Plant Protection Equipment :: Working Principle :: Crop Protection
Operation principle of spray equipment
Some sort of energy are used for conversion of liquid of spray into drops.
This technique uses many sources of kinetic energy like hydraulic, gaseous and centrifugal.
The type of sprayer and the nozzles or atomisers can be classed depending on the energy employed.
Hydraulic Power Hydraulics is the study of fluids.
A reciprocating pump actuated mechanically by a lever. Compressed to pressurise.
This pressure causes the liquid to be sprayed out of the nozzle as particles.
Energy, gaseous
The blower produces high-velocity air. A dust or liquid is injected into an air flow to be conveyed to the target.
Energy of centrifugation
The spray liquid is atomised by a high speed spinning disc (flat, concave or cage or perforated cylinder) to produce fine spray droplets.
Amount to Spray
In theory, by knowing the appropriate diameter of droplets and desired density of droplets, the smallest volume of pesticide spray per unit area may be computed. Such a computation of optimum droplet density is tricky as efficiency of the droplet is reliant on many other aspects.
However, the following broad hints can serve as a general guide:
5-10 droplets/cm2 – for translocated herbicides .
20 drops/cm2 — for most pesticides and systemic fungi-cides.
50-70 droplets/cm2 – for non-systemic fungicides
In the case of insecticides, optimal biological efficiency requires at least 20 droplets per square centimetre, irrespective of their size.
To achieve efficient deposition, a narrower droplet spectrum is needed to reduce the losses produced by droplets bigger than 300 microns and also by droplets smaller than 100 microns.
The biggest droplets with their high terminal velocity and quick fall are without question the cause of most of the wastage of pesticide.
Even if enormous drops hit the target, they will probably bounce off.
A 400 micron droplet will have 1000 times the dosage of a 40 micron droplet If this is not kept to the aim there will be a lot of wastage.
In practical use, the volume of spray applied is always higher than the value derived from the theoretical calculation. The reasons are the loss of spray volume due to leak and drift, the deposition of spray on non-target areas and also due to unpredictable distribution of droplets on target surfaces .
The spraying efficiency can be calculated as follows:
Spraying efficiency (%) = Minimum spray volume needed X 100%
Actual volume of spray applied
Area for spray application
Must spray the target afflicted by an insect, pest, disease or weed.
The spray application area is in general different from the land area, except for the scenario where pre-sowing treatment is necessary on soil and the land area matches the area to be sprayed.
The area to be sprayed depends on the distance between rows of plants, the spacing between the plants in the same row and the growth of the crop.
This is shown diagramatically.
Calibration of spray
In order to ensure consistent application of pesticide on crops, it is necessary to carry out sprayer calibration activity before undertaking actual spraying work. Sprayer calibration can also be used to lay out the required spray volume.
The spray volume can be measured theoretically by the formula .
But practical method called Area/Volume ( Volume used on marked area) is easier for regular farmer to follow.
Application Rate in Litre per Acre or Hectare = Plot Area X Spray Volume applied to marked area. Marked Area
Droplets of spray
Pesticides are generally applied to the target as spray droplets. The droplets from a hydraulic nozzle are not all the same size.
Sprays have small and coarse droplets. They are characterised by their diameter and density on the target.
Big Drops
narrow strip
Less Leaf Cover
More volume needed for spray
Particles combine and flee
Low penetration into the crop
Less loss by wind, thermal current.
Poor biological activity
Spray pattern in shower.
Fine Droplets
Broader swath
More leaves
Requires less spray volume
Particles don’t stick together and run off
Good penetration into crop
Loss of wind, thermal current more
Good biological efficiency.
Mist-like spray pattern
In order to understand how spray application equipment distributes pesticides to the target, knowledge of the physical qualities and behaviour of droplets is important.
The droplet size and density (number of droplets per unit area of target) are two critical criteria for efficient spraying.
Density and diameter of droplets are critical knowledge to have for efficient usage of insecticides.
The diameter of the droplets in a given spray can be measured as the median of the volume or number of droplets.
Median Diameter by Volume
The Volume Median Diameter (VMD) is defined as that droplet diameter which splits the volume of spray into two equal sections i.e. the volume of spray with droplets of a diameter smaller than VMD is equal to the volume of droplets with a diameter bigger than the VMD.
The numerical median diameter (NMD)
The Number Median Diameter (NMD) is the diameter of the droplet where the number of droplets with diameters larger than the NMD is equal to the number of droplets with diameters smaller than the NMD.
