Citrus Experiment Station Mimeo Series No. 60-12
SOME USEFUL INFORMATION FOR CALIBRATING SPRAY
EQUIPMENT FOR APPLYING WEED CONTROL CHEMICALS
Dale W. Kretchman
Citrus Experiment Station
Lake Alfred, Florida
Most weed control recommendations are given as the amount of herbicide to
be used per acre. This requires accurate calibration of spray equipment to
determine the number of gallons being applied per acre, so the correct amount of
herbicide can be added to the spray tank to obtain the recommended rate.
The number of gallons per acre a sprayer will discharge depends upon:
1. Ground speed
2. Nozzle pressure
3. Nozzle spacing on the spray boom
4. Size of the nozzle opening (orifice)
1. Ground Speed.--A speed of 2 to 7 miles per hour (M.P.H.) should
cover the range of most spray work. To determine the speed on
tractors without a speedometer or to check those equipped with
a speedometer the formula below is useful:
M.P.H. = 0.682 x test run in feet
time in seconds
As an example; it takes 40 seconds for a tractor pulling a full
sprayer to cover 250 feet.
: Therefore: M.P.H. m 0.682 x 250 M.P.H. 4.26
; M.P.H. = 4.26
The formula is based upon the fact that movement of 88 feet in
1 minute equals 1 mile per hour.
2. Nozzle Pressure.--Pressures of 20 to 100 pounds per square inch
are generally satisfactory for herbicidal spray applications.
This can be determined by fitting a pressure gauge on the spray
boom near the discharge nozzles.
3. Nozzle Spacing on the Spray Boom.--The nozzle spacing should be
chosen to best fit the job to be done. For broadcast spraying, :
most spray charts published in manufactures' catalogs are based "
upon a nozzle spacing of 20 inches with the boom operated 17 to,/'" .
23 inches above the ground. //I
DEC 8 1959W
CES # 94 7 DWK 10/20/59 ''
4. Size of the Nozzle Opening.--It is possible, with a set of dif-
ferent nozzle sizes, to vary the gallons per acre over wide limits
with only minor changes in speed and pressure.
A formula that can be used to calculate the gallons applied per acre (G.P.A.)
at various speeds, with various nozzle spacings and with various size nozzles
G.P.A. = 5,940 x G.P.M.
I.P.H. x W.
5,940 a constant figure
G.P.M. the rate of discharge per nozzle in
gallons per minute with a specific
nozzle size and pressure. This
figure can be obtained from the cata-
log of the nozzle manufacturer.
M.P.H. miles per hour as calculated previously.
W. spacing of the nozzles on the spray boom
As an example: TeeJet No. 8004 nozzles at 30 pounds pressure deliver
.35 G.P.M. per nozzle. The nozzle spacing is 10 inches and the speed of travel
is 3 M.P.H.
Therefore: G.P.A. = 5940 x .35 ; G.P.A. = 69.3 gallons
3 x 10
Another means of determining G.P.A. is to make a trial run with the equip-
ment over an area of a certain size and determine the number of gallons applied
to that area. Then use this information to calculate G.P.A. To accomplish this,
fill the sprayer full of water, set the tractor throttle for a desirable rate of
speed, adjust the sprayer pressure regulator to deliver a favorable pressure
and spray the pre-measured area. Then measure the amount of water necessary for
refilling the sprayer and calculate the square feet in the area sprayed. This
gives the number of gallons used for spraying the test area at the set speed
and pressure of the equipment.
The G.P.A. can now be calculated:
G.P.A. = 43,560 x number of gallons used in test area
test area size in square feet
As an example: Fifteen gallons of water were used to spray an area 500 feet
long and 10 feet wide with the equipment operating at a set rate of speed and
Therefore: G.P.A. = 43,560 x 15 ; G.P.A. = 130.68 gallons
When the calibration is completed, add the proper amount of herbicide to
the spray tank to obtain the correct rate per acre with the equipment being used.
' 4 947a DWK 10/20/59
Other useful equations:
1. Acres per
2. Spray time
3. Spray rate
in gallons =
STank capacity in gallons
Spray rate in G.P.A.
Spray swath x Speed x 88
in feet in M.P.H.
Spray rate in G.P.A.
Spray time in
minutes per acre
CES # 947b DWK 10/20/59