1、BRITISH STANDARD BS 6993-2: 1990 Incorporating Amendment No. 1 Thermal and radiometric properties of glazing Part 2: Method for direct measurement of U-value (thermal transmittance)BS6993-2:1990 This British Standard, having been prepared under the directionof the Basic Data and Performance Criteria
2、 for Civil Engineering and Building Structures Standards Policy Committee, was published underthe authority of the BoardofBSI and comes into effect on 31 July1990 BSI 08-1999 The following BSI references relate to the work on this standard: Committee reference BDB/2 Draft for comment 88/13482 DC ISB
3、N 0 580 17934 6 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Basic Data and Performance Criteria for Civil Engineering and Building Structures Standards Policy Committee (BDB/-) to Technical Committee BDB/2, upon which the following b
4、odies were represented: Aggregate Concrete Block Association Department of the Environment (Construction Association for the Conservation of Energy Industries Directorate) Association of Building Component Department of the Environment for Northern Manufacturers Ltd. Ireland Association of County Co
5、uncils Electricity Supply Industry in England and Association of District Councils Wales Association of Metropolitan Authorities Engineering Equipment and Materials Users Autoclaved Aerated Concrete Products Association Association Eurisol UK Mineral Wool Association Brick Development Association Fl
6、at Glass Manufacturers Association British Board of Agrement Gypsum Products Development Association British Cement Association Hevac Association British Gas plc Incorporated Association of Architects and British Precast Concrete Federation Ltd. Surveyors Building Employers Confederation Institution
7、 of Civil Engineers Building Services Research and Information Institution of Gas Engineers Association Institution of Structural Engineers Chartered Institute of Building Lighting Industry Federation Ltd. Chartered Institution of Building Services Ministry of Defence Engineers National House-buildi
8、ng Council Concrete Society National Illumination Committee of Great Consumer Policy Committee of BSI Britain Cranfield Institute of Technology Royal Institute of British Architects Department of Education and Science Royal Institution of Chartered Surveyors Department of Energy (Energy Efficiency S
9、cottish Development Department Office) Timber Research and Development Association Department of Health Trades Union Congress Department of the Environment (Building Watt Committee on Energy Ltd. Research Establishment) Amendments issued since publication Amd. No. Date of issue Comments 8931 Decembe
10、r 1995 Indicated by a sideline in the marginBS6993-2:1990 BSI 08-1999 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 Symbols 1 4 Equipment 1 5 Conditions for test 2 6 Determination of measurements following calibration 2 Appendix A Values to be used
11、 in U-value calculations 4 Appendix B Worked example of correction to standardized total surface resistance 5 Figure 1 Location of test element in surround panel 5 Figure 2 Interpolation of total average surface resistances 7 Publications referred to Inside back coverBS6993-2:1990 ii BSI 08-1999 For
12、eword This Part of BS 6993 has been prepared under the direction of the Basic Data and Performance Criteria for Civil Engineering and Building Structures Standards Policy Committee. Part 1 of this Standard describes a method for the calculation of the steady-state U-value of glazing. This Part of BS
13、 6993 describes an alternative method, of equal status, to establish the U-value of the central part of the glazing by direct measurement. A further Part now being developed is: Part 3: Method for measurement of photometric and radiometric properties in the solar spectral region. This Part of BS 699
14、3 describes the methodology to obtain U-values by measurement, on a basis comparable to those obtained by calculation. It is based on BS874-3.1 which describes the construction and use of a hot-box apparatus for the measurement of U-values. A British Standard does not purport to include all the nece
15、ssary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages
16、1 to 8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS6993-2:1990 BSI 08-1999 1 1 Scope This Part of BS 6993 describes a method for measuri
17、ng the U-value (thermal transmittance) of the central area of single, double or multiple glazing using hot-box apparatus complying with BS874-3.1. The edge effect, due to the thermal bridge through the spacer of double or multiple glazing, is effectively eliminated in this method. The method is desi
18、gned to measure the U-value of the glazing alone, i.e.the glass (or alternative glazing material) and any enclosed air space, and the additional information required in the test reports of clause8 and A.9 of BS874-3.1:1987. Appendix A specifies values to be used in the calculation of a U-value in ac
19、cordance with BS6993-1, when such a calculated value is to be compared with that obtained by direct measurement in accordance with this Part of BS6993. Appendix A gives a worked example, which illustrates the use of this Part of BS 6993. NOTEThe titles of the publications referred to in this standar
