BS EN 60068-3-2-1999 Environmental testing - Guidance - Combined temperature low air pressure tests - Combined temperature low air pressure tests《环境试验 指南 组合温度 低气压试验》.pdf

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

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