Field sprayer calibration steps

Posted: June 9, 2005

• The basics of sprayer calibration
• The art and science of assessing nozzle wear
• More efficient spray application saves money

The key operations to check in calibrating a field sprayer include pressure gauge accuracy, flow rate of nozzles, and tractor speed, says Brian Storozynsky, field sprayer technologist.

Regardless of whether sprayer systems are equipped with pressure gauges or electronic auto rate controllers or a combination of two the performance of various components needs to be confirmed.

Pressure Gauges: For sprayers not equipped with auto rate controllers, producers should routinely check the pressure gauge, says Storozynsky. Just because the gauge in the tractor cab says 40 psi, for example, that doesn't mean 40 psi is being delivered to the nozzles on the boom.

He recommends producers use a second pressure gauge that's only used for calibration purposes. This gauge should be attached to the spray boom on a fitting located near a nozzle, preferably on each boom section. Doing this provides the operator with an indication of pressure at the nozzles and across the entire boom width. The simple process is to pressurize the system and see if the calibration pressure gauge reads the same as the pressure gauge in the tractor cab.

"If the calibration gauge reads only 35 psi, for example, while the tractor-cab gauge is 40 psi, a quick fix is to increase the pressure at the cab to 45 psi," says Storozynsky. "But, several things should be checked before doing so. The difference between the pressure gauge readings could be due to different factors."

First, the tractor-cab gauge may simply be out. To check, tee the calibration gauge near the tractor-cab gauge and compare. If it shows a similar reading, continue checking.

Second, a pressure loss between the tractor-cab pressure gauge tap and nozzle may exist. The pressure loss may be due to an inadequately sized hose, a pinched hose, or loss through nozzle (line) check and diaphragm valves.

And, third, the tractor-cab pressure gauge tap, the actual location of the source for pressure, may be in a location that does not adequately indicate nozzle pressure.

"Nozzle pressure is seldom measured at the nozzle due to the cost and inconvenience of having a pressure gauge for each boom section on the sprayer," he says. "Most sprayer manufacturers place the pressure gauge tap upstream in a manifold or main hose feeding the individual boom sections.

"It is up to the operator to find out what's causing the pressure loss," he says. "Without an auto rate controller, the operator must check several things. With an auto rate controller, the difference may still be one of the problems noted above, but is easily corrected or adjusted because of the monitor outputs."

Auto Rate Control Systems: Sprayer operation is easier to monitor with an auto rate controller. However producers need to confirm at least once per season that pesticide is being applied at the proper rate and pressure.

If nozzles are worn, the auto rate controller will compensate to ensure flow rate is maintained. But, even that should be checked, says Storozynsky.

"A controller programmed to maintain an application rate of 112 L/ha, for example, will maintain that rate even if nozzles are worn, but the indicated pressure will be lower," he explains. "So instead of 40 psi, the pressure drops to 35 psi. You're getting the proper flow rate, but with some nozzle types, coverage and spray pattern may be affected because the pressure is reduced."

Check tractor speed

Several Internet Web Sites including Ontario Ministry of Agriculture and Food (OMAF) Website at http://www.gov.on.ca/OMAFRA/english/crops/sprayer/ep75.htm ; Colorado State University Web site at: http://www.ext.colostate.edu/pubs/farmmgt/05003.html ; and North Dakota State University Web Site at: http://www.ext.nodak.edu/extpubs/ageng/machine/ae73-1.htm all explain how to measure tractor speed as part of the calibration process.

Common steps to confirm tractor speed:

• Place 2 stakes 50 metres apart in the field
• Select the gear and throttle setting (r.p.m.) which will be used to spray
• Fill the sprayer half full of water.
• Drive the distance between the stakes three times, timing each pass. Each time, make sure the tractor is at the desired speed as the first stake is passed. Keep driving at this speed to reach the second stake.
• Take the average time of the three passes.

Nozzle output

If the boom pressure is correct and the tractor travel speed correct, check the output of the individual nozzles. Checking every nozzle may not be practical during the spraying season, says Storozynsky, noting it would mean checking 54 nozzles on an average 90-foot boom, or 72 nozzles on a 120-foot boom.

