Drift Causes and Control

When applying crop protection chemicals, spray drift is a term used for those droplets containing the active ingredients that are not deposited on the target area. The droplets most prone to spray drift are usually small in size, less than 200 µm micron in diamter and easily moved off the target area by wind or other climatic conditions. Drift can cause crop protection chemicals to be deposited in undesirable areas with serious consequences, such as:

  • Damage to sensitive adjoining crops.
  • Surface water contamination.
  • Health risks for animals and people.
  • Possible contamination to the target area and adjacent areas or possible over-application within the target area.

    Causes of Spray Drift

A number of variables contribute to spray drift; these are predominantly due to the spray equipment system and meteorological factors.

  • Droplet Size

    Within the spray equipment system, drop size is the most influential factor related to drift.

    When a liquid solution is sprayed under pressure it is atomized into droplets of varying sizes: The smaller the nozzle size and the greater the spray pressure, the smaller the droplets and therefore the greater the proportion of driftable droplets.

  • Spray Heights

    As the distance between the nozzle and the target area increases, the greater impact wind velocity can have on drift. The influence of wind can increase the proportion of smaller drops being carried off target and considered drift.

    Do not spray at greater heights than those recommended by the spray tip manufacturer, while taking care not to spray below the minimum recommended spray heights.

  • Operating Speed

    Increased operating speeds can cause the spray to be diverted back into upward wind currents and vortexes behind the sprayer, which trap small droplets and can contribute to drift.

    Apply crop protection chemicals according to good, professional practices at maximum operating speeds of 4 to 6 mph / 6 to 8 km/h (with air induction type nozzles - up to 6 mph / 10 km/h). As wind velocities increase, reduce operating speed.*

    *Liquid fertilizer applications using the TeeJet tips with very coarse droplets can be performed at higher operating speeds.

  • Wind Velocity

    Among the meteorological factors affecting drift, wind velocity has the greatest impact. Increased wind speeds cause increased spray drift. It is common knowledge that in most parts of the world the wind velocity is variable throughout the day (see Figure 1). Therefore, it is important for spraying to take place during the relatively calm hours of the day. The early morning and early evening are usually the most calm. Refer to chemical label for velocity recommendations. When spraying with the traditional techniques the following rules of thumb apply:

    In low wind velocity situations, spraying can be performed at recommended nozzle pressures.

    As wind velocities increase up to 17 mph (3 m/s), spray pressure should be reduced and nozzle size increased to obtain larger droplets that are less prone to drift. Wind measurements should be taken throughout the spraying operation with a wind meter or anemometer. As the risk of spray drift increases, selecting designed to more coarse droplets that are less prone to drift is extremely important. Some such TeeJet nozzles that fit into this category are: DG TeeJet, Turbo TeeJet, AI TeeJet, Turbo TeeJet Induction, and AIXR TeeJet.

    When wind velocities exceed 5 m/s, spraying operation should not be performed.

  • Air Temperature and Humidity

    In ambient temperatures over 77°F/25°C with low relative humidity, small droplets are especially prone to drift due to the effects of evaporation.

    High temperature during the spraying application may necessitate system changes, such as nozzles that produce a coarser droplet or suspending spraying.

  • Crop Protection Chemicals and Carrier Volumes

    Before applying crop protection chemicals, the applicator should read and follow all instructions provided by the manufacturer. Since extremely low carrier volume usually necessitates the use of small nozzle sizes, the drift potential is increased. As high a carrier volume as practicial is recommended. 

Figure 1.
Development
of wind velocity,
air temperature,
and relative air
humidity (ex.).
From: Malberg


    Application Regulations for Spray Drift Control

In several European countries, regulatory authorities have issued application regulations in the use of crop protection chemicals to protect the environment. In order to protect the surface waters and the field buffer areas (examples are: hedges and grassy areas of a certain width) distance requirements must be kept because of spray drift. Inside the European Union (EU) there is a directive for the harmonization of crop protection. In this respect the procedures that have been implemented in Germany, England, and the Netherlands will be established in other EU countries in the coming years.

To reach the objectives for environmental protection, spray drift reducing measures have been integrated as a central instrument in the practice of risk evaluation. For example, buffer zones may be reduced in width if certain spraying techniques or equipment is used that have been approved and certified by certain regulatory agencies. Many of the TeeJet nozzles designed for reducing spray drift have been approved and certified in several EU countries. The certification of those registrars fits into a drift reduction category, such as 90%, 75%, or 50% (90/75/50) control of drift. This rating is related to the comparision of the BCPC reference nozzle capacity of 03 at 3 bar (43.5 psi).


  

Figure 2.
Dv0.1 for XR, DG,
and TT nozzles
with lower limits
of 10% DV for
various wind speeds

 

*Dv0.1 of the
XR11002 at 36 psi.


    Nozzles for Spray Drift Control

Drift potential can be minimized even when it is necessary to use small size nozzles by selecting the appropriate style. Nozzles such as the Turbo TeeJet (TT), Air Induction TeeJet (AI), and the Drift Guard TeeJet (DG) produce medium to coarse sprays even in the smaller sizes. Large size droplets are much less susceptible to drift, but in some cases target coverage may be reduced due to a reduction in the number of drops. This needs to be taken into account, especially when using contact crop protection chemicals.

Wide angle flat spray nozzles with pre-orifice technology can achieve a larger drop size range at equal pressures without a reduction in flow rate. The DG, AI, TT, TTI, and AIXR incorporate pre-orifice technology, which performs the primary flow metering function. THe larger exit orifice provides secondary metering and pattern formation (see Figure 2).

Venturi-type nozzles, such as the AI, TTI, and AIXR, use a pre-orifice to create a high velocity liquid stream and then draw air into the stream through a side opening. This mixture of air and liquid is then discharged at a low exit velocity thus creating very coarse droplets with air inclusion. However, air-filled droplets only occur with chemicals containing a sufficient concentration of surfactants.

Figure 3 demonstrates the difference in droplet sizes between the TeeJet XR, DG, and TT nozzles on the basis of VMD. From this figure, the following conclusions can be drawn:

  • The DG nozzle in comparsion to the XR achieves 30% higher VMD (Dv0.5) values. However, as the pressure increases, the percentage difference decreases.
  • The TT nozzle achieves about 10-20% higher VMD (Dv0.5) values than the DG at equal pressures.
  • The VMD (Dv0.5) values for the TT nozzle at 15 psi (1.0 bar) pressure is about 70% higher than the XR.


  

Figure 3.
VMD achieved by
XR, DG, and TT nozzles
relative to pressure.

Measurement conditions:
• Laser-Doppler Test 
• Continuous PDPA
   measurement over 
   entire flat spray width
   (PDPA) 
• Distance 20"
   (measured along
   spray tip axis) 
• Water temp. 70°F


    Summary

Drift can be managed successfully with the right knowledge of the equipment and the factors influencing drift. Every application must be balanced between managing drift and maintaining effective crop protection. Below is a list of factors that must be considered to ensure a safe, accurate spray application.

  • Spray Pressure
  • Nozzle Size
  • Application Rate
  • Spray Nozzle Height
  • Operating Speed
  • Wind Velocity
  • Air Temperature and Relative Humidity
  • Buffer Zones (safe distances from sensitive areas)
  • Instructions from the Crop Protection Chemical Manufacturer

Having taken into account all the variables that can have an impact on the drift potential, it may still be necessary to consider the use of drift control nozzles, such as the AI, TTI, or AIXR.

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