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An aspirated screen is a metal box with two temperature sensors. One measures the greenhouse temperature (the wet bulb) while the other measures the humidity (the dry bulb). By use of a ventilator in the aspirated screen greenhouse air is being sucked along the sensors constantly, so that it can be measured. The aspirated screen sends the measured values to the climate computer, after which the climate computer calculates the relative humidity (RH). Based on this the greenhouse climate can be adjusted, creating more equality.

An aspirated screen consists of a ventilator, a wet bulb and a dry bulb. The wet bulb consists of a thermometer surrounded by a fabric sleeve. This sleeve extracts water from a water reservoir in the aspirated screen. As the water evaporates in the sleeve, heat is extracted from the temperature sensor whereupon the temperature of the wet bulb drops. The ventilator ensures a constant air flow along the wet bulb, realizing constant evaporation so the wet bulb won't become saturated. The ventilator does not cool the wet bulb by the way. 

During the evaporation also water molecules are issued to the greenhouse air. The lower the RH (relative humidity), the more water can be issued to the greenhouse air, so the more the wet bulb cools off. But if the relative humidity in the greenhouse is high, less water can be issued to the greenhouse air, whereupon the wet bulb will cool off less. In other words: as the humidity increases so will the temperature of the wet bulb.  

The temperature of the dry bulb is always higher than that of the wet bulb, due to the evaporation. The climate computer measures the difference in temperature and can, as such, determine the relative humidity in the greenhouse (because: RH = temperature dry bulb – temperature wet bulb). The bigger the difference, the lower the RV; the smaller the difference, the higher the RV.

It is important to hang an aspirated screen in an adequate location in the greenhouse. The greenhouse climate is not the same everywhere in the greenhouse; some spots are warmer or colder than average and in some places the RH is higher or lower. This can never be entirely prevented. When positioning the aspirated screens, pay attention that you do not hang them in those locations, because they are not representative for the entire greenhouse . The best location is usually the center of the department.  

In order to get proper measuring results, it is important to maintain the aspirated screen on a monthly basis. The sleeve of the wet bulb will dry up as a consequence of the pollution in the water reservoir, that clogs the pores. Therefore, the sleeve should be replace every month (when doing this ensure that the sleeve does not get dirty during replacement) and clean the water reservoir every month. You can do this by using chlorine, but make sure that you thoroughly rinse the reservoir afterwards. Subsequently fill the reservoir with demineralized water (not tap water!) and replenish it during the month to prevent the sleeve from drying out. In order to extend the sleeve's lifespan, you can also use Aquanex aspirated screen fluid. 

Also check if the ventilator is able to rotate. If it makes a lot of noise, then this may indicate wear. In that case replace the ventilator. 

Additionally check if the temperature sensors are functioning properly. you do this by taking off the sleeve, cleaning and drying the sensor and then assess if the sensors still show the exact same temperature. If that is not the case then one of the sensors is probably defective, or the cabling is creating a problem. 

Also have an aspirated screen checked every year to prevent defects.

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