Heat Flow Meter (HFM) and Guarded Hot Plate (GHP) Methods As Thermal Conductivity Measurements of Concrete

Introduction – HFM and GHP Techniques and Related Standards

HFM and GHP instruments can be used to measure the thermal performance of rigid and higher thermal conductivity (lower thermal resistance) masonry products such as concrete, gypsum board, stone, lumber. HFM 436/3 Lambda and GHP 456 Titan from Netzsch are shown as examples in Figures 1 and 2.

HFM 436/3 Lambda

Figure 1. HFM 436/3 Lambda

GHP 456 Titan

Figure 2. GHP 456 Titan

These techniques are standardized test methods and the following standards outline the test procedures:

  • DIN EN 12667/12939:2001: Thermal performance of building materials and products – Outlines the procedure to determine thermal resistance using HFM and GHP methods for (thick) products of high and medium thermal resistance
  • ASTM C177 - Standard test method to measure steady-state heat flux and thermal transmission properties using the GHP apparatus
  • ASTM C518 - Standard test method to measure steady-state heat flux and thermal transmission properties using the HFM apparatus
  • ISO 8302:1991: Thermal insulation – Test method to determine steady-state thermal resistance and related properties by means of the GHP apparatus
  • ISO 8301:1991: Thermal insulation - Test method to determine steady-state thermal resistance and related properties by means of the HFM apparatus

It is possible to achieve an accuracy of ±2% using the absolute GHP method. The instrument needs to be calibrated as in the case of the HFM method, which can achieve accuracies of ±2% depending on the reference material.

Method to Handle Rigid Samples Having Rough Surfaces

For both HFM and GHP methods, careful sample preparation and special techniques may be required to obtain accurate surface temperature measurements. The afore mentioned building materials may have rough surfaces and therefore preparation of samples with highly flat and parallel surfaces is a challenging task. This may result in a considerable interface thermal resistance (temperature drop) in any air gaps between the sample surfaces and instrument plates. If this thermal resistance is higher than the thermal resistance of the sample, then temperature sensors mounted in the plate surface become ineffective to measure the temperature difference throughout the sample. This issue can be addressed by mounting additional small diameter thermocouples on the surface of the sample and put a compliant interface sheet such as silicone rubber in between the sample surfaces and the instrument plates as illustrated in Figure 3.

HFM and GHP arrangement for sample surface thermocouples

Figure 3. HFM and GHP arrangement for sample surface thermocouples

Test Procedure

In this example, three pairs of concrete samples (305 mm by 305 mm by roughly 50 mm thickness) were analyzed with the GHP technique (double-sided), following by testing of individual samples by means of the HFM method. Silicone rubber interface sheets with a thickness of roughly 2 mm and sample surface mounted thermocouples were used for each method.

The calibration of the HFM 436 was performed with the NIST 1450b (Standard Reference Material) fiberglass board of 25 mm thickness. The temperature was measured by plugging the sample thermocouples into the data acquisition channels utilized for the plate thermocouples, followed by adjusting the plate temperatures to obtain the specified sample temperature difference during the test using the automatic offset adjustment in the software. The equilibrium parameters were set to 1% (rough) and 0.1% (fine).

The tests were performed at room temperature (mean sample temperature). The temperature difference between the two HFM plates was roughly 18 K with 8 K temperature difference across the sample. For the GHP, plate temperature difference was roughly 26 K with 12 K across the sample.

Test Results

The results of thermal conductivity measurements of concrete by HFM and GHP are shown in Table 1. For the higher density concrete sample C, the thermal conductivity value was 1.8 W/(m•K), which is much higher than the value of 1.2 -1.3 W/(m•K) for A and B, as anticipated. The two methods are in good agreement, especially with regards to the low thermal resistance of the samples and imperfect surfaces. The average of the thermal conductivity determined by the HFM method for the individual samples ranges from 2.4% higher to 4.1% lower when compared to the measurements obtained by the GHP method for both samples.

Table 1. Thermal conductivity measurements of concrete by GHP and HFM

Sample Thickness (mm) Density (Kg/m3) Mean temperature (°C) Thermal conductivity (W/(m.K)) Thermal resistance (m.K/W)
A1, A2 (GHP) 52.6 1896 24.1 1.36 0.0387
A1 (HFM) 53.6 1897 23.9 1.38 0.0387
A2 (HFM) 51.6 1895 23.9 1.23 0.0421
A1, A2 (avg., HFM) 52.6 1896 23.9 1.31 0.0404
Variation -4.0%

B1, B2 (GHP) 51.1 1909 25.0 1.27 0.0402
B1 (HFM) 51.1 1935 23.9 1.23 0.0416
B2 (HFM) 51.0 1882 24.1 1.21 0.0423
B1, B2 (avg., HFM) 51.1 1909 24.0 1.22 0.0419
Variation -4.1%

C1, C2 (GHP) 51.4 2297 25.2 1.76 0.0292
C1 (HFM) 51.7 2298 23.4 1.92 0.0269
C2 (HFM) 51.1 2296 23.8 1.69 0.0303
C1, C2 (avg., HFM) 51.4 2297 23.6 1.80 0.0286
Variation 2.4%

Conclusion

From the results, it is evident that both the relative HFM and the absolute GHP methods are qualified for determination of the thermal resistance and thermal conductivity of rigid and higher thermal conductivity (>1 W/(m•K)) building materials and even masonry products with rough surfaces. Moreover, the use of additional thermocouples and compliant sheets between the sample and plates enabled achieving accurate surface temperature measurements. The narrow difference between the HFM and GHP test results demonstrates the high performance capability of both techniques.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.

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