1、BRITISH STANDARD Environmental testing - Part 3: Background information - Section 2: Combined temperaturehow air pressure tests The European Standard EN 6O0683-21999 has the status of a British Standard ICs 19.040 BS EN 6068-3-2: 1999 IEC 60068-3-2:1976 NO COPYING WITHOUT BSI PERMISSION EXCEPT AS PE
2、RMITTED BY COPYRIGHT LAW National foreword direction of the Electrotechnicai Amd. No. Da Sector Committee, was published This British Standard is the English language version of EN 6006852: 1999. It is identical with IEC 600683-2:19976. It supersedes BS 20113WAM is still a very small fraction of the
3、 resulting dimensions. STD-BSI BS EN bOOb8-3-2-ENGL 1999 m Lb24bb9 0792959 243 m Page 6 EN 60068-3-2:1999 3. Environmental effects 3.1 The following effects on a specimen are considered to be due to the combination of temperature and low air pressure: a) increase of surface temperature and changes o
4、f temperature gradients of heat-dissipating specimens in comparison with the values achieved with Tests A or B are due to the decrease of the convection coefficient ae, at low air pressure (low air density). Although the increase of surface temperatures can be achieved by using a higher test tempera
5、ture at normal air pressure, this test temperature value cannot be properly established and the correct temperature gradients cannot be achieved without combining with low air pressure; b) changes in functional or safety characteristics of specimens due to changes with pressure and temperature of di
6、electric properties of air (air density and ion mobility effects). At low pressures, particularly when combined with high temperature, there is a marked reduction of air dielectric strength with the consequent increased risk of arc, surface or corona discharges. Table I gives multiplying factors for
7、 obtaining sparkover voltages for plane and parallel electrodes with air gaps greater than 5 mm. The factors convert values at standard conditions (25 “C and 1 013 mbar) to the pressures and temperatures given in the table. TABLE I 1 I r Temperature in degrees Celsius Pressure (mbar) - 20 O 20 40 60
8、 I70 340 510 675 850 0.26 0.24 0.23 0.21 0.20 0.19 0.47 O .44 0.42 0.39 0.31 0.24 0.68 0.64 0.60 0.56 0.53 0.50 0.87 0.82 0.77 0.72 0.68 0.64 1 .O7 0.99 0.93 0.87 0.82 0.17 c) changes in material characteristics (brittleness, plasticity) due to cold or heat increase the risk that sealed equipments o
9、r packages will be deformed or cracked at low air pressures. 3.2 Other effects due essentially to temperature but remarkably accelerated by low air pressures (so that a test at normal air pressure can give the same effects only after a very long test time) are: a) volatilization of plasticizers and
10、of degradation products from plastics, with changes in the mechanical and electrical characteristics of individual parts of the specimen; these evaporated products can, in addition, condense on nearby surfaces, producing changes of characteristics or corrosion and degradation; b) evaporation of lubr
11、icants with consequent blocking of moving parts; this process can be exaggerated by dimensional changes due to temperature; c) dissolved gas evolution from liquids; the reduced pressure can result in transitory boiling, with a possible leakage of liquid. At much lower air pressures (such as the ones
12、 existing in outer space), other phenomena become very important. A special testing philosophy must then be used for these latter conditions. 4. Testing facilities The composition of the atmosphere in the testing chamber is generally not truly representative of the natural conditions. The compositio
13、n is determined essentially by the type of pumps used to reduce the air pressure. A con- biderable quantity of water vapour is generally present (the humidity can be measured by a dew point instrument). Page 6 EN 60068-3 - 2: 1999 The error in the convection coefficient a, due to differences between
14、 natural and simulated air composition is The following factors are more important for possible side effects on specimens being tested: not more than 10%. a) chamber atmosphere contamination from vapours of the pump working fluid and from vapours released from chamber accessories (valves, heat insul
15、ators, etc.); b) contamination by dust or water introduced by the air when pressure is restored to normal. 5. Measurements of environmental parameters 5.1 Temperature measurements air temperature monitoring is reduced by the a, decrease. The efficiency of thermal exchange between test atmosphere and
16、 sensing elements of the thermometer used for As a consequence: a) the response time of thermometers to temperature changes is greater than at normal pressure; b) errors due to the thermal conduction between the thermometer and the chamber exterior can become important. In testing of heat-dissipatin
17、g specimens, there is another measurement error (caused by the reduced a, value) which may be larger at low air pressure than at normal air pressure; the thermometer can give incorrect readings if affected by energy radiated from the specimen. The use of radiation shields around the thermometer can
18、reduce this error. 5.2 Air pressure measurements In the pressure range under consideration, the pressure inside the chamber is generally measured with man- ometers connected to the working space by tubes. In stationary conditions, such tubes do not result in significant measurement errors even for l