The NMD is usually smaller than the VMD since most pesticide sprays comprise huge numbers of very small droplets.
The VMD is dominated by relatively few large droplets while the NMD is more sensitive to small droplets .
The ratio of VMD and NMD tends to unity with increasing uniformity of droplet size.
In normal course, the spray droplets are spherical in shape.
To ease the calculation and grasp the mathematical reasoning, the droplets may be considered in the shape of a cube instead of a sphere.
Suppose the ideal spraying has been done, so that all the droplets are of similar size in the shape of a cube having all the sides of same dimension say 2 mm.
The volume of a droplet is the total of length, breadth and height multiplied together i.e. a cubic relation.
If the droplet size is lowered from 2 mm to 1 mm, the number of droplets produced from the same volume will be 8 times.
The area occupied by the droplets is the product of length and breath i.e. a square relationship.
As the size of droplet decreases, the contact area of droplet on the target rises.
Hence, the contact area can be doubled for the same spray volume while reducing the droplet size from 2 mm3 to 1 mm3.
To conclude, for the same amount of pesticide if we lower the size of the droplet from 2 mm3 to 1 mm3 (i.e. a factor of two):
Eight times more droplets can be made. Thus the number of droplets increases by the cube of the factor by which the size is reduced.
Double contact area of droplets on targets is possible. Thus, the contact area is increased by the same factor as the reduction in size.
The spray droplet density will be eight times larger, i.e. increases by the cube of the factor of size decrease.
Best Drop Size
Generally, the optimum droplet size for pesticide application is given as a range of droplet diameter,
Optimum droplet size for application of pesticide to Agricultural Pests may not be defined precisely, due to biological complexity of target.
In addition to this, various factors determine the fate of droplets from the time of their production by a nozzle until their deposition on a target such as:
Droplet ejection velocity
Gravitational attraction
Speed of the wind
Air turbulence produced by thermal motion
Liquid volatility spray and
Characteristics of the target surface
For effective application with least contamination of surroundings the most favourable droplet size is most necessary. A 50 micron droplet has 1/1000 Lethal dosage of a 500 micron droplet. Small range of droplet spectrum is important to prevent wastage.
The coarse droplets are dominated by gravity and are significantly less affected by turbulence. Fine drops will be carried away by wind and turbulence and will tend to drift off.
Objective
Size of droplets (Microns)
Insects that Fly
10-50
Insect in leaves
30-50
Leaves
40-100
Application to soil (minimise drift)
250-500
Most often utilised spraying techniques are High Volume (HV), Low Volume (LV) and Ultra Low Volume (ULV) .
The graphic illustrates the range of droplets created by the spraying procedures.
Droplets of sizes more than 300 microns are lost by drip while droplets of sizes less than 100 microns are lost by drift.
HV and ULV spray techniques respectively have higher losses from drip and drift.
Leaf Area Index (LAI)
The leaf area to be treated as a target may be very much larger than the ground area. The Leaf Area Index (LAI) is defined as the Leaf Area divided by the Ground Area.
Leaf Area Index (LAI) = Leaf Area Ground Area
It will differ from crop to crop according on the growth of the plants.
The LAI ratio seldom surpasses 6 or 7, depending on the crop.
This is the reason why per acre need of water in a spray solution varies from crop to crop depending upon total leaf area to be sprayed.
The total leaf area to be covered with droplets is determined by the real pest, i.e. leaf of a weed, a very mobile foliage bug or a non-mobile fungus infection on the leaf and the method of action of the pesticides.
Theoretical droplet density from 1 litre of pesticide sprayed over 1 hectare (assuming uniform dispersion and no loss on non-target areas)
Diameter of droplets
Number of droplets per cm2 at LAI levels 1-7*
1 2 3 4 5 6 7 10 19,100 9,550 6,364 4,773 3,818 3,182 2,717 20 2,380 1,190 795 596 477 398 341 50 153 77 51 38 31 26 22 100 19 9.5 6 5 4 3.2 2.7 200 2.4 1.2 0.8 0.6 0.5 0.4 0.3 400 0.3 0.15 0.1 0.08 0.06 0.05 0.04
The droplet density per cm2 should be divided by two if both the upper and bottom surfaces are examined.
To illustrate the correlations between droplet diameter, droplet density, LAI and spray volume it is assumed that all droplets are of one size, distribution on the target is uniform and no pesticide loss occurs on non-target surface. This is of course never the case in practice.
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