This standard is divided into three sections, representing the most comprehensive assemblage of information-required to use the guarded hot plate apparatus, i.e.:
1 General;
2 Apparatus and Error Evaluation;
3 Test Procedures.
While the user of the method specified in this standard for test purposes may need to concentrate only on Clause 3, he must also be familiar with the other two sections in order to obtain accurate results. He must be particularly knowledgeable .about the general requirements. Clause 2 is directed towards the designer of the apparatus, but he also, in order to provide good apparatus, must be concerned with the other sections of this method. Thus, the method will serve its purpose well.
0.2 Heat transfer and measured properties
A large proportion of thermal- testing is undertaken on light density porous materials. In such cases, the actual heat transfer within them can involve a complex combination of different contributions of radiation, conduction both in the solid and gas phase, and convection (in some operating conditions), plus their interactions together with mass transfer,- especially in moist materials. For such materials, the heat transfer property, very often wrongly called “thermal conductivity”; calculated from a defined formula and the results of measurements of heat flow-rate, temperature difference and dimensions, for a specimen may be not an intrinsic property of the material itself. This property, in accordance with ISO 9288, should therefore be called “transfer factor” as it may depend on the -test conditions (the transfer factor is often referred to elsewhere as apparent or effective thermal conductivity). Transfer factor may have a significant dependence on the thickness of the specimen and/or on the temperature difference for the same mean test temperature.
Heat transfer by radiation is the first source of dependence of transfer factor on specimen thickness. As a consequence, not only material properties influence results, but also the radiative characteristics of the surfaces adjoining those of the specimen. Heat transfer by radiation also contributes to the dependence of transfer factor on temperature differences. This dependence can be experimentally detected for each type of material and for each mean test temperature when the temperature difference exceeds defined limits. Thermal resistance is therefore the property that better describes the thermal behaviour of the specimen, provided it is accompanied by information on the radiative characteristics of the adjoining surfaces. If there is the possibility of the onset of convection within the specimen (e.g. in light mineral wool for low temperatures), the apparatus orientation, the thickness and the temperature difference can influence both the transfer factor and the thermal resistance. In such cases, as a minimum it is required to fully specify the geometry and the boundary conditions of the specimen tested, even though information supplied in Clause 3 on test procedures does not cover these test conditions in detail. In addition, it will take considerable knowledge to evaluate the measurement, as such, especially when applying the measured values in practice.
The influence of moisture within a specimen on the heat transfer during a measurement is also a very complex matter. Therefore, dried specimens only shall be tested according to standard procedures. Measurements on moist materials need additional precautions not covered in detail in this standard.
The knowledge of the physical principles mentioned is also extremely important when a heat transfer property, determined by this test method, is used to predict the thermal behaviour of a specific material in a practical application even though other factors such as workmanship can influence this behaviour.
0.3 Background required
The design and subsequent correct operation of a guarded hot plate to obtain correct results and the interpretation of experimental results is a complex subject requiring great care. It is recommended that the designer, operator and the user of measured data of the guarded hot plate should have a thorough background of knowledge of heat transfer mechanism in the materials, products and systems being evaluated, coupled with experience of electrical and temperature measurements, particularly at low signal levels. Good laboratory practice in accordance with general test procedures should also be maintained.
The in-depth knowledge in each area mentioned may be different for the designer, operator and data user.
0.4 Design, size and national standards
Many different designs’ of guarded hot plate apparatus exist worldwide which conform to present national standards. Continuing research and development is in progress to improve the apparatus and measurement techniques. Thus, it is not practical to mandate a specific design or size of apparatus, especially as total requirements may vary quite widely.
