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Identifying methods of controlling spray drift when using increased boom heights. - PS2020

A number of studies relating to the risk of drift from agricultural crop sprayers have identified boom height as one of the most important variables that must be controlled if the risk of drift is to be minimised. Timeliness is a key feature influencing the efficacy of plant protection products and this is related to sprayer work rate and the range of conditions in which a machine can operate. For a boom sprayer, work rate is a function of spraying speed, boom width, application volume and filling/loading time. The trends towards the use of wider booms and higher spraying speeds to achieve higher work rates can make operations with boom heights of 500 mm or less difficult to achieve. Boom suspension design and the use of automated boom height control systems can improve the opportunities for operating machines with boom heights at or below the threshold value for good drift control of 500 mm. However there are likely to be conditions, particularly involving undulating terrain, when maintaining the correct boom height for spray drift control is very difficult. This project sets out to identify strategies that can be used to minimise the risk of drift when boom heights of greater than those associated with good drift control have to be used for operational reasons.

Spray angle is an important variable in determining the boom height at which a uniform volume distribution pattern will be achieved at crop canopy level. Spray angles of 110 to 120 degrees have become an adopted standard within the industry enabling boom heights of less than 500 mm to be used to give a uniform volume distribution pattern. Results from computer simulation studies have confirmed that, if boom heights of less than 500 mm can be used with 110 degree nozzles then this is a better option for drift control than using a narrower 80 degree spray angle and boom heights of 500 mm or more. However, using 110 degree nozzles at boom heights of more than 500 mm gives substantial increases in the risk of drift. The project will therefore use a combination of wind tunnel, field and computer simulation studies to establish relationships between drift and boom height for different spray angles.

The risk of drift can also be reduced by reducing spraying speed. With a conventional sprayer fitted with a pressure control system, operating at a slower speed reduces the nozzle pressure as well as the air flow around the spray. While previous work has suggested that the drift reductions due to the reduced air flow around the spray are the most important, it is likely that both effects will be a function of spray angle. The work will therefore seek to obtain information about the relationships between spray angle, pressure, boom height and forward speed on the risks of drift. Air-induction nozzles are also an established method of reducing the drift from boom sprayers as demonstrated by the LERAP Low Drift star ratings that have been recognised for this nozzle design. The experimental work will therefore use both conventional and air-induction nozzle designs.

Wind tunnel experiments will involve mounting a five nozzle boom statically in the tunnel and monitoring the airborne spray 5.0 m downwind of the boom with an array of passive sampling line collectors. Experiments with conventional and air induction agricultural flat fan nozzles will examine the effects of boom height, spray angle, air speed around the nozzle and operating pressure. The ranges and combinations of variables to be used in the study will relate to the practical operation of boom sprayers fitted with pressure control systems and a nozzle spacing of 0.5 m. Wind tunnel measurements effectively provide a measure of the droplets detrained from the main part of the spray. The work will relate this detrainment to the risk of spray drift by making direct measurements of drift in field conditions examining the effects of boom height, spray angle, forward speed and nozzle pressure. Computer modelling approaches will also be used to study the relationships between spray detrainment and the risk of drift.

The range of spray angles is limited particularly for many designs of air-induction nozzle. The work will use computer modelling techniques to extend the results from field and wind tunnel experiments and investigate the possible advantages of a wider range of nozzle angles.

The main output from the project will be a decision tree that will help operators select appropriate drift control strategies when boom heights greater than the optimum for drift control have to be used. This will be based on the relationships established in the project work. Results from the project will also add to the understanding of the mechanisms and factors leading to drift from boom sprayers.
The objectives of the work are:

A. To assess the extent to which the increase in spray drift associated with operating spray booms above the optimum height can be mitigated by:

(a) selecting nozzles with an appropriate spray fan angle; and/or
(b) reducing forward speed and operating pressure as would be implemented in a conventional spray rate control system.

B. To define relationships between spray fan angle, boom height forward speed, nozzle pressure and the risk of drift from different nozzle designs integrated into a decision tree that will enable drift to be controlled when operating conditions require a boom height above the optimum to be used.

The technical and scientific aims of the research are therefore:

(1) To establish relationships between spray detrainment, measured in wind tunnel conditions, and the sprayer operating variables of boom height, forward speed and spraying pressure for both conventional and air-induction nozzles having a range of spray fan angles;

(2) To obtain information about the relationship between detrainment as measured in wind tunnel conditions and drift risk under field conditions from data obtained with selected combinations of boom height, spray fan angle, forward speed, nozzle designs and operating pressures;

(3) To establish the risk of drift for boom sprayers operating with different nozzle designs, spray fan angles, boom heights, forward speeds and operating pressures using computer modelling approaches;

(4) From an analysis of the results from wind tunnel, field experiments and modelling studies:

(i) provide a decision tree that could be used to provide guidance relating to the control of drift when boom heights above the optimum have to be used;

(ii) define relationships between spray angle, boom height and the risk of drift for different nozzle designs and operating conditions that can be used as part of the decision tree above and in drift risk assessments.

Project Documents
• FRP - Final Report : PS2020 final report   (337k)
Time-Scale and Cost
From: 2008

To: 2009

Cost: £74,887
Contractor / Funded Organisations
Silsoe Spray Application Unit
Agricultural Land              
Decision Support Tools              
Environment and Health              
Environmental Impact              
Environmental monitoring              
Environmental Protection              
Equipment Design              
Monitoring and evaluation              
Pesticide use              
Fields of Study
Pesticide Safety