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The problem of fogging up the glass of the display cases in the Southern Museum has long been a troubling problem. Especially in the wet season, the fog is even more severe, which brings great inconvenience to visitors.Two days ago, Beijing suddenly cooled down, I got up early to go to work, drove out of the underground garage, and after a few minutes, the car glass was full of fog, not only the front windshield, but also the left and right side windows were foggy, and I couldn't see anything clearly.
Immediately turn on the (MAX front windshield) defogging mode.
Interestingly, at the same time, the linkage turned on a c and closed the inner circulation, and soon the water mist was removed, not only the fog on the front windshield, but also on the side windows.
This process reminds me that when I used to go to museums in the southern region, I would often find water mist in the display cases, and the principle must be the same as this process.
The first thing we need to figure out is why it fogs. In fact, the principle is very simple, all fogging is due to the large temperature difference, which causes the water vapor in the air to condense into water droplets.
In the winter of the north, the temperature difference between indoor and outdoor is large, the temperature of the basement can generally reach minus 3-5 °, when the car is parked in the basement overnight, and the ground is driven on the ground in the morning, the ground temperature is generally minus 5-6 °, and because the car is seated in the person at this time, the human body heat dissipation and human breathing bring out more moisture, which will further lead to the temperature difference between the inside and outside of the car and the humidity in the car, and finally form fog on the inner surface of the car glass.
When the "defogging mode" is turned on, it is essentially pumping out the moisture-filled air in the car as quickly as possible, which is why once the "defogging mode" is turned on, the internal circulation will be automatically turned off.
The same principle, when viewed on the display case, the formation of temperature difference and humidity is still different from the situation of automobiles.
In the southern region, especially in summer, due to the humid climate, the whole environment is in a state of relatively high humidity and high heat, and museums usually turn on the air conditioning to cool down during the day, which leads to low temperature outside the cultural relics display cabinet and high temperature inside the cabinet; At the same time, the high humidity in the cabinet is still the key, because the display cabinet is not sealed enough, the high humidity air outside the cabinet will continue to enter the cabinet, a large number of water molecules are encircled in the cabinet, once the temperature difference between the inside and outside of the cabinet reaches the condition, the fog will appear.
The tightness of the cultural relics display cabinet is crucial and is the basis of all performance, which has been repeatedly mentioned in my previous articles [High standard air tightness: how important is it to the display case of the museum!] (baidu.com)】。
Many well-known museum display cabinet manufacturers in Europe and the United States have proposed that high-quality display cabinets must reduce air exchange to a maximum of 10% of the volume per 24 hours, while China's current standard is far lower than this.
The standard is still so, due to the lack of standardization of the entire industry, there are many products below this standard in actual operation.
If the display cabinet is not tightly sealed, the inner surface of the display cabinet glass will not only fog up, but also highlight various uneven marks on the glass surface.
If the humidity rises further, the water droplet diameter will be larger. The H+ in the water droplets undergoes an ion exchange reaction with the Na+ in the glass, which eventually corrodes the glass.
The poor sealing of the display cabinet not only allows moisture to enter the cabinet, but also the dust-rich unclean air is easy to enter, and there is even no shortage of flies, mosquitoes, and geckos.
Micro-environment climate regulation systems for display cabinets are generally divided into two categories, active control and passive control.
Active climate control means that purified humidity-controlled air or nitrogen is injected into the display case space as a "one-way system". With the active climate control system, a constant slight overpressure is generated, which continuously "washes" the display area.
I think that the slight overpressure generated by the active climate control system prevents the air from outside the cabinet from circulating into the cabinet, which in turn prevents water molecules from entering the cabinet, which is a good way to control the humidity inside the cabinet.
Of course, the active climate control system must have a high standard of air tightness as the foundation, otherwise the burden of forming positive pressure will be very large, which is directly reflected in the high energy consumption, and the museum equipment should be kept on 24 hours a day in principle, if the energy consumption is too high, it will bring great pressure to the institution.
China has a vast territory and huge differences in climate, so it is necessary to customize suitable display equipment according to the local climate characteristics.