Design and Function of the Guarded Hot Plate Apparatus

The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) is a test tool based on an Embedded-PC for absolute value measurements on guarded hot plate method according to ISO 8302, DIN EN 1946-2, DIN EN 12667 and ASTM C177 (DIN 52612). The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) measures the sample thickness d of the inserted sample, the temperature difference ΔT over the sample and the heat flux Q which is equivalent to the electrical power of the measuring heating P (= U ⋅ I). The thermal conductivity is determined based on the defined measurement area A and the one-dimensional thermal conduction as follows:

It is not a heat flow meter and therefore it offers a higher accuracy induced by principle than the most other offered test tools. It is designed for long-term use and does not require calibration even after years.

The use of modern technologies for designing the sensor plates enables to go without a measurement chamber environment, which is standard otherwise. It is therefore easier to handle.

The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) does not require a thermo stationary controlled measurement environment. It must be plugged into the regular power supply and can be used in any room or office.

It makes very little noise (like a PC) and dissipates very little heat. The advanced control algorithm of the test tool calculates for each measurement the optimal measurement parameters and reduces the measurement time to a minimum.

The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) is a compact desktop tool.

Image 1 - Design λ-Meter EP500e

The lower and middle components contain the sensor plates. They are built up concentric. Centrepiece of these plates are computer optimated aluminium units, 40 mm in thickness, required for the maintenance of an isothermal temperature. Air-cooled high-performance Peltier modules are responsible for bringing the temperature of the sensor plates to any temperature between -15°C and 65°C so that thermal conductivity measurements within the temperature range of 10°C and 40 °C at a temperature difference of the sensor plates of 5 K and 15 K can be made. Contrary to other conventional test tools, the temperature measurements on both sample surfaces is done not spotty with thermocouples but cover integrally the entire measurement surface. It ensured such a high measurement accuracy also for inhomogeneous samples.

Ideal specimens are sheets of 500 x 500 mm² in size. The actual testing area of the tool is located right in the centre of the specimen. Its size depends on the type of tool (e.g. 200 x 200 mm²). For a specimen 500 x 500 mm² in size the tool will measure the thermal conductivity that applies to the central area. The adjacent outer material will not be considered for the test result. It is however necessary to have an outer layer. It is needed to bring about thermal conditions that will ensure a one-dimensional, stationary temperature field.

The upper component contains the whole electronic modules, the electrical hoist cylinder for driving the middle component, the measuring instrument for sample thickness and test pressure as well as all display and control units. Handling of the tool is surprisingly convenient and simple. The electric hoist mechanism is managed by the two pushbuttons on the front plate. It will lift the upper sensor plate so that the sample can be inserted. To allow easy insertion, the test tool is sidewise opened. During lowering of the upper sensor plate the hoist mechanism will slow down to a crawl mode – occurred by bottom out of the little pin respectively the coupled light barrier on the lower side of the upper sensor plate. This mode provides higher precision and prevents possible damage. The mechanism is automatically cut-off if the desired test pressure is reached. Measurement of the sample's thickness complies with standards DIN 18164 and DIN 18165 which stipulate the requirement of a specific surface pressure (ranging from 0.05 to 2.5 kN/m² resp. 50 … 2500 Pa) exerted on the sample.

Design and Functions of Sensor Plates - Thermal Conditions within the Specimen by a λ10-test

A single-sample apparatus must have an additional heating plate, a so-called counter heater, situated above the upper sensor plate. This counter heater provides a thermal barrier and makes sure that all heat will dissipate into the sample. The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) also includes a highly sensitive heat flux plate used for detection of possible heat convection between heating plate and counter heater. Precise thermal sealing can be achieved, this would not be possible with thermocouples. Deflection of transverse heat between the measurement zone and the protective heating ring is highly advanced and managed as follows: The protective heating ring is not controlled by measurement of temperature difference with only a few thermocouples, but more than 100 thermocouples forming a chain are evenly spread over the gap between the measurement zone and protective heating ring. They can detect even the smallest transverse heat flows. The protective ring will then compensate those unwanted heat flows. It is surrounded by another protective heating ring and again by a cooling resp. tempering ring. Heat flows between the two rings are measured by thermocouple chains (heat flow sensors). The absolute temperature will be measured at various locations. The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) will calculate from these data, including the programmed test regime, the sample's thickness and room temperature, the temperature field in the sample and also the control variables for the protective heating rings and the cooling resp. tempering ring.

Image 2 - conventional temperature field of a guarded hot plate

The principle figures shown below (Image 3 and Image 4) shall illustrate the thermal conditions within a measured sample for a λ10-measurement of a 120 mm sample taken with two different conductivity measurement tools both according to ISO 8302. The first figure (Image 3) shows the pattern for a conventional guarded hot plate apparatus according to ISO 8302 para. 2.1.3 figure 5 a. The second figure (Image 4) illustrate the temperature field which is induced within the present Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus). It would be the same field, which is induced, if the design according to ISO 8302 para 2.1.3 figure 5 c isupgraded with an additional heating and cooling ring.

An even, one-dimensional and stationary temperature field only exists if the temperature in the lateral zones equals the sample's mean temperature (see Image 3). The another set-up (Image 4) where additional heating and cooling rings on either side of the sample provide a heat barrier and creates such a temperature field that is independent of the lateral conditions (i.e. room temperature) one-dimensional and stationary. 

Image 3 - exemplary temperature field of λ-Meter EP500e

For the presented device construction of the Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) in accordance to ISO 8302, para 2.1.3., fig. 5-c. during a λ10-measurement, the test tool cools down the border area of the sample from both sides in this way that the higher border front surface temperatures (room temperature) will not permeate the inside of the sample. Within the specified ranges of measurement temperature and sample thickness for the test tool, an exact one-dimensional, stationary temperature field will be built up for the range of the measuring heating and protective heating zone. A thermostatic measuring chamber is not necessary for it!

The additional cooling ring functions like a “humidity sink”. Air humidity and eventual humidity in the border area of the sample shift towards the cooling ring and has in this way no influence to the measurement result. Additionally the measurement time will be decrease – both an more decisive advantage in comparison with conventional guarded hot plate apparatus.

An intelligent control mechanism can determine the ideal parameters for a measurement and reduce runtimes. The Thermal Conductivity Test Tool λ-Meter EP500e (guarded hot plate apparatus) can be used in any room and does not require a constant temperature environment.