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1、BRITISH STANDARD BS 874-2.2: 1988 Methods for Determining thermal insulating properties Part 2: Tests for thermal conductivity and related properties Section 2.2 Unguarded hot-plate method UDC 536.212.3.001.4 Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33
2、:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 This British Standard, having been prepared under the direction of the Refrigeration, Heating and Air Conditioning Standards Committee, was published under the authority of the Board of BSI and comes into effect on 30 September 1988 BSI
3、08-1999 First published September 1939 First revision August 1956 Second revision November 1965 Third revision November 1973 First Part revision (as 874-2.2) September 1988 The following BSI references relate to the work on this standard: Committee reference RHE/9 Draft for comment 86/76700 DC ISBN
4、0 580 16565 5 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Refrigeration, Heating and Air Conditioning Standards Committee (RHE/-) to Technical Committee RHE/9, upon which the following bodies were represented: British Ceramic Researc
5、h Ltd British Gas Plc Chartered Institution of Building Services Engineers Combustion Engineering Association Cranfield Institute of Technology Department of Health and Social Security Department of the Environment (Building Research Establishment) Department of Trade and Industry (National Physical
6、 Laboratory) Electricity Supply Industry in England and Wales Engineering Equipment and Materials Users Association Eurisol (UK) Association of Manufacturers of Mineral Insulation Fibres Gypsum Products Development Association Institution of Gas Engineers Phenolic Foam Manufacturers Association Refr
7、igeration Industry Board Royal Institute of British Architects Structural Insulation Association Thermal Insulation Manufacturers and Suppliers Association (TIMSA) Thermal Insulations Contractors Association Water-tube Boilermakers Association The following bodies were also represented in the drafti
8、ng of the standard, through subcommittees and panels: Aggregate Concrete Block Association Association of Lightweight Aggregate Manufacturers Autoclaved Aerated Concrete Products Association British Board of Agrment British Precast Concrete Federation Ltd. Cement and Concrete Association Department
9、of Trade and Industry (National Engineering Laboratory) Flat Glass Manufacturers Association Fulmer Yarsley Ltd. Institute of Refrigeration Institution of Chemical Engineers University of Salford Coopted member Amendments issued since publication Amd. No.Date of issueComments Licensed Copy: London S
10、outh Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 BSI 08-1999i Contents Page Committees responsibleInside front cover Forewordii 0Introduction1 1Scope1 2Definitions1 3Principle1 4Laboratory environment2 5Apparatus2 6Tes
11、t specimens6 7Procedure7 8Edge-loss correction8 9Expression of results9 10Test report10 Appendix A Description of composite materials11 Figure 1 Unguarded hot-plate apparatus2 Figure 2 Unguarded heater plate3 Figure 3 Alternative cold plate designs4 Figure 4 Typical thermocouple arrangements on spec
12、imen surfaces5 Publications referred toInside back cover Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 ii BSI 08-1999 Foreword This Section of BS 874 was prepared under the direction of the Re
13、frigeration, Heating and Air Conditioning Standards Committee. Previous editions of BS 874 summarized the methods available for determining thermal insulating properties, and indicated which methods were applicable to various materials intended for use in a variety of temperature ranges, but the met
14、hods themselves were not defined in detail. The growing importance of energy conservation and the need to have reliable information about the insulating properties of materials required detailed specifications of the test methods. Accordingly, a complete revision of the standard was undertaken, with
15、 each test method being fully specified. The full revision of BS 874 will take several years to complete, and it has therefore been decided to issue each test method as a separate Section of BS 874 as and when they are completed, thus gradually replacing the 1973 edition. This Section constitutes a
16、revision of 4.2.2 of BS 874:1973 which it supersedes and which was deleted by amendment. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of
17、 itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 12, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will
