Before getting into the thick of the discourse, it is necessary to ask what is the goal of our treatment. The effectiveness, efficiency, environmental protection and cost effectiveness are the parameters to which we must refer when analyzing the technical aspects that lead to the choice of a machine rather than another one or to the definition of its operating parameters.
This is useful to understand that the uniformity of treatment is the key that determines its effectiveness.
In this sense, we can imagine that as in the varnishing, the distribution of the sanitary product in question must be as homogeneous as possible; but, equally, we consider that unlike the varnishing usually distributed on a surface, here we operate on a volume that should be ‘penetrated’ uniformly in all its points.
“In the presence of large volumes, will meet the greater difficulty with backpack sprayers with motor but without a fan, whose ability to convey the product in the ‘volume’ of vegetation is reduced – said David Facchinetti researcher at the Department of Agricultural Sciences environmental and University of Milan, an expert in agricultural engineering, environmental and systems.
Even lances connected to wheelbarrows, motor pumps or sprayers connected to the tractor by a big pipe, do not lend themselves to the treatment of thick vegetative volumes, without resorting to very high pressures, of the order of several tens if not hundreds of bars, because create a repository for the most superficial of the product.
If, by placing the equipment mentioned above, on decide to use strong pressure, we must consider that the population of droplets generated will be large enough to cause significant losses for direct evaporation, compared to a non-optimal penetration as what might be with a current of air. “
In addition to the vegetative volume, will also be considered the kind of treatment. It will make difference the use of a plant protection cover or a systemic action one that does not generate significant problems of uniformity and, to ensure a good result in terms of cost and effectiveness, requires the maximization of the deposit on the leaf respect to the losses.
In this case, especially working on walls vegetal not particularly deep, as well clarified by Facchinetti, the air current may even be counterproductive. In fact, you can see a penetration of the product that exceeds the vegetable mass and, without going to the target, it disperses.
“Having to treat a vineyard guyot at the beginning of the vegetative phase – explains the researcher -, when there is a reduced number of leaves, it is easier to maximize the foliar deposit with simple vertical bars free of air current, than using a classic mist blower whose current is excessive even minimizing the quantity of air. “
The situation is different when using a covering product such as copper-based or sulfur preparations, as it happens in the treatments contemplated by the biological disciplinary but also in fruit growing and viticulture, where, for reasons of cost, most of the treatments with pesticides are active for contact.
In this case, it becomes essential to have a current of air capable of penetrating the layer of vegetation; current which must be much more powerful the greater the thickness to be treated.
“The advantage of an air flow backpack mist blower, with mechanical pulverization and nozzle, is to be able to adjust the air speed by decreasing it, this thing is not possible with the pneumatic pulverization.
It will be useful, conversely, have a machine with pneumatic working, for example in the treatment of apple trees in pots or on the botrite so as to penetrate, with contact fungicides, deeply into the cluster”.
To everyone his own nozzle
Even in this case, the optimal choice of the type of nozzle comes from careful evaluation of a series of parameters.
“In systemic treatments – explaines Facchinetti – always with the objective of maximizing the percentage of foliar deposit, it is better to choose a greater caliber nozzles to work with a population of larger droplets and less prone to drift or evaporation.
If we talk about treatment coverage, then we’ll have to choose finer nozzles for droplets with diameters between 100 and 300 microns. “
According to the information provided by the researcher, we can say that by using a traditional nozzle associated at the mechanical pulverization and classical techniques as pressure values, we will have a fairly uniform population of droplets with an average size ranging between 100 and 300 microns; while, with pneumatic pulverization machines, the population of droplets will have average size of between 80 to 200 microns – under this value the droplet is not well formed and the machine no longer functions optimally – inversely proportional to the speed of the air whose range of adjustment, however, is rather limited.
“For many types of wall – says researcher – machines with with pneumatic working, also the minimum air velocity is already excessive.
These machines are very subject to drift and not surprisingly, in several states in northern Europe are unsuitable for the bands of respect of several tens of meters required by law. “
In the market for some years, the induction air nozzles, generate droplets that are actually air bubbles which, once in contact with the vegetation, should explode, forming smaller droplets, less prone to drift and capable of better coverage.
“This – clarifies Facchinetti – is only partly true. The same effect of reduction of the drift, in fact, is also obtained using a larger nozzle with a lower pressure capable of generating the same volume with larger droplets.
Anyway, we can affirm that on the mist blower, the nozzle most used is the hollow cone which is able to create a more uniform population of droplets from the dimensional point of view.
In some very rare cases – he specific – and I am referring to machines not aeroassistite, there is the possibility to choose the classic fan nozzle for spraying from which we obtain, however, a population of droplets very uneven and difficult to convey to a target “.
This last choice becomes functional with the spraying bar that having a very large opening angle, 110 degrees with nozzles spaced about 50 centimeters between them, when it passes in horizontal treatments at a distance of 50-60 cm above the vegetation, generates a diagram distribution practically perfect.
“Everything change if we speak of vertical treatments – clarifies Facchinetti -, in this case, the droplets within the opening angle, are larger and heavier and they fall for first while the lateral ones are much more subject to drift.”
Wide also the choice of materials for the nozzles.
Aluminium subject to rapid wear, has been replaced by plastic, much cheaper and suitable for machines ‘low-end’ that operate at pressures below 10 bar.
Brass is an economical choice but low life; between options also steel and, in the end, the ceramic is distinguished by its greater durability.
“Steel and Ceramic – specifies the researcher – on equal pressure and shape of the hole, they generate the same population of droplets; ceramics – he concludes – has the advantage of last twice with a cost absolutely superimposable if not, for certain types of nozzle, lower. “