19、arge temperature differences between manometer and working space. Errors can, however, be introduced by variations of elastic characteristics of the sensing element due to heating or cooling by the gas in the test chamber, if the manometer temperature becomes appreciably different from the calibrati
20、on temperature. Short and large connecting tubes are detrimental in this respect. A time delay in manometer indications can be introdud by long and/or small bore tubes during pressure changes. Page 7 EN 60068-3-2:1999 (“Cl 4 20 is 10 5 o -5 -10 0.h Air calme Free air 1,O m/s 2,O m/s 6.1 37/76 Ar = k
21、art de la temprature de surface du spcimen (valeur Ar = deviation from specimen surface temperature (mean moyenne) par rapport rique normale (1 bar) la valeur pression atmosph& value) at normal air pressure (i bar) p = pression de lair dans la chambre dessai p = air pressure in the test chamber Temp
22、rature de lair: 45 “C Emissivir: paroi de la chambre: e = 1 Surface du spcimen: O, 12 m2 Dissipation dnergie du spcimen: 43 W surface du spkcimcn: E = 0,7 Air temperature: 45 “C Emissivity: chamber walls: e = 1 Specimen surface area: 0.12 mz Specimen heat dissipation: 43 W specimen surface: e = 0.7
23、Le diagramme a t tabli conformment aux formules donnes dans McAdams ( Heat Transmission s (McGraw-Hill, 1954). The diagram has been calculated according to formulae given in McAdams “Heat Transmission“ (McGraw-Hill, 1954). FIG. 1. - Effets combins de la pression atmosphrique et de la vitesse de lair
24、 sur ia temprature de surface du spcimen. Combined effects of air pressure and air velocity on specimen surface temperature. Annex ZA (normative) Normative references to international publications with their corresponding European publications This European Standard incorporates by dated or undated
25、reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when inco
26、rporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments). NOTE: When an international publication has been modified by common modifications, indicated by (mod), the relevant ENIHD applies. Year Title -_I_ Publica
27、tion Year - EN/HD IEC 60068-3-1 1974 Environmental testing + IEC 60068-3-1 A Part 3: Background information EN 60068-3-1 i 999 Section 1 : Cold and dry heat tests BS EN 60068-3-2: 1999 IEC 60068-3-2:1976 BSI 389 Chiswick High Road London w4 4AL BSI - British Standards Institution BSI is the independ
28、ent national body responsible for preparing British standards. It presents the K view on standards in Europe and at the international level. It is incorporated by Royal Charter. Revisions British Standards are updated by amendment or revision. Users of British Standards should make sure that they po
29、ssess the latest amendments or editions. It is the constant aim of BSI to improve the quality of our products and services. We would be grateful if anyone nding an inaccuracy or ambiguity while using this British Standard would inform the Secretary of the technical committee responsible, the identit
30、y of which can be found on the inside kont cover. Tel 020 8996 9000. Fax: 020 8996 7400. BSI offers members an individual updating Service called PLUS which ensures that subscribers automatically receive the latest editions of standards. Buying standards Orders for aii BSI, international and foreign
31、 standards publications should be adressed to Customer Services. Tel 020 8996 9001. Fax: 020 8996 7001. In response to orders for international standards, it is BSI policy to supply the BSI implementation of those that have been published as British Standards, unless otherwise requested. Information
32、 on standards BSI provides a wide range of information on national, European and internationai standards through its Library and its Technid Help to Exporters Service. Various BSI elech-onic information services are also available which give details on ali its products andservices. Contact the Infor
33、mation Centre. Tel O20 8396 7111. Fax: O20 8996 7048. Subscribing members of BSI are kept up to date with standards developments and receive substantial discounts on the purchase price of standards. For details of these and other benefits contact Membership Admjnislxation. Tel 020 8996 7002. Fax: O2
34、0 8996 7001. Copyright Copyright subsists in ail BSI publications. BSI also holds the copyright, in the UK, of the publications of the international standardization bodies. Except as permitted under the Copyright, Designs and Patents Act 1988 no extract may be reproduced, stored in a retrieval syste
35、m or transmitted in any form or by any means - electronic, photocopying, recording or otherwise - without prior wri-n permission from BSL This does not preclude the free use, in the course of implementing the standard, of necessary details such as symbols, and size, type or grade designations. If th
36、ese details are to be used for any other purpose than implementation then the prior written permission of BSI must be obtained. If permission is granted, the terms may include royalty payments or a licensing agreement Details and advice can be obtained from the Copyright Manager. Tel O20 8996 7070.
copyright@ 2008-2019 麦多课文库（www.mydoc123.com）网站版权所有
备案/许可证编号：苏ICP备17064731号-1