20、d are listed on the inside back cover. 2 Definitions For the purpose of this Part of BS 6993 the following definitions apply. 2.1 thermal transmittance, U-value 1) the heat flux density through a given structure divided by the difference in environmental temperatures on either side of the structure
21、under steady-state conditions 2.2 environmental temperature, effective ambient temperature 1) the environmental temperature influencing a surface is a suitably weighted mean of the adjacent air (or other fluid) temperature and the mean radiant temperature of the surroundings, in order to indicate co
22、rrectly the change in heat flow rate to or from the surface for a small change in surface temperature. For a fluid opaque to radiation, the effective ambient temperature is the same as the fluid temperature 3 Symbols 4 Equipment 4.1 Apparatus The construction and operation of the apparatus shall be
23、in accordance with BS874-3.1, except where modified by this Part of BS 6993. 4.2 Surround panels The surround panel shall be not less than100mm thick and constructed of material of conductivity not greater than0.04W/mK. Plywood facing on either side of the surround panel to provide rigidity is permi
24、tted, but no material of thermal conductivity greater than0.04W/mK (other than thin tape) shall bridge the aperture. 1) Repeated from BS 874-1. Symbol Description Unit d thickness of layers of glass (or alternative glazing materials) m q heat flux density W/m 2 r thermal resistivity of glass (glazin
25、g material) mK/W R surface resistance m 2 K/W T temperature C or K T m mean temperature of gas space C T temperature difference K U U-value (thermal transmittance) W/m 2 K U m measured U-value W/m 2 K w gas space width m w gas space width difference between centre and edge m thermal conductance W/m
26、2 K Subscripts a environmental (effective ambient) g gas space s surface t total i interior (hot side) e exterior (cold side)BS6993-2:1990 2 BSI 08-1999 4.3 Test element The test element is a sample glazing unit. To ensure consistency of measurement the test element dimensions and location in the su
27、rround panel shall be as follows. a) The test element shall be square with sides measuring 1.2 m. b) The sample shall be mounted in the aperture in the surround panel as shown in Figure 1, with all edges of the glazing unit insulated as shown to a depth of50mm with material of conductivity less than
28、0.04W/mK (such that the exposed area is1.1m 1.1m). c) The cold side face shall be 25 mm in from the edge of the aperture, as shown in Figure 1. 4.4 Calibration specimens Calibration specimens are required to set up the test conditions as specified in clause 5 and enable edge and surround panel effec
29、ts to be corrected. Each calibration specimen shall consist of two flat pieces of glass measuring1.2m 1.2m square, and of minimum nominal thickness4mm, enclosing an insulating material with stable and homogeneous thermal properties. Calibration specimens shall have a uniform thermal conductance not
30、greater than6W/m 2 K, calculated from the known thermal conductivities and thicknesses of the components. At least two calibration specimens spanning the range of thermal conductances of the test elements shall be used (see BS874-3.1). The thermal conductance of the calibration specimens shall be es
31、tablished by either measuring equivalent smaller specimens on a hot-plate apparatus at a laboratory accredited by NAMAS (the National Measurement Accreditation Service) for the measurement of thermal conductivity in accordance with BS874-2.1, or by obtaining calibration specimens with certified prop
32、erties directly from the National Physical Laboratory (NPL). Calibration specimens shall be mounted in the surround panel aperture in the same manner as test elements (see4.3 and Figure 1). 5 Conditions for test NOTE 1The U-value calculation method described in Part 1 of BS6993 specifies the conditi
33、ons for standardized values and presentations. The temperature difference %T sbetween the gas space bounding surfaces of the glazing required for Part 1 is15 K and the mean temperature of glazing T mis10 C. Additionally the prescribed surface resistances, derived from the reciprocal of the heat tran
34、sfer coefficients, are0.12m 2 K/W on the hot side (conventionally corresponding to natural convection) and0.06m 2 K/W on the cold side (corresponding to air velocities of about2m/s). It is not practical to meet these conditions exactly in a hot-box test without specifying a wide range of complicated
35、 measurement conditions. The following test conditions shall apply. a) The environmental temperature on the cold side (T ae ) shall be between 0 C and 5 C, and on the hot side (T ai ) between20 C and25 C and held constant to within1% of the air to air temperature difference across the sample as spec
36、ified in BS874-3.1. The air temperature difference (T a ) between hot and cold sides shall be20 1K. b) The calibration tests shall be used to set up temperature and air velocity conditions to give a total average surface resistance (hot plus cold side) of0.18 0.01m 2 K/W. The total surface resistanc
37、e shall be determined from the mean surface temperatures of the calibration sample and the environmental temperatures on either side. c) For the test element measurements the apparatus shall be operated under exactly the same temperature and air velocity conditions set up for the calibration tests.
