All PO film products adopt imported coating liquid, which has the advantage of a short initial dripping time and can form a water film immediately after the greenhouse is covered with the film. If the greenhouse is covered in late autumn and early winter, and the temperature drops right after covering, with air vents narrowed, the temperature difference between the inside and outside of the greenhouse increases, and the humidity inside the greenhouse is ≥70%, the mirror-like water film can be maintained all the time.
On Ventilation Issues
At night, as the air vents are closed and the thermal quilts are put down, the air is not circulating and there is no temperature difference between the inside and outside. Only a small amount of water vapor close to the film will condense into water droplets, resulting in dense water droplets forming on the film surface. Therefore, there will be many water droplets on the film surface after the thermal quilts are pulled open in the morning.
Ventilation for PO Film
In autumn, the temperature difference between day and night is large and the temperature is relatively high during the day, so ventilation is carried out in the daytime. In some places, the upper air vent is opened wide, about 1.5 meters in width, and in some places, both the upper and lower air vents are opened at the same time. At this time, due to air circulation, water vapor is carried away and will not condense on the film surface. As a result, the suspended droplets on the film surface will gradually evaporate and shrink in size, and the film surface will be air-dried around 11 o'clock.
On the Problem of Surface Suspended Droplets
Suspended Droplets on PO Film Surface
The above pictures were taken at 9 a.m. in Shouguang. The air vent of this greenhouse was opened about 1.5 meters wide, the humidity inside the greenhouse was only about 50%, and the film surface was air-dried at noon. The surface suspended droplets caused by this reason will turn into a mirror-like water film when the temperature drops, the temperature difference between the inside and outside increases, the air vents are narrowed, and the humidity inside the greenhouse is ≥70%.
Suspended Droplets on PO Film Surface
The above pictures were taken at 9:30 a.m. in Shouguang. The air vent was opened more than 1 meter in the morning, and the film surface was air-dried at noon. The water had run off at the front edge of the greenhouse with almost no suspended droplets left, while there were many suspended droplets near the air vent. This is because only the upper air vent was opened, leading to poor air circulation and more water vapor at the front edge, while the water vapor near the air vent escaped and could not continue to condense on the film, so the water droplets became smaller but did not fall off, and the film surface was air-dried at noon.
Suspended Droplets on PO Film Surface
Drip irrigation is used in many greenhouses with mulch film covered on the ground. When the seedlings are small, the transpiration is insufficient, and the mulch film results in low humidity inside the greenhouse. If this problem occurs, the air vents can be appropriately narrowed to alleviate it. Of course, the problem will disappear when the temperature drops, the air vents are narrowed and the seedlings grow a little bigger, and a mirror-like water film will form instead.
Distinguishing Between Suspended Droplets and White Droplets
Distinguishing Suspended Droplets and White Droplets on PO Film
Viewed from outside the greenhouse, if the suspended droplets are white, it means the anti-dripping function has failed, which can usually be seen in areas rubbed by iron wires, and the film can no longer be used. If the droplets are transparent, they will definitely flow into a mirror-like film when the temperature drops and both humidity and temperature difference rise.
Summary
1.If the film surface is completely white after covering the greenhouse, one reason is that the film is installed reversely, which can be solved by turning it over; the other reason is the failure of the anti-dripping function, and the film needs to be replaced directly.
2.If the anti-dripping function works well when the greenhouse is covered, all subsequent problems such as suspended droplets and dense water droplets will be resolved on their own.
3.The problems of excessive suspended droplets and dense water droplets mostly occur between October 20 and November 10 every year. During this period, the temperature drops gradually, the temperature difference between day and night is large, the air vents are closed at night and opened wide during the day, leading to the appearance of suspended droplets.
4.After November 10, as the temperature drops, the temperature difference increases and the air vents are narrowed, a mirror-like water film will form on the film surface.