6.4.3 Operation and Calibration

With motorised mistblowers, collection of the spray in an air stream is virtually impossible; simply measuring the flow of liquid in the formulation line past the restrictor will always give a substantial (often >30%) under- estimate of operational flow since there is no tank pressure or “suction effect” at the twin-fluid nozzle. 

The flow rate can also vary substantially with the angle at which the nozzle tube is directed (e.g. spraying upwards into trees vs. horizontally into crops). 

Tank pressurisation may thus be inadequate for consistent formulation flow, and we recommend that sprayers should be selected with an independent pump. 

Accurate calibration involves the following procedure:

1. Place the sprayer on a firm horizontal surface and note (or mark) a level in the upper half of the pesticide tank;
2. make sure that the formulation tap is off; fill the tank with clean water (or blank formulation) to the reference level;
3. start the engine and operate at normal operating speed (full throttle);
4. spray normally, with the nozzle directed at a typical working height and angle (on the crop itself if possible), for a measured length of time (usually 2 minutes);
5. turn the engine off and place the sprayer on the same horizontal surface as in (1);
6. using a measuring cylinder, carefully find out how much water is needed re-fill to the reference mark;
7. calculate flow rate F = volume/time (e.g. 700 ml in 2 minutes = 350 ml/min).

The canopy volume of trees and bushes can vary enormously between cocoa fields at different stages of development. This makes single rates for volume application and amount of pesticide inappropriate (recommended mixing rates for chemical pesticide are therefore usually given as a concentration or ratio rather than per hectare). 

Spraying may be confined to a single row and the volume per tree calculated on the basis of the time needed to project spray to all sides of a tree. Sufficient time should be given for the volume of air in each tree to be replaced with the air carrying spray droplets.

An examination has been made on how to improve the dose transfer process with motorised mist-blowers by assessing spray to target efficiency [1]. Two spray techniques, every row and alternate row, were examined at different VAR. Comparative deposition on key biological targets, such as cocoa pods, was measured using a spectrophotometric technique with two commercial food dyes for the different application regimes. 

Based on this work, the most efficient spraying takes place when spray operators are trained to reduce flow rates and walk along every row to improve uniformity of coverage. In practical terms, the reduction in VAR by using flow rates of <0.5x but spraying every row represents a reduction of one tank-load per hectare. 

The reduced cost of chemical and time reduction for tank-filling may well help to mitigate the unquestionable increase in time taken to walk up every row. However, the greatest benefit should be seen as an increase in the efficiency of the spraying and an increase in uniformity of deposition. 

This means that there is a greater likelihood of providing efficacious control of pest populations and therefore increases in productivity and quality.