18、 be indicated in the amendment table on the inside front cover. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 BSI 08-19991 0 Introduction This Section describes an unguarded hot-plate method f
19、or determining the steady-state thermal conductivity of homogeneous insulating solids of medium conductivity such as rubbers, dense plastics, glasses and masonry materials in the temperature range 20 C to 100 C. It is not an absolute method since a correction has to be applied for lateral heat flow
20、to, or from the specimen and heater plate edges, based on a calibration of the apparatus using specimens of known conductivity. Nevertheless, as the method is restricted to a limited thermal resistance/conductivity range over which this correction is relatively small, it produces results of comparab
21、le accuracy to the guarded hot-plate method described in BS 874-2.1. To facilitate calibration by direct exchange of specimens with Section 2.1, the present Section specifies the same basic plate dimensions as the guarded hot-plate apparatus, i.e. 305 mm square. Other essential dimensions, tolerance
22、s, and design features are detailed in the text and minimum performance requirements are specified for the associated electrical and temperature measuring instruments. The measurement procedures, test conditions and specimen preparation methods, especially for masonry materials, are described and th
23、e information required in test reports is specified. Although the precautions needed to ensure accuracy with different types of test material have been detailed in this Section, experience has shown that heat transfer measurements are far from straightforward and care should be taken to ensure that
24、the execution of its provisions is entrusted to appropriately qualified and trained people. 1 Scope This Section of BS 874 describes a method for determining the steady-state thermal resistance, conductance, conductivity and resistivity of homogeneous insulating solids in slab form from 20 C to 100
25、C using the 305 mm unguarded hot-plate apparatus. NOTE 1In the context of this Section, thermally homogeneous materials are materials whose measured thermal conductivity or resistivity, at any given temperature, is not affected by a change in the temperature gradient, or the thickness or area of the
26、 test specimens. Materials which contain a random distribution of inclusions or cavities whose dimensions are small relative to the specimen thickness are regarded as homogeneous, but layered structures that appreciably distort the heat flux distribution are not. The method is applicable over the fo
27、llowing, mutually inclusive, ranges, i.e. test specimens have to satisfy all four conditions a) to d): a) thermal resistance, R, between 0.025 m2 K/W and 0.33 m2K/W; b) thermal conductance, C = 1/R, between 3 W/(m2K) and 40 W/(m2K); c) thermal conductivity, 2, between 0.15 W/(mK) and 2 W/(mK); d) th
28、ermal resistivity, r = 1/2, between 0.5 mK/W and 6.7 mK/W. NOTE 2It is recommended that this Section be read in conjunction with BS 874-1. NOTE 3The titles of publications referred to in this standard are listed on the inside back cover. 2 Definitions For the purpose of this Section of BS 874, the d
29、efinitions given in BS 874-1 apply. 3 Principle The determination of thermal conductivity and related properties by this method involves the measurement of the mean temperature difference between the opposite faces of two identical, parallel-faced specimens at steady-state when a constant heat flux
30、density of known magnitude passes through them. The specimen surface temperatures are measured by means of thermocouples mounted on, or within, the surfaces and the heat flux density passing through them is determined from their effective area and the power supplied to the unguarded heater plate, co
31、rrected for a small lateral component of heat flow associated with heat losses or gains from the specimen and heater-plate edges. The magnitude of this correction for any given set of experimental conditions is evaluated from a pre-calibration of the apparatus involving measurements on reference spe
32、cimens of low thermal conductivity. Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 2 BSI 08-1999 With reference to Figure 1, two specimens of known thickness and area, having the specified numb
33、er of thermocouples mounted on their surfaces, are placed on either side of the heater plate and clamped in the horizontal plane between two isothermal cold plates. Good thermal contact between the specimen and plate surfaces is facilitated by placing thin sheets of compressible rubber between them.
