From the point of view of processing quality, there are two pressure values: permissible (minimum and maximum) and optimal.
Before processing the area, you need to understand what the application rate will be, the selection of the type of sprayer and the choice of nozzles for work depends on it. Obviously, at a rate of 50 liters per hectare, nozzles with a smaller orifice will be needed, and the speed of movement of the sprayer may be higher.
But immediately it should be noted that when the sprayer speeds up more than 20 km/h, fertilizer consumption increases due to wind drift (including swirling streams behind the rods and barrel) and evaporation. Too small drops are very susceptible to these processes. Therefore, purely in terms of droplet size, it is better to choose such a dilution of any preparation in which the working fluid should be made at least 200 liters per hectare. In this case, the size of the droplets allows them to «resist» the upward flow of warm air from the ground, wind, evaporation and fully convey the active substance to the destination. Small droplets of working fluid (up to 200 microns) are literally «money down the drain».
Reducing the drop diameter by 2 times reduces its weight by 8 times. The increase in the weight of a drop increases the possible supply of its kinetic energy. Reducing the size increases air resistance to movement. Therefore, the larger the drops, the more manageable they are, and small ones, on the contrary, behave like an uncontrollable aerosol.
If we are talking about a spout of 500 l/ha, you will have to use other nozzles and reduce the speed of movement in order to have time to pour out the desired volume.
If the pressure at the time of application falls below the permissible minimum, then the spray plume on the sprayer will not appear. The drops simply will run down and will not be able hold onto the surface of the sheet. It should be noted. Large droplets can only be useful for application of liquid fertilizers in the ground.
If the pressure exceeds the permissible maximum, then instead of droplets you will receive an aerosol that does not settle on the leaves, is blown away by the wind (and by an upward air flow - a thermal) and is subject to evaporation. Thus, it is impossible to simply raise the pressure in the system in order to increase the rate of outflow - the working fluid will actually leave the sprayer tank faster, but will not fall on the ground or the leaves.
It is necessary to work in the range between the minimum and maximum allowable values of pressure. Between them is the optimal pressure (for a given application rate). Its value will differ in different models of sprayers.
It is necessary to take into account the fact that to obtain at any pressure strictly identical droplet size is impossible. Rather, we can talk about the spread of the size of the droplets, the range of values that can be in the range from 10 to 700 microns and depends on the type and quality of the sprayer. The average size of all droplets shifts to smaller values with increasing pressure and, conversely, to larger ones with decreasing. Considering such features of fluid behavior, it is not surprising that today there are more than twenty methods for calculating the average size of droplets.
Drops smaller than 50 microns behave like an aerosol. They wiil not contact with the leaves because the wind and ascending air will blow them to the side or hold them in the air until they evaporate.
Size 150-200 already allows drops to settle on the leaves, very well penetrate into the crown of vegetation. They can fly away due to wind and evaporation losses is great.
The optimal size is 200-400 microns, since it is this size that settles well on the leaves and lingers on them. This size is much more resistant to demolition by the wind. Drops larger than 400 microns are already subject to rolling from the sheet. Although drops with size 400-800 microns will not roll due to villi on a sheet of potatoes.
Add a drop of tar in our calculations - immediately after leaving the spray nozzle a drop begins to lose water due to evaporation. Therefore, we can talk about the size of the drop, only at the time of emergence from the dispenser (nozzle). After flying through the air, its size will change unpredictably.
The rate of evaporation of droplets depends on the humidity and temperature. Therefore, in coastal regions and simply at high humidity, the calculations will be alone, and at low humidity, others.
Since the cost of a laser device to determine the size of droplets starts from 600 thousand euros, it is more economical to choose the pressure and number of the nozzles based on previous experience in estimating yield and other factors.
In contrast to the slit, injector nozzles produce droplets with air bubbles. Such drops are useful in that when they come into contact with a leaf, they burst, improving the quality of leaf processing. In addition, the injection sprayers provide a more uniform spectrum in droplet size, they make less of smaller in size drops, which gives a better resistance to wind impact.
The size of the nozzles must be chosen on the basis of the planned rate of outflow and the type of sprayer. An example for the device «Vodoley» can be seen here.
The computer of spraying should monitor when the sprayer is operating to keep the pressure at the desired level - as close as possible to the optimum one. After that, we can expect that on each hectare will be applicated as much as it was planned.