, with 4G connectivity)

On-site measurement of glass color uniformity

Glass color uniformity is important to building facade aesthetics. It is possible to instrumentally measure the glass color uniformity on-site with a handheld spectrophotometer (a type of instrument for color measurement).

The principle

There are two steps in measuring glass color uniformity:

  1. Determination a reference color
  2. Determination of color differences between the sample glasses and the reference color

Reference color

The reference color cannot be defined numerically (refer to the inter-instrument error section for the reasons) and it must be defined physically. There are two possible ways:

  • A phyical glass sample (i.e. a control glass provided by the client)
  • A group of glasses identified on-site (e.g. 10 installed glass selected by the client)

For the second option, the average color of the selected glasses is used as the reference color. According to ASTM C1376, a minimum of 10 glasses are required. The second option is often used in practice, as it is more convenient to use installed glasses as the reference.

Color difference between a sample glass and the reference color

In the CIELAB color space, a color is expressed as 3 values: L*, a and b:

  • L*: represents the lightness of a color, with the range 0 – 100 (0: black; 100: white)
  • a: represents the position of a color between red and green (a > 0: redish; a < 0: greenish), with the typical range -128 – 127
  • b: represents the position of a color between yellow and blue (b > 0: yellowish; b < 0: bluish), with the typical range -128 – 127

The color difference, ΔE, between the reference color, L*abref, and a specific sample color, L*absample, is calculated as:

If the color difference (ΔE) is smaller than the criteria, the sample glass color is close to the reference color, i.e. in good color uniformity; otherwise, the color uniformity is poor. The criteria defined in ASTM C1376 is ΔE < 4.0.

Inter-instrument error

The glass color measurement instrument (typically a handheld spectrophotometer) measures the color in CIELAB color space.

Due to the wide range of CIELAB color space (typical ranges: L*: 0 – 100; a: -128 – 127; b: -128 – 127), ordinary color measurement instruments cannot measure colors with sufficient accuracy.

In contrast, the range of color difference (ΔE) is small (typical range: ΔE < 10, as larger color differences can be easily perceived by human eyes). Most color measurements can measure color differences with sufficient accuracy (e.g. better than ±0.2).

In order to achieve satisfactory measurement accuracy of color difference (ΔE), the same instrument shall be used in both reference color and sample color measurement. If the reference color and sample color are measured by different instruments, the inter-instrument error makes the color difference (ΔE) results very unreliable. This is the reason that, in the reference color section, physical colors need to be used as the reference color.

All-in-one weather station for weather and air quality monitoring

Shown below is an all-in-one weather station supplied by OTM for weather and air quality monitoring.

The system consists of the following components:

The system is fully integrated and completely independent. The data are uploaded to Delta Ohm cloud every 2 minutes. OTM also provides 2-year maintenance services to the instruments.