Paul
D. Ayers, Bruce Bosley
Colorado State University Cooperative Extension
Quick
Facts
- Inaccurate
pesticide application rates, spray patterns and
droplet size can lead to the movement of pesticides
from the targeted area and reduce pesticide effectiveness.
- The
first step in sprayer calibration is to determine
the correct nozzle type and size.
- Nozzle
material selection is important in reducing inaccurate
applications due to nozzle wear.
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Due to timeliness and effectiveness, chemical pesticide application
has become a leading method of weed and insect control in U.S.
agricultural production. The continued use of pesticides in
the agricultural industry has led to concerns of chemical trespassing
by groundwater contamination or drift.
Inaccurate
pesticide application rates, spray patterns and droplet size
can lead to pesticide movement from the targeted area and
reduce the effectiveness of the pesticide. A recent study
in Nebraska revealed that two-thirds of the applicators were
applying pesticides improperly (application rate errors greater
than 5 percent). A similar study conducted in North Dakota
indicated that 60 percent of tested sprayers had calibration
errors greater than 10 percent. Although inaccurate tank mixing
causes some of these errors, a majority of the problems result
from improper spray equipment calibration and worn nozzles.
The first step in sprayer calibration is to determine the correct
nozzle type and size (flow rate). Flat-fan nozzles are used
for broadcast spraying of most herbicides and some insecticides
where a medium droplet size distribution is obtained. Flat-fan
nozzles are used for banding herbicides. Flooding type and full
cone nozzles used for pre-plant herbicides produce drift resistant
large droplets, and wide nozzle spacing can be used. Hollow
cone nozzles produce smaller droplets and are used to apply
insecticides and contact herbicides that need to penetrate the
canopy.
Inaccurate
applications can be due to nozzle wear, therefore it's important
to select the correct nozzle material. Wear-resistant materials
such as tungsten, carbide, ceramic and hardened stainless
steel help nozzles maintain a constant flow rate after a long
period of usage. Nozzles made from less durable materials
(plastic, brass) demonstrate increased flow rates after only
a short period of spraying. For example, after 50 hours of
spraying, a brass nozzle can exhibit an increased flow rate
of 10 to 15 percent, whereas a hardened stainless steel nozzle
will increase only about 2 percent. The increased flow rates
result from an increased nozzle orifice area. The added cost
to purchase a more durable nozzle can pay for itself many
times over by reducing over-application that results from
nozzle wear.
Nozzle
size depends on the desired application rate, ground speed
and nozzle spacing. For each nozzle type and spray angle,
the manufacturer recommends spray height and nozzle spacing.
Nozzle spacings of 20 and 30 inches are most common. The desired
flow rate from the nozzle can be determined from the following
equation:
GPM
= (GPA x MPH x w) / 5940
where:
GPM
is the nozzle flow rate in gallons per minute,
GPA is the application rate in gallons per acre,
MPH is the ground speed of the sprayer in miles per
hour (MPH = (ft/min)/88), and
w is the nozzle spacing in inches for broadcast
spraying.
Spray Rig
Preparation
- Clean
spray rig thoroughly. Check for signs of rust, leaks or
other problems.
- Determine
gallons needed to apply per acre based on recommended rate
from pesticide label, tank size, pesticide container size,
and rate of pesticide application per acre.
- Calculate
a rough estimate of nozzle application rates based on planned
application speed and boom pressure.
- Check
all nozzles on spray boom for signs of wear and nozzle size.
Replace worn nozzles and nozzles of the wrong size for the
desired application.
- Half-fill
spray tank with water and go to prepared field.
One Way to Calibrate
a Sprayer
- Measure
ground speed of rig with sprayer implement in place. (Average
travel time of tractor in seconds over 300 feet in the field
for two separate passes.)
- Calculate
ground speed.
- Measure
distance in inches between spray nozzles on the boom.
- Calculate
desired nozzle output (ounces or gallons).
- Catch
one minute's worth of water from one or two nozzles at the
operating pressure.
- Adjust
pump pressure or ground speed until desired output is reached.
- Calculate
acreage covered on one tank of spray mixture.
- Finish
filling spray tank with pesticide and carrier (usually water).
Apply about one-half tankful of spray and determine if the
correct amount of acreage has been covered.
- Continue
spray application; recalibrate if the first half tankful
didn't cover the correct acreage.
Example
The field is prepared and spray tanks, booms and nozzles have
been cleaned and checked. The pesticide label recommends that
1 quart per acre of chemical and a minimum application of 10
gallons of mixture per acre be applied. The pesticide comes
in 2-1/2 gallon containers; the spray tank holds 350 gallons.
