This standard provides the technical basis for temperature/low pressure (≥1kPa) comprehensive test and the data on the influence of temperature and pressure on air characteristics and products, and provides guidance for the measurement of temperature and pressure and the application of test chambers. GB/T 2424.15-1992 Basic environmental test procedures for electrical and electronic products Temperature/low pressure comprehensive test guide GB/T2424.15-1992 Standard download decompression password: www.bzxz.net

National Standard of the People's Republic of China
Basic enviromental testing procedures forelectric and electronic productsGuide for combined temperature/low air pressureare testsGB/T 2424.15--92
Replaces CB2424.15---81
This standard is equivalent to the International Electrotechnical Commission IEC68-3-2 (1976) "Basic Environmental Testing Procedures Part 3 Background Information Part 2 Temperature/Low Air Pressure Combined Test". 1 Subject Content and Scope of Application
This standard provides the technical basis for conducting temperature/low air pressure (≥1kPa) combined tests and the information on the influence of temperature and air pressure on air characteristics and products, and provides guidance for the measurement of temperature and air pressure and the application of test chambers. This standard applies to the following three temperature/low pressure comprehensive test standards: GB/T2423.25-92 "Basic environmental test procedures for electric and electronic products Test Z/AM Low temperature/low pressure comprehensive test"; b. GB/T2423.26--92 "Basic environmental test procedures for electric and electronic products Test Z/BM High temperature/low pressure comprehensive test"; c. GB2423.27--81 "Basic environmental test procedures for electric and electronic products Test Z/AMD Low temperature/low pressure/humid heat continuous comprehensive test method".
2 Reference standards
GB2421 General rules for basic environmental testing procedures for electric and electronic products GB2422 Terminology of basic environmental testing procedures for electric and electronic products GB2424.1 Basic environmental testing procedures for electric and electronic products Guidelines for high and low temperature tests 3 General description
In the temperature/low pressure comprehensive test, the physical properties of air and the heat transfer mode have a great influence on the test results. In the process of formulating the temperature/low pressure comprehensive test standards and conducting the temperature/low pressure comprehensive test, the difference between the air properties at abnormal temperature and normal pressure and the air properties at normal temperature and normal pressure should be taken into account.
Within the low pressure range (>1kPa) specified in this standard, the changes in air density and heat transfer do not have a decisive influence on some basic properties of air, which are described in 3.1~~3.3. These rules also apply to the temperature/low pressure comprehensive test. 3.1 Thermal conductivity, viscosity and flow characteristics of air Within the air density range specified in this standard, the mean free path of air molecules is always a few tenths of a millimeter, so the thermal conductivity and viscosity of air are independent of air pressure. Air flow is generally of stagnant or end flow type, and it obeys various laws under normal air pressure. 3.2 Convective characteristics of air Within the air density range specified in this standard, the basic laws of heat transfer of free air convection and forced air convection are the same as those under normal atmospheric pressure. Therefore, the guidance on convection in GB2423.1 is also applicable to the temperature/low pressure comprehensive test. When the air density P decreases, the convection heat transfer coefficient α of the air can be greatly reduced. Whether it is free convection or forced convection, the relationship between the convection coefficient and air density is shown in the following formula. α. = f(p\)n = 0. 5 ~ 0. 7Although α. has decreased under low pressure, the convective heat transfer effect of air still needs to be considered. 3.3 Radiative heat transfer
Under low pressure conditions, more attention should be paid to the role of radiative heat transfer. Attention should be paid to controlling the emissivity and temperature of the box wall. As the convection and heat conduction effects of the air decrease, the radiative heat transfer effect remains unchanged, and the influence of radiative heat transfer increases. 3.4 Test of heat dissipation test samples
3.4.1 Air conditionsbzxz.net
Heat dissipation test samples-General provisions use "free air" conditions without forced air circulation or use lower air speed. The air speed should be as low as possible so that the additional cooling effect brought by air flow can be ignored. Within the air pressure range specified in this standard, the surface temperature of the test sample under strong air flow conditions will be much lower than that under "free air" conditions. When the air temperature and heat dissipation conditions remain unchanged, the curve of the average surface temperature of the test sample with air flow speed and air pressure is shown in Figure 1. This figure can be used to find the surface temperature of the product when forced air flow is used. 3.4.2 Number of test samples
Under low pressure conditions, the mutual thermal radiation between heat dissipation test samples increases, which will affect the reproducibility of the test. Generally, only one test sample is loaded in the test chamber for comprehensive testing at a time. If it is known that the thermal radiation between heat dissipation samples is not significant and will not affect the reproducibility of the test, multiple test samples are allowed to be tested in the same chamber at the same time. 15 +
()2
Free air
100Air pressure in the chamber, kPa
Figure 1 Comprehensive effect of air pressure and air velocity on the surface temperature of the test sample Note: The air temperature of the test chamber is 45°; the heat dissipation of the test sample is 43W; the emissivity of the chamber wall ε is 1; the emissivity of the test sample surface ε is 0.7 and the surface area of ​​the test sample is 0.12m2.
