Due to the gravitational force of the earth, the air has a certain weight, forming atmospheric pressure. The atmospheric pressure at a certain altitude is the heaviest in the entire column of air per unit area perpendicular to the ground above that point. The air becomes thinner and the atmospheric pressure decreases as altitude increases. According to actual measurements, within 3,000 kilometers of sea level, the air pressure drops by 100 Pa for every 10 meters of altitude increase, and the atmospheric pressure near 31 kilometers is 1/100 of the standard atmospheric pressure at sea level. Weather changes are also influenced by atmospheric pressure, which is related to altitude. At the same place, the pressure is higher in sunny weather and lower in cloudy weather, and the air pressure in winter is higher than that in summer.
In China, about 50% of the Earth’s surface is 1,000 meters above sea level, and about 25% is 2,000 meters above sea level. It can be seen that in places where the surface of the earth is higher than sea level, equipment stored, transported, and used in the air will inevitably encounter the low-level atmospheric environment and be affected by the low-level atmospheric activity environment. For dry aviation audio products, since the maximum and minimum flight altitudes of the aircraft are also kilometers, it generally needs to fly nearly 10,000 meters or more, with a maximum of 30 kilometers. Therefore, airborne equipment will be subjected to more severe low-pressure effects than plateau equipment.
1. What is an altitude simulation test?
The high-altitude simulation test is to put the test sample into the high-altitude test box, then reduce the air pressure in the high-altitude test box to the value specified in the relevant standards, and keep the specified duration for the test. Its main purpose is to test the environmental adaptability and reliability of instruments, electrical products, materials, parts, and equipment under low pressure, high temperature, and low temperature alone or simultaneously in aviation, aerospace, information, electronics, and other fields.
2. Influence of low-pressure environment on equipment
The impact of the low-pressure environment on the equipment is multifaceted: including the direct mechanical impact of the pressure difference caused by the decrease in air pressure, and the impact of the decrease in air density on the heat dissipation and thrust of the power equipment and the electrical performance. Additional effects and detrimental effects on volatile substances caused by damage to seals caused by pressure differences in electrical equipment.
1) Directly use the shell to destroy the sealed product
Under the action of low air pressure, the shell of the sealed product with shell will be directly damaged due to the excessive pressure difference between the inside and outside, and the existence of the pressure difference will also cause the seal to be damaged.
2) Reduce electrical performance
Under normal atmospheric conditions, the air is a better insulating medium, and many electrical products use air as an insulating medium. When these products are used at high altitudes or as airborne equipment, partial discharges often occur near electrodes with high electric field strength due to the reduction of atmospheric pressure. What’s more serious is that air gap breakdown sometimes occurs, which means that the normal operation of the equipment is destroyed. Therefore, the low-voltage environment will also have an impact on the electrical performance of electronic and electrical products, especially for equipment that uses air as the insulating medium, the impact of low voltage is more significant.
3) Lead to a large temperature rise of heat dissipation products
The so-called heat dissipation product refers to the test product whose surface temperature at the hottest point is more than 5°C different from the ambient temperature after the temperature of the test product reaches stability under free air conditions and specified atmospheric pressure. A considerable part of electronic and electrical products are heat dissipation products, such as motors, transformers, etc. These products consume part of the electric energy during use and convert it into heat energy, which increases the temperature of the product. The temperature rise of cooling products increases with the increase in altitude (decrease of atmospheric pressure). The temperature rise and altitude are roughly linear, and the slope depends on factors such as its own structure, heat dissipation, and ambient temperature.
4) Lead to the loss of volatile substances
The liquid’s boiling point is lowered as a result of this decrease in pressure. For those liquids whose saturated vapor pressure is high under normal atmospheric conditions at sea level, the low pressure causes them to vaporize or even boil. The evaporation process of liquid is an equilibrium process, that is, the number of liquid molecules volatilized into the air through energy is balanced with the number of molecules combined with air molecules hitting the liquid surface.
When the atmospheric pressure decreases, the density of the air decreases, and the possibility of the volatile molecules of the liquid entering the air being knocked back to the liquid surface is greatly reduced. Therefore, under low-pressure conditions, the rate of liquid evaporation will be greatly accelerated. This is the case with lubricating oil or grease. The reduction of the pressure will accelerate the volatilization of lubricating oil (or grease), aggravate the friction of moving parts, and accelerate the wear of the surface of moving parts. Plasticizers in organic materials will also accelerate volatilization due to reduced air pressure.
The volatilization of plasticizers promotes the aging of organic materials, changing their mechanical or electrical properties. Volatilization of volatile matter will also pollute the product and its surroundings, causing the product or object to corrode or become contaminated. Based on the impact of the above low-pressure environment on equipment, typical low-pressure environment effects include gas or liquid leakage from the sealed enclosure; deformation, cracking, or explosion of the sealed container; changes in the physical and chemical properties of low-density substances; equipment damage caused by low-voltage arcs or corona discharges Faults or faults; under low pressure, the heat transfer efficiency decreases, resulting in overheating of the equipment; the lubricating oil volatilizes; the engine starts and burns unstable, the thrust or traction decreases, and the airtight seal fails, etc.
3. Test equipment Requirement
It is very important to conduct high-altitude simulation tests on products. Here are some common requirements for the test equipment:
1) General requirements
- The low-voltage test equipment is to be able to generate and maintain the low pressure required for the test, and be equipped with necessary auxiliary instruments capable of monitoring the low pressure;
- The low-pressure chamber is to be equipped with a device for continuously recording the pressure of the test chamber:
- The resolution of the data readout device is not less than 2% of its full scale;
- Pay attention to prevent the air in the box from being polluted by volatile substances such as air pumps, valves, and insulating materials of the test equipment;
- When repressurizing, pay attention to preventing external dust and water vapor from entering the box and causing pollution.
2) Rapid decompression explosion decompression equipment
Based on the existing low-pressure test equipment, it is very difficult to improve the vacuuming capacity of the vacuum system and realize the rapid decompression test, because a vacuum pump system with strong vacuuming capacity is required, not only the investment is large, but also the test room can be tested within 15 seconds or 0.1 seconds. It is very difficult to reduce the pressure from 75KPa to 188KPa.
At present, the auxiliary vacuum tank method is generally used, even if the laboratory is connected with another large-volume vacuum tank or low-pressure tank through the pipeline solenoid valve. Vacuum tanks and low-pressure tanks are evacuated. When a rapid decompression test is required, Quickly open the solenoid valve, connect the test room and the vacuum tank, and balance the pressure of the test room and the vacuum tank to achieve the desired purpose.