Calibrating a constant-rate sprayer that varies pressure can be more complicated than calibrating a system with a set pressure. The constant-rate sprayer won't turn the boom on until the ATV is traveling fast enough for the selected application rate. The Spray-Mate II controller includes a test speed function for performing stationary calibration, but we wanted to ensure the computer was functioning correctly. You can calibrate the sprayer using the following procedure.

Fill the sprayer tank with clean water. Enter the boom width and target rate into the controller. The boom width is the number of nozzles on the boom times the nozzle spacing in inches. Follow the nozzle manufacturer's recommendation for nozzle spacing and distance from the ground. The target rate is the desired application rate. The target rate should be within the recommended application rate listed on the herbicide label. Use the nozzle manufacturer's data to calculate the approximate rate the sprayer can apply.

The sprayer's output is limited by the flow and pressure of the pump. Remember, the flow and pressure listed by the pump manufacturer do not include line losses. The pressure and flow at the nozzle will be somewhat less than shown on the pump label.

Set the controller in automatic mode and the selector switch to the target rate position. Turn on the sprayer while traveling within the desired travel speed (typically 2 to 4 miles per hour) to verify that the system can apply the target rate. If the system can apply water at the target rate at the low end of the speed range, but can't at the high end, set a lower target rate. If the system can apply the target rate at the upper end of the speed range, but not at the lower end, set a higher target rate.

After adjusting the desired target rate, verify that the controller can lock onto this number at both ends of the speed range. An application rate within 10 percent of the desired target rate is probably acceptable at the upper or lower ends of the speed range. If the target rate is not within label requirements, the speed range or nozzle configuration needs to be changed.

Measure the width of the spray swath in feet while spraying at the target rate. The width should be close to that specified by the nozzle manufacturer.

Lay out a 1∕8-acre test plot (5,445 square feet). Divide 5,445 by the width of the spray swath to calculate the distance that must be traveled to cover 1∕8 acre.

As an example: The Boominator 1400FM boomless nozzle has a spray swath of 22 feet. To cover 1∕8 acre, the sprayer must travel a distance of 5,445/22 feet = 247.5 feet.

Mark the beginning and end of the travel distance, in our case 247.5 ft.

Securely attach a bucket under one spray nozzle and make sure all the nozzle's flow will be directed into the bucket.

While traveling within the desired speed range, turn on the spray boom at the beginning marker. Turn the boom off as you pass the end marker.

Determine the gallons of liquid in the bucket using a liquid measuring container or by weighing the water in pounds and dividing by 8.33 to convert the pounds of water to gallons. Don't forget to subtract the weight of the bucket.

To calculate the gallons per acre, multiply the gallons collected in 1∕8 acre by the number of nozzles times eight.

When you are using more than one nozzle, it is a good practice to verify that the volume sprayed from each nozzle is within 5 percent of the volume sprayed from other nozzles.