Three hundred gallons can be applied before refilling.
In this
situation, applying pesticide to 30 acres with one tankful
would be in compliance with the label. Solid applied herbicides
generally work better with larger volumes of spray mixture.
One full container of chemical will cover 10 acres. If 15
gallons of carrier per acre are applied, the applicator would
get 20 acres per refill and use two containers of pesticides.
The
tractor with spray rig is set as if spraying the first 300-foot
pass in 42.5 seconds. The second pass is a bit faster at 42.7
seconds. The average time is 42.6 seconds.
MPH
= 300 ft / (1.47 x 42.6 sec) = 4.8 MPH
Spray
nozzles are spaced at 30 inches. Using the formula acreage
output rates to nozzle output, application will be approximately
0.364 gallons per minute per nozzle.
GPM
= (15 GPA x 4.8 MPH x 30 inches) / 5940 = 0.364 GPM
Experience
shows that the pump can handle this volume and nozzles are
rated for this application. Field application is now ready.
The
nozzle output now can be checked at the field's edge. Once
adjustments are made and each nozzle checks within 5 percent
of the desired output, fill the tank with pesticide and water.
Ten
acres should be covered by the time the half-tank level is
reached using the example above.
A standard
nozzle with a flow rate of 0.4 gallons per minute at 40 psi
is easy to obtain. The 15-gallon per acre application rate
can be obtained by operating the sprayer at the recommended
40 psi and a higher ground speed.
MPH
= (GPM x 5940) /( GPA x w)
= (0.4 GPM x 5940) / (15 GPA x 30 in)
= 5.3 MPH
Or the ground
speed can be kept constant at 4.8 miles per hour, and the nozzle
pressure reduced using the following relationship.
Pressure
new / pressure rated =
(GPM desired / GPM rated)2 =
Pressure new / 40 psi =
(.364 GPM / .4 GPM)2
The new
pressure is 33 psi. Use small adjustments in pressure to obtain
the desired nozzle flow rate within the recommended operating
pressure. Operating a nozzle at excessively high pressures will
produce small spray droplets susceptible to drift. Operating
at excessively low pressures produces larger, less- effective
spray droplets and poor spray pattern uniformity down the length
of the boom.
If calibrating
with water and spraying solutions that are heavier or lighter
than water (8.3 pounds per gallon), conversion factors must
be used (Table 1). In the
example above, to obtain a nozzle flow rate of 0.364 GPM with
a solution that weighs 10 pounds per gallon, the nozzle should
produce 0.364 GPM x 1.10 or about 0.40 GPM when spraying water.
After all the adjustments are made, fill the sprayer with water
and measure the nozzle flow rates by catching the nozzle output
for 1 minute. Divide the number of ounces by 128 (128 ounces
in a gallon) to obtain the flow rate in gallons per minute.
For example, 67 ounces caught in 1 minute produces a flow rate
of 67/128 or 0.52 GPM. Another method of measuring nozzle flow
rates is with a spray tip tester. Maintaining the desired application
rate is essential. Over-application results in wasted pesticide,
potential groundwater contamination and possible crop injury.
Under-application can produce ineffective pest control.
Erroneous
flow rates can result from damaged, worn or plugged nozzles
or strainers, and spray hose restrictions between the pressure
gauge and the nozzle. Clean nozzles with a toothbrush, not
a pocket knife. Never blow out a nozzle with the mouth.
Check
the pressure along the length of the boom. If a large pressure
difference is found, look for restrictions or install a larger
diameter spray hose (see Table
2). An accurate pressure gauge is worth the extra cost.
Conduct field calibration when spraying the pesticide. Start
with the tank full of solution, spray a known distance in the
field (at least 3000 feet) and determine the number of gallons
needed to refill the tank. Determine the application rate (GPA)
with the following formula. GPA = (gallons sprayed x 43.560)
/ (Boom width (ft.) x distance (ft.))
Spray distribution uniformity is important for broadcast spraying.
Uniform spray coverage eliminates weed streaking and crop injury.
Concentrations up to four times the recommended amount can result
from non-uniform applications. To obtain even coverage, make
sure all the nozzles are the same and are equally spaced along
the boom. Check each nozzle to make sure the flow rates are
correct. Replace nozzles if the flow rates are in error of 10
percent. The boom height should be adjusted to the recommended
height (Table 3). Spray boom
bounce should be minimized with support members.