4 Effects of temperature and low pressure
4.1 Comprehensive effects
GB/T 2424.1592
4.1.1 Raising the surface temperature of the heat dissipation sample and changing the temperature gradient Due to the decrease of the convective heat transfer coefficient α under low pressure, the surface temperature of the heat dissipation sample during the temperature and low pressure comprehensive test is higher than that under the normal pressure temperature test, and the temperature gradient inside and outside the product is changed. In this case, the normal pressure temperature test cannot be used as a substitute, because although the increase in the test temperature can be used to simulate the increase in surface temperature, it is difficult to determine how much the increase is. In addition, it is impossible to obtain accurate temperature gradient conditions without combining with low pressure. 4.1.2 Changing the dielectric properties of air
Under low pressure and high and low temperatures, due to the decrease in air density and changes in ion migration ability, the dielectric strength of air is significantly reduced, increasing the risk of arcing, surface discharge or corona discharge, thereby affecting the function and safety characteristics of the product. Table 1 shows the amplification factor of the breakdown voltage of parallel plate electrodes with an air gap greater than 5mm. The breakdown voltage under the pressure and temperature can be obtained by multiplying the amplification factor corresponding to a certain pressure and temperature in the table by the breakdown voltage under standard conditions (35℃ and 101.3kPa). Note: For electrical professional products, especially those with a voltage of more than 1000V, the correction factor of the environmental conditions should be in accordance with the relevant standards. 4.1.3 Changes in material properties
Temperature changes will cause changes in the brittleness, plasticity and other characteristics of the material, increasing the risk of deformation or cracking of sealed equipment or components under low pressure.
Table 1 Multiplication factors of breakdown voltage under various temperatures and pressures Pressure
4.2 Other effects
0, 68
Other effects refer to effects that are mainly caused by temperature but are significantly aggravated by low pressure. This effect can only reach the same level under low pressure after a long test time under normal pressure. The main effects are: 1. "Plasticizers and plastic degradation products volatilize, causing changes in the mechanical and electrical properties of various parts of the test sample. In addition, these volatiles condense on nearby surfaces, causing performance changes, corrosion and deterioration; b. Lubricant evaporation causes active parts to get stuck, and changes in part size caused by temperature further aggravate this phenomenon; c. Gases dissolved in liquids escape, and some liquids will boil under low pressure, causing liquid loss. 5 Test equipment || tt || 5.1 Temperature/low pressure comprehensive test equipment should be able to change temperature and air pressure at the rate or requirements specified in the relevant standards pressure and other conditions, and can keep these conditions individually or comprehensively within the tolerance range specified in the relevant standards. 5.2 The air composition in the comprehensive test chamber generally cannot truly represent the natural atmospheric conditions. It is mainly affected by the type of pressure reducing pump. The air in the chamber generally contains a large amount of water vapor (its humidity can be measured by a dew point meter), which will affect the convection coefficient α of the air. However, this effect will not exceed 10%.
5.3 When conducting comprehensive tests, pay attention to prevent the air in the chamber from being polluted to prevent pollution from affecting the product test results. The main pollution is: 9
GB/T 2424.15—92
a. The air is polluted by the steam of the working fluid of the pump and the steam from the accessories of the test chamber (valves, insulation, etc.); b. The air is polluted by dust or water introduced into the air during the process of restoring the air pressure. 6 Measurement of temperature and air pressure
Under the conditions of low air pressure and high and low temperatures, the measurement error of temperature and air pressure parameters will increase due to the decrease in air heat exchange efficiency and the influence of thermal expansion and contraction of materials. These factors should be taken into account when formulating relevant comprehensive test standards and conducting comprehensive tests. 6.1 Increased temperature measurement error
Under low air pressure, due to the decrease in air density in the chamber, the heat exchange rate between the sensor and the air in the chamber decreases, resulting in a longer response time of the thermometer to temperature changes, and an increase in the error caused by heat conduction between the thermometer and the outside of the chamber. When testing heat dissipation samples, the decrease of α also causes measurement errors. This error is greater under low air pressure than under normal pressure. When testing heat dissipation samples, the thermometer should have a shielding cover to reduce the interference of heat radiation from the heat dissipation sample on the measurement. 6.2 Air pressure measurement
When using a barometer to measure air pressure, use a tube to connect the barometer to the space inside the box. Under normal circumstances, even if the temperature of the barometer is very different from the temperature inside the box, this tube will not cause a large measurement error. If the temperature of the barometer is very different from the temperature when it was calibrated, it is possible that errors will be introduced due to elastic changes in the sensor element caused by heating or cooling of the gas in the test chamber. Therefore, care should be taken not to use short and thick tubes, and try to use slender connecting tubes, but at the same time, consider the disadvantage that slender tubes react slowly to changes in air pressure, causing delayed indication time. Additional notes:
This standard was proposed and coordinated by the National Technical Committee for Standardization of Environmental Conditions and Environmental Testing of Electrical and Electronic Products. This standard was drafted by the 30th Institute and the 60th Institute of the Ministry of Aeronautics and Astronautics, and the Guangzhou Institute of Electrical Science of the Ministry of Machinery and Electronics.
The main drafters of this standard are Zhu Yaochang, Xu Guobao, and Zhang Leshan. 600
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