Thermal Conductivity Testing Insulation - Traditional Steady State & Accelerated Transient Methods
Thermal insulation materials function as a barrier to minimize heat loss, and have applications across a wide range of fields, including construction, aerospace, packaging and more. As such, a critical property of these types of materials are their relatively low thermal conductivities (k). This performance for insulation products is often expressed in its reciprocal form R-Value or Thermal Resistance, which is dependant on material thickness. For a detailed explanation of the reciprocal relationship between R-Value and Thermal Conductivity click here.
As thermal conductivity is such critical performance characteristic of any insulation product, there is the need to periodically test the performance of such materials in manufacturing as well as in the initial development and qualification process of the product. Low k materials (or materials with a high thermal resistance) can take on a variety of forms such as thick batt-type (i.e. fiberglass bundles), thin sheets (i.e. ultra-thin aerogel rolls) or even loose-fill, unconsolidated solids (i.e. blown cellulose). Their form is highly dependent on their application, method of use and size requirements.
Traditionally, industry has standardized around characterizing the performance of insulation products with steady state methods such as ASTM C518 Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. Steady state thermal analysis (i.e. Heat Flow Meter (HFM)) is widely considered as the gold standard method for determining material thermal properties such as thermal conductivity (k). C-Therm’s HFM offers industry a reliable, high-precision, durable tool for thermal conductivity characterization that is very easy to use. All commercially available heat flow meters do however have the same noteworthy limitations in that they tend to take much longer to measure thermal conductivity compared to transient methods - typically a minimum sampling time of 30 minutes vs seconds. Also, the samples for an HFM must comply with the dimensional requirements of the plates in the system. Transient methods, such as C-Therm’s Modified Transient Plane Source (MTPS), have been shown to provide a complimentary capability with high quality measurements in a fraction of the time of classical steady state methods while also enabling the characterization or testing of more variable-shaped samples. This opens up potential to test samples more quickly in development, and assists in addressing a significant bottleneck in manufacturing.
Are MTPS measurements as accurate as steady state measurements? The performance is very similar. Chart 1 shows a comparison between MTPS and HFM results on a series of foam materials. Both methods provided results within (2%) of the expected value, however the MTPS accomplished 5 full measurements in (~5 mins), whereas the HFM required (>1 hour) for the same. While both methods are clearly well-suited for thermal conductivity analysis of foam samples, the MTPS offers a comparable level of measurement performance.
Chart 1: Comparing MTPS and HFM measurements of a variety of foam and insulative materials. These types of materials have applications towards aerospace, home insultation and factory applications.
In quality control and R&D environments, speed of results can be a significant factor in selecting the preferred test method. As seen from the above results, thermal conductivities were within 1-2% of the expected value on the manufacturers datasheet, suggesting that both methods are highly accurate for determining thermal conductivity. Specifically, in the case of the NIST EPS sample (Chart 2) (provided from the National Institute of Standards and Technology), the expected value at RT was 0.0333 W/mK. MTPS measurements resulted in a thermal conductivity value of 0.0338 W/mK (1.5% difference) while HFM measurements resulted in a value of 0.0335 W/mK (0.6% difference). While the HFM does achieve a more accurate result, MTPS measurements are within 1% of the HFM measurements and achieve these results 12.5 times faster.
Chart 2: Comparing the test time for both MTPS and HFM measurements on NIST EPS.
Overall, heat flow meters offer the best accuracy and are referenced within industry as the consensus standard. However, transient methods such as C-Therm’s MTPS method offer a complimentary capability in assisting with the accelerating characterization and data collection. Furthermore, the MTPS method offers a much more dynamic range of materials it can test beyond insulation – suitable for testing also liquids, powders, pastes and higher conductivity solids.
For more information on the C-Therm’s portfolio of tools for thermal conductivity characterization of insulation materials please visit: https://ctherm.com/ or contact us directly at: (506) 457-1515.