Check
spray uniformity by spraying water on a concrete surface and
observing the amount of streaking that occurs when the water
dries. Spray patterns that result in excessive accumulation
below the nozzle are produced by:
- nozzle
wear,
- low
boom height,
- low
operating pressure, and
- large
nozzle spacing.
Irregular
spray patterns result from damaged nozzle tips, mismatched nozzles
and uneven booms.
Pesticide
drift is a major concern of land application of pesticides.
In addition to reduction in effectiveness, pesticide drift
can damage non-target areas. One method to decrease drift
is to use a low volatile formulation that is less likely to
volatize and drift.
Pesticide
drift also can be controlled by reducing the number of small
droplets emitted from the sprayer. Nozzle type, angle and
orientation, boom height, and operating pressure can influence
the production of driftable drops. A droplet of 100 microns
in diameter can drift about 50 feet in a 3-mile per hour breeze
where a 10 micron droplet can drift 3000 feet. Spray thickeners
can reduce drift as well as spraying at low temperatures and
high humidity.
1 acre = 43,560 square feet
1 gallon = 128 fluid ounces
1 pint = 16 fluid ounces
1 pound = 16 ounces of weight. (16 fluid ounces of water at
39 degrees Fahrenheit weighs 1 pound)
Gallons
per acre = (5940 x gallons/minute/nozzle) / (MPH x nozzle
spacing)
Gallons per minute per nozzle = (gallons/acre x MPH x nozzle
spacing) / 5940
Ounces per minute per nozzle = (gallons/acre x MPH x nozzle
spacing x 32) / 1485
Miles
per hour = distance traveled (ft) / (88 x minutes) = distance
traveled (ft) / (.47 x seconds)
Table
1.
Spraying solution conversion factors
Weight
of solution |
Specific
gravity |
Conversion
factors |
7.0
lbs. per gallon |
.84 |
.92 |
8.0
lbs. per gallon |
.96 |
.98 |
8.3
lbs. per gallon* |
1.00 |
1.00 |
9.0
lbs. per gallon |
1.08 |
1.04 |
10.0
lbs. per gallon |
1.20 |
1.10 |
11.0
lbs. per gallon |
1.32 |
1.15 |
12.0
lbs. per gallon |
1.44 |
1.20 |
14.0
lbs. per gallon |
1.68 |
1.30 |
*
Suitable for most water soluble pesticides. |
Table
2.
Pressure drop through various hose sizes
Flow
in GPM |
Pressure
drop in PSI (in 10-foot length) without couplings |
1/4"
I.D. |
3/8"
I.D. |
1/2"
I.D. |
3/4"
I.D. |
1"
I.D. |
0.5 |
1.4 |
.2 |
--- |
--- |
--- |
1.0 |
--- |
.7 |
--- |
--- |
--- |
1.5 |
--- |
1.4 |
.4 |
--- |
--- |
2.0 |
--- |
2.4 |
.6 |
--- |
--- |
2.5 |
--- |
3.4 |
.9 |
--- |
--- |
3.0 |
--- |
--- |
1.2 |
--- |
--- |
4.0 |
--- |
--- |
2.0 |
--- |
--- |
5.0 |
--- |
--- |
2.9 |
.4 |
--- |
6.0 |
--- |
--- |
4.0 |
.6 |
--- |
8.0 |
--- |
--- |
--- |
.9 |
.3 |
10.0 |
--- |
--- |
--- |
1.4 |
.4 |
(1)
From Spraying Systems. |
Table
3.
Nozzle height (inches) for flat-fan nozzles
Spray angle (degree) |
20-inch spacing |
30-inch spacing |
65 |
21-23 |
32-34 |
73 |
20-21 |
27-29 |
80 |
17-19 |
25-27 |
110 |
10-12 |
14-18 |
Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
Service
in Action
5.003
,
Cooperative Extension, Colorado State University. Published
December 1992. Reviewed September 1992. Copyright 1992. For
more information, contact your county Cooperative Extension
office.
Paul D. Ayers, Colorado State University Cooperative Extension
agricultural engineer, agricultural and chemical engineering;
and Bruce Bosley, Cooperative Extension County Director,
Morgan County Cooperative Extension.
Issued in furtherance of Cooperative Extension work, Acts
of May 8 and June 30, 1914, in cooperation with the U.S.
Department of Agriculture, Milan Rewerts, interim director
of Cooperative Extension, Colorado State University, Fort
Collins, Colorado. Cooperative Extension programs are available
to all without discrimination. No endorsement of products
named is intended nor is criticism implied of products not
mentioned.
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