Engineering case study: atmospheric pressure indicators that cannot be ignored

In electronic products, the most common way to write the technical indicator of atmospheric pressure is 750-1060 hPa (some use mmHg, millibar), which is a common and easily overlooked role. Even if many manufacturers write it, they actually turn a blind eye to it, but this indicator is indeed more than just a decoration. Just in the past few days, this phenomenon has unfortunately been repeated in a large enterprise ranked in the top 100 (or even the top 50) in the world.

As we all know, the atmospheric pressure at sea level is 1060mmH2O. As the altitude increases, the atmospheric pressure decreases at a slope of 11.2mbar (1120Pa) for every 100m increase in altitude. In this case, 750hPa corresponds to an altitude of about 2700m. Xining in Qinghai is basically at this altitude. In other words, this marking method cannot be used in areas above Xining. Even if it can be used, the failure mechanism will be greatly increased.

The purpose of this article is to give a few examples to illustrate the seriousness of the problem and to arouse everyone's awareness of the problem.

Atmospheric pressure changes. For devices with internal pressure, such as electrolytic capacitors, the internal heat needs to be dissipated during operation. However, when the heat is too large to be dissipated in time, the internal medium will expand. When the altitude is low, the external pressure is strong. Even if the internal pressure expands, △P will still be relatively small, and the shell of the device is sufficient to overcome this stress. However, when the altitude is high, the external pressure is small, and △P will increase. When it is strong enough to overcome the strength of the device shell, the device will inevitably crack. In summary, electrolytic capacitors in high altitude areas are prone to explosion.

Another typical phenomenon is that for equipment with air and water sources, there is pressurized gas or liquid in the pipe. Due to the change in altitude, the pressure difference between the inside and outside changes, which will also cause the accuracy error of air and liquid supply or the risk of bursting and leakage. Such as respiratory therapy equipment, sealing equipment (sealed electrical switch cabinet), etc.

From the above, it can be seen that the atmospheric pressure indicator is still very useful and must not be ignored. Its impact is not only on the entire system, but also on mechanical piping and the device itself. Therefore, the use scope of the equipment must be strictly controlled, and relevant tests must be conducted in the factory to verify that the product will not cause problems due to changes in air pressure. Otherwise, when the product project dies, you won’t even know how it died.

Hot discussion among netizens:

Shenyucha: Xining Railway Station is 2100m, Xining Nanchuan Factory is 2300m, the air pressure is 760 millibars. Not the 760mmHg at sea level. The hemoglobin level in Beijing is 9.5 and gradually rises to 16.5 in Xining. Can frequent trips to and from the plateau cause hemoglobin fatigue?

Ke Lianghu: Let me say it again. According to my investigation, there are two reasons why low air pressure causes failure. One is heat. The air's heat dissipation capacity decreases under low pressure, which causes high-power equipment to be insufficiently cooled and burn out. I saw the altitude requirements marked on Agilent's maximum power supply. I can't remember the exact number, but it roughly means that if it is below a certain altitude, it can be used at full capacity. After exceeding it, the rating will be reduced by a certain percentage for each increase in altitude. So this derating should not be due to the capacitor bursting problem. The other is cosmic rays. Electronic equipment on the plateau is more likely to be affected by cosmic rays, which may lead to execution errors. The problem of cosmic rays should be considered on equipment that requires high reliability, just like it should be considered on airplanes.