Absolute Measurement of Thermal Conductivity –
7 Things that Make for a Good Guarded Hot Plate

Insulating materials are becoming increasingly important in a variety of applications including the insulation of buildings. Better insulation lowers energy consumption and thus the heating costs for any individual household or industrial company. This also reduces CO2 emissions – an essential contribution toward keeping global warming in check.

The temperature-dependent thermal conductivity, λ, is the key parameter since this material property determines the energy flow through the insulation. By means of the NETZSCH GHP 456 Titan® Guarded Hot Plate apparatus (see figure 1), the entire thermal resistance of large, relatively thick samples can be easily determined, yielding a representative value of the thermal conductivity.

1) Absolute Method

The big advantage of the GHP method is that it is an absolute method; i.e., no calibration or correction is required at all. The thermal conductivity values result, in the stationary state, simply from the entire heat output measured, Q, the average sample thickness, d, the measurement area, A, and the set and measured mean temperature gradient, ∆T, along the sample or the two samples, as the case may be (the factor 2 results for two samples):

λ
=(Qd)/(2A • ∆T)

2) Wide Temperature Range

Insulating materials can be applied in an extremely wide temperature range, e.g., as cryo-insulation or insulation of high-temperature furnaces. This is the reason why the NETZSCH GHP 456 Titan® is available in two versions: For the range from -160°C to 250°C and for the range from -160°C to 600°C. 

Fig. 1: NETZSCH GHP 456 Titan® in open position. A sample or two identical samples (not shown) are located between the hot plate (1) and guard ring (2), or between the lower (3) and upper cold plates (4), respectively. Additionally shown are the three-paFig. 1: NETZSCH GHP 456 Titan® in open position. A sample or two identical samples (not shown) are located between the hot plate (1) and guard ring (2), or between the lower (3) and upper cold plates (4), respectively. Additionally shown are the three-part surrounding furnace (5), insulation (6), feed-throughs (7), hoisting device (8) and gas connection (9).

3) Vacuum

The vacuum-tight design of the NETZSCH GHP 456 Titan® is a prerequisite for defined atmospheres at the samples’ site: normal conditions, dry air or inert, oxygen-free purge gas can therefore be applied. Moreover, it is also possible to measure in a vacuum under pressures of up to 10-4 mbar. All these possibilities are particularly interesting for porous or fibrous insulations since in these cases, the thermal conductivity of the atmosphere in the free sample volume represents a significant portion of the sample’s total effective thermal conductivity. 

4) Conformity to Standards

The setup and usage of a GHP apparatus are described in international standards such as ISO 8302 or ASTM C 177; for the high-temperature range, the technical specification DIN CNT/TS 15548-1 exists. The setup, dimensions and temperature sensors of the NETZSCH GHP 456 Titan® are based on these standards. Adherence to the accuracy levels specified by the standards for the thermal conductivity values is, of course, decisive. 

5) Ease-of-Use

In line with the simple measuring principle, operation of the NETZSCH GHP 456 Titan® is also very easy: The apparatus is opened and closed by means of an electronic hoisting device; in between, the operator inserts the sample(s) from the front. Measurement and the generation of a complete measurement report is all handled by the software. 

6) Robustness

The NETZSCH GHP 456 Titan® features robust mechanics and temperature stability – prerequisites for good reproducibility of the measurements. Maintenance requirements are relatively low. 

7) Measurement Accuracy

Does a GHP deliver correct thermal conductivity values? Are these values within the tolerances required by the standard? These important questions can only be answered by comparing measured data with trusted literature values [1]. In the field of insulating materials, certified reference values are available; e.g., for the materials NIST SRM 1450D (NIST = National Institute of Standards, USA) in the temperature range from 7°C to 67°C and IRMM440 (IRMM = Institute of Reference Materials and Measurement, Belgium) in the temperature range from -170°C to 50°C. The thermal conductivities of both materials can be measured with high accuracy by means of the NETZSCH GHP 456 Titan®. This is demonstrated for IRMM440 over a very wide temperature range (see figure 2). 

Above 67°C, i.e., in the entire high-temperature range, there are unfortunately no appropriate certified materials. However, insulating materials with sufficiently accurate published thermal conductivity values do exist: Figure 3 shows a comparison between the well-known VDI/keymark values for an expanded glass granulate (Liaver GmbH & Co. KG) and the measured values obtained by means of the NETZSCH GHP 456 Titan®. The agreement in the range from 50°C to 500°C is better than 3%. 

Fig 2: Thermal conductivity of IRMM440, measured with the NETZSCH GHP 456 Titan® in comparison with the values certified by IRMM (solid line). The dotted lines represent the extended uncertainty budget of the IRMM values (±5% below -10°C; ±1% above -1Fig 2: Thermal conductivity of IRMM440, measured with the NETZSCH GHP 456 Titan® in comparison with the values certified by IRMM (solid line). The dotted lines represent the extended uncertainty budget of the IRMM values (±5% below -10°C; ±1% above -10°C) while the error bars reflect the combined measurement uncertainties.
Fig. 3: Thermal conductivity of an expanded glass granulate measured by means of the NETZSCH GHP 456 Titan® in comparison with the published VDI/keymark values (solid line). The dotted lines represent the standard uncertainty of the VDI/keymark values (Fig. 3: Thermal conductivity of an expanded glass granulate measured by means of the NETZSCH GHP 456 Titan® in comparison with the published VDI/keymark values (solid line). The dotted lines represent the standard uncertainty of the VDI/keymark values (±3%) while the error bars reflect the combined standard measurement uncertainties.

Porous calcium silicate SilCal1100 (CALSITHERM Silikatbaustoffe GmbH) had also already been investigated in detail and round-robin results had been published. Of course, this material was also measured by means of the NETZSCH GHP 456 Titan® (see figure 4): The agreement with the round-robin values is approx. 1-2% at 100°C and approx. 5% at 600°C.

Along with the accuracy, also the combined uncertainty of each individual GHP measurement is an issue that generally depends on the measurement conditions (mean sample temperature, temperature gradient used) and the sample’s properties (thermal conductivity, thickness). As a result, optimization of the measurement parameters allows for minimization of the uncertainty and also for an increase in the accuracy to a certain extent.

Fig. 4: Thermal conductivity of SilCal1100, measured by means of the NETZSCH GHP 456 Titan® in comparison with the published round-robin values (solid line). The dotted lines represent the standard uncertainty of the round-robin values (±3% increasing uFig. 4: Thermal conductivity of SilCal1100, measured by means of the NETZSCH GHP 456 Titan® in comparison with the published round-robin values (solid line). The dotted lines represent the standard uncertainty of the round-robin values (±3% increasing up to ±7%) while the error bars reflect the combined standard measurement uncertainties.
Guarded Hot Plate GHP 456 Titan®Guarded Hot Plate GHP 456 Titan®

With the examples of IRMM440, expanded glass granulate and SilCal1100, it was demonstrated that the NETZSCH GHP 456 Titan® fullfills the accuracy requirements of ±2% at room temperature and ±5% across the entire temperature range, as stipulated by standard ISO 8302.

The GHP 456 Titan® is thus a high-performance instrument for the absolute measurement of thermal conductivity. Additionally, NETZSCH offers the certified standard materials NIST SRM 1450D and IRMM 440 as well as SilCal1100 (incl. works certificate).

[1] A. Schindler, G. Neumann, D. Stobitzer and S. Vidi, Accuracy of a guarded hot plate (GHP) in the temperature range between -160°C and 700°C, High Temperatures–High Pressures, Vol. 45, 2016, pp. 81-96