34、 These also serve to insulate the thermocouples from the metal plates. To minimize heat exchange with the environment, the assembly is surrounded by a thick layer of insulation housed in a container which can be sealed when measurements are required with cold plate temperatures near to, or lower tha
35、n the dew-point of the ambient air. With the cold plates maintained at an appropriate constant temperature by fluid circulation from a thermostatically controlled bath, stabilized electrical power is supplied to the heater plate to establish, at steady- state, a suitable temperature difference at co
36、nstant heat flow rate through the specimens. The power supplied to the heater plate is measured potentiometrically using precision instruments of high input impedance. A small correction is applied to compensate for the edge heat losses or gains appropriate to the conditions of test as described in
37、clause 8. The average temperature difference across the specimens at steady-state is determined from the thermocouple e.m.fs. The requisite thermal property is evaluated from these measured values using the equations given in clause 9. 4 Laboratory environment The ambient air temperature of the labo
38、ratory shall be maintained constant to within 2 C and the relative humidity between 25 % and 65 % during the test. NOTECloser control of the laboratory relative humidity or the use of an environmental chamber may be required to comply with the conditioning requirements specified for some materials.
39、See also 5.4 and 6.5.1. 5 Apparatus 5.1 Heater plate The heater plate shall be a double-sided unit consisting of a heating element securely sandwiched between two metal surface plates of high conductivity metal, such as copper or aluminium, some 3 mm to 5 mm thick. The plate shall be nominally 305 m
40、m square and its working surfaces shall be flat within 0.1 mm. NOTE 1Guidance on the determination of flatness may be sought from BS 817. The heater element shall be uniformly wound and be embedded in heat-resisting, electrically insulating material within the surface plates to ensure good thermal c
41、ontact. The windings shall be symmetrical with respect to the two working surfaces of the heater plate and shall be of sufficiently small pitch to ensure uniform surface temperature on both sides (see Figure 2). The resistance between the heater elements and plates shall be greater than 1 M7. Figure
42、 1 Unguarded hot-plate apparatus Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 BSI 08-19993 The heater plate shall be supplied with stabilized electrical power, usually d.c., using equipment c
43、apable of holding the power dissipated in the plate constant to at least 0.1 % over the duration of a test. The uncertainty in the measurement of the power shall not exceed 0.25 %. NOTE 2In Figure 2, the heaters are wound with resistance tape on thin insulating sheets. Printed circuit techniques off
44、er an attractive alternative method of achieving uniform winding configuration. 5.2 Cold plates The cold plates shall be fabricated from heavy gauge, high conductivity metal such as copper or aluminium and shall have the same nominal dimensions as the heater plate, i.e. 305 mm square. Their working
45、surfaces shall be flat to within 0.1 mm. The plates shall be cooled by liquid flow. The temperature of the cooling liquid shall be thermostatically controlled, and the system shall be capable of holding the surface temperature of the cold plates constant to better than 0.1 C for the duration of the
46、test. NOTETo improve temperature uniformity, the liquid circuits in the plates should preferably be in the form of a double spiral which allows the liquid to circulate in opposite directions beneath the surface. Typical designs are shown in Figure 3. 5.3 Temperature measurements 5.3.1 General. Base-
47、metal thermocouples of diameter equal to or less than 0.2 mm shall be used to measure temperatures and temperature differences. They shall be fabricated from a stock of calibrated thermocouple wire or wire that has been certified by the supplier to comply with the internationally-accepted reference
48、tables given in BS 4937 to better than 0.4 %, or be individually calibrated. Thermocouple cold junctions shall be maintained at a constant uniform temperature either by immersion in a Dewar flask containing melting ice or by attachment to an isothermal block in a suitably designed reference chamber.
49、 Solid-state units shall be used to compensate for the junction temperature if it is different from 0 C. Figure 2 Unguarded heater plate Licensed Copy: London South Bank University, London South Bank University, Fri Dec 08 05:33:37 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS 874-2.2:1988 4 BSI 08-1999 A good quality voltmeter capable of resolving 1 4V, or better, shall be used to measure the output of the thermocouples. The uncertainty in the measurement of the temperature difference between the hot and cold faces of the test
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