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All methods provide the same answer – a sprayer calibration value expressed in gallons per acre.  This refers to the amount of spray mixture needed to spray one acre.  A reasonable sprayer output is 15 to 30 gallons per acre.  The formula used to calibrate a broadcast sprayer is:

GPA = (GPM*5940)/(speed*nozzle spacing)

The definitions of terms in this equation are:

GPA - Sprayer output in gallons/A.

GPM – Average flow rate per nozzle in the spray boom in gallons/min.

5940 – Mathematical constant that is the result of many unit conversion factors.  (Accept this constant as is.  Trust me – too much explanation will be pointless and overly confusing.)

Speed – Calculated ground speed of the ATV in miles/hour (refer to the first article in this series).

Spacing - Spacing of nozzle tips along the spray boom expressed in inches (refer to the first article in this series).

PROCEDURE

Step 1.  Several items are needed for calibration.  A device to accurately measure water in fluid ounces (or milliliters) is needed.  For example, an inexpensive clear plastic measuring cup with graduations in fluid ounces and total capacity of about 16 fl.oz. will work.  For safety reasons, dedicate this measuring device exclusively for sprayer use.  Several containers are needed to collect spray output from each nozzle tip.  Disposable drink cups or metal cans will work, but for safety reasons these should be discarded after use.  A watch with a stopwatch feature is needed to accurately measure time in seconds.  Finally, a handheld calculator is needed for the mathematical computations.

Step 2.  Assemble and mount the sprayer to the ATV (refer to the first article in this series).  Fill the spray tank with water alone (no herbicides).  With the ATV stationary, turn on the sprayer and adjust the pressure to the desired level, if pressure adjustments are possible.  Run the sprayer for a minute or so to make sure that spray is being discharged from all tips and there are no leaks in the lines and couplings.  Use the cups to collect the spray from each nozzle tip along the sprayer boom for one minute.  Measure the output volume from each nozzle tip and calculate the average output across all nozzle tips.  Convert the average output to gallons/minute using either of the following conversion factors:

(fl. oz./min)/128 = gal./min.

or

(ml/min.)/3785 = gal./min.

Other useful conversion factors are listed in Table 1.

Step 3.  Substitute the appropriate terms into the calibration equation.  It is essential that each term in the equation be in the correct units.  GPM must be in gallons/minute.  Speed must be in miles/hour.  Nozzle spacing must be inches.  Using the wrong units will result in gross errors in calibration.

Step 4.  Check your work, by repeating the calibration procedure and all computations.

EXAMPLE

A sprayer is assembled and mounted on an ATV.  A comfortable and repeatable throttle setting has been determined and with those settings, the ground speed of the ATV was found to be 3 miles/hour.  The sprayer is configured with seven TeeJet® 8002 flat fan spray tips spaced 19 inches apart along the boom.  The sprayer pump does not have adjustable pressure, so we use whatever pressure the pump generates.

With the spray tank filled with water and pump operational, we run the sprayer for one minute and collect the following volumes from each nozzle:

Notice the output collected from Nozzle 5.  Compared to the other sprayer tips, something is obviously wrong.  The nozzle assembly (nozzle tip, strainer, and nozzle body) is disassembled and a piece of metal filing is removed from the nozzle tip.  The process is repeated and gives the following results:

The output from all nozzle tips is now similar, less than 10% variation among all seven tips.  This is acceptable.

            The next step is to calculate a simple average output per nozzle tip on the sprayer boom.

(26 + 25 + 25 + 27 + 26 + 27 + 25)/7 = 25.86 fl. oz./min.

Convert the output from fl.oz./min. to gal./min.

(25.86 fl. oz./min.)/(128 fl. oz./gal.) = 0.2020 gal./min.

(This answer is one of the critical terms in the calibration equation.)

Refer back to the original calibration equation and begin substituting.

GPA = (GPM*5940)/(speed*spacing)

GPA = (0.2020*5940)/(3*19)

GPA = 1200/57

GPA = 21 gal./A

The sprayer in this example is now calibrated.  It will apply 21 gallons of water uniformly over one acre.  This value is used in the next step of sprayer calibration – mixing, dilution, and application.  That is the topic of the next article in this series.

Table 1.  Useful conversions for sprayer calibration.