Sprayer Calibration

The purpose of sprayer calibration is to control the
pesticide dosage by delivering a known volume of spray
liquid, and therefore a fixed amount of pesticide to a
specified area of orchard. To do this, the sprayer travel
speed in miles per hour (mph), pump discharge rate in
gallons per minute (gpm) and area covered (swath =
distance between tree rows in feet) must be held constant.
Any change in these factors affects the application rate.
The following procedures outline the basic steps for
calibrating andadjusting airblast sprayers for maximum
performance, and should be used in conjunction with the
manufacturer's manual for your particular sprayer.

Calculate travel speed.Most tractor speedometers
are geared to tachometers and are not accurate under field
operating conditions. Use a stopwatch and a measured
course to accurately determine the travel speed of your
tractor and sprayer unit:

feet traveled in 1 minute _{= mph}


Calculate pump discharge rate.Calculate the
pump discharge rate (gpm) needed to deliver the correct
number of gallons per acre (gpa) (based on your earlier
calculation of TRV or Table 14) at a fixed travel speed
(mph) and planting distance between rows (swath):

proportions of liquid distribution for large and small trees.
Note that for small trees that are less than 12 feet tall, one
third of the spray volume is delivered to the top half of the
tree. This arrangement is necessary to provide adequate
coverage in the tops of trees without depositing excessive
amounts on lower limbs. The worst possible
arrangement uses the same size nozzle throughout the
manifold. Refer to the sprayer manufacturer's tables on
nozzle performance at various operating pressures and
select the number and size of nozzles needed to obtain the
right output at the selected operating pressure. When
arranging the nozzles in the air stream, make sure that
adjacent nozzle patterns merge before the spray stream
reaches the nearest foliage of the tree canopies.

Figure 1. Proportional distribution of airblast spray required for
good coverage.

Test the Calibration Setup.Nozzle one side of the
sprayer and determine the number of gallons delivered per
minute by operating the machine for 3to 5 minutes and
then measuring the amount of water required to refill the
tank to the starting level. For one side, this should be
one-half the gpm discharge rate determined earlier.
Differences between the desired rate and the measured rate
may require only a slight pressure adjustment. Differences
of more than 10 percent should be corrected by changing
the size of the nozzles. Once you are satisfied with the
performance of one side of the sprayer, nozzle the other
side in a similar fashion and run a final check operating
the full system.

Check for Coverage.Where a major change in
spraying practice is planned, conduct the calibration
procedure in the late summer or fall when the setup can be
tested on trees with a full complement of foliage that
represents the most difficult target for coverage. Spray
targets (cards, film strips, or water-sensitive papers)
should be placed within the tree canopy to assess spray
coverage. For a quick assessment of uniform spray
distribution in the effective airblast and to see whether
travel speed can be adjusted to improve this distribution,
use water sensitive spray paper and clothespins. Clip
papers to small branches in the lower, middle and top
portions of the inside of the tree, then spray with water.
Deposits on the paper should be reasonably uniform from
top to bottom. If they are not, recheck the nozzle
arrangement or the travel speed. Before putting the
sprayer away, make a diagram of the sprayer manifold,
noting the number, location, and size of all nozzles and

gpa x swath x mph

= gpm

If the gpm is already known, then the gpa can be
determined for various orchards and travel speeds by
substituting these values in the following formula:

gpm x 495

= gpa

Define the Effective Airblast.The effective airblast
of a sprayer is that portion of the spray stream actually
involved in covering the trees. It will vary with tree size
and distance between the tree and the sprayer manifold
(Figure 1). Place the machine at the normal spraying
distance from a tree of typical size and shape for the
orchard, and turn off all nozzles not actually used to cover
the tree. The spray stream should just overshoot the top
of the tree by several feet.

Measure the Operating Pressure.The operating
pressure for a sprayer, measured in pounds per square inch
(psi), should be measured at the discharge manifold by
replacing one nozzle with a reliable pressure gauge. This
pressure value will be needed to select nozzles of the
proper size and provide the right discharge rate.
Nozzle the Sprayer.Uniform coverage of large trees
require that two thirds of the spray volume be applied to
the top one third of the tree. Figure 1 shows the

30

1998 New Jersey Commercial Tree Fruit Production Guide