Test Overview
Basic concept of high altitude low pressure testing: Air pressure refers to the atmospheric pressure acting on a unit area, numerically equal to the gravity of the vertical air column extending upward from a unit area to the upper boundary of the atmosphere. The international unit is the pascal, abbreviated Pa. From this definition, it can be seen that in the natural environment, air pressure decreases gradually with increasing altitude. At an altitude close to 5000 m, the atmospheric pressure drops to about half of the standard atmospheric pressure at sea level; at an altitude close to 16000 m it drops to about 1/10, and at an altitude close to 31000 m it drops to about 1/100 of the sea-level standard atmospheric pressure. The atmospheric pressure at the same altitude is also affected by temperature and latitude. Temperature affects the movement of gas molecules in the atmosphere: the higher the temperature, the more intense the movement, and under otherwise identical conditions, the higher the temperature, the higher the atmospheric pressure. Different latitudes are subject to different centrifugal forces of the Earth: the lower the latitude, the stronger the effect, and under otherwise identical conditions, the lower the latitude, the higher the atmospheric pressure.
Effects of Low Pressure
Effects of low pressure on products: The direct effect of reduced air pressure on products is mainly the pressure action caused by pressure changes. For sealed products, the enclosure is subjected to a pressure under which there is a risk of seal failure. However, the main effect of reduced pressure lies in the fact that the accompanying reduction in air density greatly affects product performance. For heat-generating products such as motors, transformers, contactors and resistors, these products generate heat during use, and the heat raises their temperature; this temperature rise is called temperature rise. The temperature rise of heat-dissipating products increases as the atmospheric pressure decreases and as the altitude increases, leading to degraded performance or unstable operation. For equipment using air as the insulating medium, the effect of low pressure is even more significant. Under normal atmospheric conditions, air is a good insulating medium, and many electrical products use air as insulation. When these products are used in high-altitude areas or as airborne equipment, the reduced atmospheric pressure often causes local discharge phenomena near stronger electrodes, known as corona. More seriously, air gap breakdown may sometimes occur. At low pressure, especially when combined with high temperature, the dielectric strength of air decreases significantly, i.e. the corona onset voltage and the breakdown voltage decrease significantly, increasing the risk of surface arc discharge or corona discharge.
Test Purpose
Low pressure testing is mainly used in aviation, aerospace, information technology, electronics and other fields to determine the environmental adaptability and reliability of instruments, electrical products, materials, components and equipment under low pressure, high temperature or low temperature, individually or in combination, and optionally to measure electrical performance parameters while the specimen is energized.
Reference Standards
High altitude low pressure testing complies with the following standards:
GB/T 2423.21-2008 Environmental testing for electric and electronic products, Part 2: Test methods, Test M: Low air pressure
GB/T 2423.24-1995 Environmental testing for electric and electronic products, Guidance for combined temperature (cold and heat), low air pressure and vibration (sinusoidal) tests; IEC 68-2-41 Basic environmental testing procedures, Part 2: Tests, Test Z/BM: Dry heat/low air pressure combined test
HB 5830.14 Environmental conditions and test methods for airborne equipment — Low air pressure (altitude)
JB 3224-83 Low air pressure test methods for electrical products used in high altitude areas; GB/T 4857.13 Packaging — Transport packages — Low pressure test method
GB 12085.5 Optics and optical instruments — Environmental test methods — Combined cold and low air pressure; GB/T 10590 Technical requirements for low temperature/low air pressure test chambers
GB/T 5170.10 Inspection methods for basic parameters of environmental testing equipment for electric and electronic products — High/low temperature and low air pressure testing equipment
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