But take a random sample of at least 15 to 25 percent of the nozzles along the boom. To measure the average nozzle output:

• Park the sprayer and install the calibration gauge at or near the nozzle body or line.
• Engage the pump.
• Adjust the pressure regulator until the calibration gauge reads 40 psi (276 kPa). For sprayers with control monitors, program the monitor to deliver the nozzle manufacturer's rated outputs. And note the pressure, flow rate, speed and application rate prior to and after collecting the flow from several or all the nozzles.
• In a measured beaker or other container, collect the output from each nozzle for one minute.
• Record the amount produced by each nozzle on a paper record, sum the numbers and divide by the number of nozzles tested. This will give the average flow rate for the nozzles on the sprayer, even if all were not tested. The total flow from multiplying the number of nozzles with the average flow should match the flow rate indicated on the monitor.
• Use this average in the application rate formulas to calculate an application rate at a desired spraying speed or conversely to calculate speed at a desired application rate. Again, this application rate should match the rate shown on the monitor.
• If not testing every nozzle, a visual inspection is important too. Look at each nozzle for a consistent spray pattern. Clean nozzles showing streaky spray patterns or replace if cleaning does not help.
• If the monitor flow, application or pressure do not match the calculated rates and measured flows, then the flow or pressure sensor calibration number should be reprogrammed to indicate the calibrated values.
• With new nozzles, replace any that are more than five percent above or below the manufacturers rated output.

Sprayer output

Once pressure, speed and individual nozzle output are checked, calculate sprayer output to determine litres/hectare, litres/acre or gallons/acre, depending on the preferred unit of measure. A couple of different formulas are used for calculating sprayer output.

Also, Internet Web-based calculators have been developed to help producers crunch the numbers. The Ontario Ministry of Agriculture and Food (OMAF) has developed an Web- based calculator, which walks producers through the steps of sprayer calibration and handles many of the required calculations. It has earned good reviews as a useful tool. The calculator can be found at: http://www.gov.on.ca/OMAFRA/english/crops/sprayer/ep75.htm

Important figures in this calculation include an average of nozzle output and the nozzle spacing on the boom. It can vary, but commonly is 20-inch spacing or .508 metres. Again, the OMAF web-based calculator speeds up the process.

Here's the OMAF formula for calculating sprayer output:

Sprayer Output	=	Average Nozzle Output (mL) x 2
(litres/hectare)		Nozzle spacing (m)

At the Alberta AgTech Centre the calculation uses a constant factor of 600 as part of the formula:

Sprayer Output = 600 x litres/minute (collection average)
(litres/hectare)        Nozzle spacing x km/hr 

(To get sprayer output in litres/ac using a factor of 240 for the calculation instead of 600.)

Sprayer Output = 240 x litres/minute (collection average)
(litres/acre)		         nozzle spacing x km/hr

"The formulas are normally used to calculate application rate after measuring nozzle flow rates," says Storozynsky. "But they can also be used to calculate ground speed for a desired application rate, or to select a nozzle size (flow) for a desired application rate and ground speed. This is done by rearranging the formula."

For example, for ground speed, the formula would be:

 km/h = (600 x L/min) / (nozzle spacing (m) x L/ha).

For nozzle size, the formula would be:

 L/min = (L/ha x km/h x nozzle spacing (m)) /600.

Area check

As a check against the actual calibration, producers should also compare the volume of spray mixture applied to a given area.

Re-check the actual sprayer calibration after each tank of spray is applied by dividing the volume sprayed by the actual area sprayed. The nature of some products may slightly alter the calibration from that of clean water.

Conversion figures

Growers who prefer to measure in litres/acre or gallons/acre, can use the following conversion guide:

• Litres per hectare x 0.4 = Litres per acre
• Litres per hectare x 0.09 = Imperial gallons per acre
• Litres per hectare x 0.11 = U.S. gallons per acre

Next: More efficient spray application saves money