ISO 8301:1991 断熱—定常状態の熱抵抗と関連する特性の決定—熱流量計装置 | ページ 3

Introduction

0.1 Document subdivision

This International Standard is divided into three sections representing the most comprehensive assembly of information required to use the heat flow meter apparatus:

Section 1: General considerations

Section 2: Apparatus and calibration

Section 3: Test procedures

While the user of the method may need to concentrate only on section 3 for test purposes, he must also be familiar with the other two in order to obtain accurate and precise results. He must be particularly knowledgeable about the general requirements. Section 2 is directed towards the constructor of the apparatus, but he also, in order to build good apparatus, must be familiar with the other sections.

0.2 Heat transfer and measured properties

A large number of tests are run 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 in the gas phase;
• convection (in some operating conditions);

plus their interaction, together with mass transfer, especially in moist materials. Therefore, the heat transfer property, very often improperly called"thermal conductivity", calculated from a defined formula and the results of measurements of heat transfer 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 the"transfer factor" as it may depend on the test conditions (the transfer factor is often referred to elsewhere as apparent or effective thermal conductivity). The 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 the transfer factor on specimen thickness. As a consequence, not only the material properties but also the radiative characteristics of the surfaces bounding the specimen influence results. Thermal resistance is therefore the property that better describes the thermal behaviour of the specimen, provided that it is accompanied by information on the bounding surfaces.

If there is any 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 that the geometry and the boundary conditions of the specimen tested be fully specified, even though information supplied in the 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. Dried specimens only therefore ought to be tested according to standard procedures. Measurements on moist materials need additional precautions not covered in detail in this International Standard.

The knowledge of the physical principles 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 heat flow meter (HFM) apparatus (see 1.6.1 and 2.2.2) 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 user of measured data of the HFM apparatus should have a thorough background of knowledge of heat transfer mechanisms 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 cited may be different for the designer, operator, and data user.

0.4 Design, size, and national standards

Many different designs of heat flow meter apparatus exist worldwide to 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.

0.5 Guidelines supplied

Considerable latitude both in the temperature range and in the geometry of the apparatus is given to the designer of new equipment since various forms have been found to give comparable results. It is recommended that designers of new apparatus carefully read the comprehensive literature cited in annex E. After completion of new apparatus it is recommended that it should be checked by undertaking tests on one or more of the various reference materials of different thermal resistance levels now available. This International Standard outlines only the mandatory requirements necessary to design and operate heat flow meter apparatus in order to provide correct results. A table summarizing limit values for the apparatus performance and testing conditions stated in this International Standard is supplied in annex A. It also includes recommended procedures and practices plus suggested specimen dimensions which together should enhance general measurement levels and assist in improving inter-laboratory comparison and collaborative measurement programmes.