38、NOTE 2For the majority of test elements with a range of U-values between1W/m 2 K and 3W/m 2 K, these conditions lead to surface temperature differences across the test element of between16K and19K. NOTE 3The procedures for deriving the parameters to be used in equivalent calculations of the U-value
39、are specified inAppendix A. 6 Determination of measurements following calibration 6.1 Conditions The conditions shall be as specified in clause 5 and in accordance with BS874-3.1. 6.2 Procedure Following calibration, place the test element in the aperture of the surround panel with the same temperat
40、ure and air velocity conditions which have been set during calibration. Measure any dishing or bowing of the unit at the centre point at room temperature. NOTEDishing and bowing are the terms used respectively for the concavity and convexity of the bounding glazing surface of a sealed unit due to pr
41、essure differences between enclosed gas space (s) and the atmosphere. Apply the following constraint in respect of dishing or bowing: |%w| 0.25w where |%w| is the total dish (ve) or bow (+ve) (side1 + side 2), at the centre; w is the gas space width at unit edges.BS6993-2:1990 BSI 08-1999 3 If |%w|
42、0.25w then reject the unit and select another for test. Repeat the procedure until an acceptable unit is selected. With the same temperature and air velocity conditions which have been set during calibration undertake test measurements in accordance with Appendix A of BS 874-3.1:1987. Measure any di
43、shing or bowing of the unit at the end of the test as close as possible to the conditions and temperatures of test (seeA.3). Correct the measured U-value to a standardized total surface resistance (hot plus cold side) of0.18m 2 K/W (see6.3). 6.3 Calculation and expression of results The U-value (in
44、W/m 2 K) for the standardized total surface resistance is calculated from the equation: U = U m 1+ 0.18 R t 1 where where 6.4 Test report The test report shall contain the information specified in clause 8 and A.9 of BS874-3.1:1987. In addition the following shall be reported. a) The U-value correct
45、ed to the standardized total surface resistance R tof0.18m 2 K/W. NOTEThis corresponds to the definition of U-value for normal exposure as given in CIBSE Guide A3. b) Mean temperature T mand environmental temperature difference T a(as calculated in accordance with Appendix A). c) Dishing (or bowing)
46、 at the centre point of the test element (sample glazing unit) at room temperature, and at the termination of the test (see 6.2 and A.3). d) Constructional details of the test elements, e.g.spacing, glass thickness and, if known, the nature of coatings, gas filling. U m is the measured U-value (in W
47、/m 2 K); R t is the total surface resistance (in m 2 K/W) as determined from the calibration tests by interpolation (seeAppendix B for worked example). R t= R e+ R i R e is the exterior (cold side) surface resistance; R i is the interior (warm side) surface resistance.BS6993-2:1990 4 BSI 08-1999 App
48、endix A Values to be used in U-value calculations NOTEThis appendix specifies the derivation of the values to be used in the method adopted in Part1 of BS6993, such that allowance is made for differences between the prescribed mean temperature and surface temperature differences therein, and conditi
49、ons applied during this test. A.1 Mean temperature of gas space The mean temperature of the gas space, T m(in K), is derived as follows. T m= “ (T ai+ T ae ) (R i R e ) U m %T a where Alternatively, if the surface temperatures are measured then T mis derived as follows. T m= “ (T si+ T se ) where A.2 Temperature difference between bounding surfaces of gas space The temperature difference between bounding surfaces of gas space, %T sg(in K), is derived as follows. %T sg= %T a 1 U m(R t+ d r) where Alternative
