Education For Environmental Responsibility

Correct sprayer calibration vital to chemical application

This article appeared in Ag Alert on June 16, 1999

By Will Stockwin, Assisant Editor

From Bill Steinke’s perspective as a University of California agricultural engineer, it’s hard to find a bigger waste of time and money than spraying an orchard with uncalibrated equipment.

"The purpose of being out in the orchard with a sprayer is not just to get the stuff out of the tank," Steinke said. "The goal should be to put the right amount of material up in the trees where it can best protect the crop."

At a field demonstration on sprayer calibration earlier this year, Steinke determined with a quick survey of grower participants that orchard application costs can run from $40-to-$60 an acre.

"That means a grower with a three-acre orchard may be looking at $120 on the low end of application costs, while somebody else with an 80-acre orchard could be spending $4,800 on the high end for his application,"Steinke said. "The application is based on a written recommendation calling for so many pounds or gallons of material to be put on with an exact amount of water, and calibrating your equipment ensures that‘s actually happening."

Steinke also pointed out that some new product labels are specifying applications be made with calibrated equipment.

"You may not have seen them yet, but you will," he said. "And if the agricultural commissioner shows up wanting to see your calibration records, showing him work sheets like the ones we’re about to generate will take care of that."

That doesn’t mean north state ag commissioners are suddenly going to be dropping out of the trees on unsuspecting growers, but stepped up inspections are definitely a future possibility in Sacramento Valley counties trying to reduce chemical runoff into surface water.

"We’re in a transition period on the dormant spray runoff issue right now, where we’re still doing voluntary things," Glenn County agricultural commissioner Ed Romano said. "But I could see where if we don’t see continued improvements that some regulatory steps might become necessary, and sprayer calibration would probably be part of that."

Romano’s colleague in Butte County, Richard Price, said "I think we’ve been slowly heading that way for about the last five years. We don’t know at this point what would be required in terms of documentation, but having signed and dated worksheets sounds reasonable."

Sutter County agricultural commissioner Mark Quisenberry agreed. "We will also probably want to know the source of whatever calibration method is used," he said. "You know: was it from a course that we or the university sponsors, or did the grower just make something up."

Steinke’s method takes roughly 30 minutes to an hour, beginning with towing the sprayer into the orchard and parking it in the drive row between two trees. Next, he stretches colored tape or string between the sprayer’s manifold and the trees on either side to describe the top and bottom of the spray arc on each side of the row.

Two more lengths of tape or string (on each side of the manifold) sketch the spray arcs into thirds. When done on a standard sprayer set up, you may end up with three nozzles in the top third, two or three in the middle, and two in the bottom third of the arc.

Steinke then estimates how much of the tree’s foliage is exposed in each third of the spray arc in order to get an idea for what percentage of the sprayer’s total gallons-per-minute (gpm) rate needs to go into each third of the arc.

The demonstration was set up in a young walnut orchard, and with the tapes in place Steinke figured he needed 25 percent of the total gpm in the top third, 50 percent in the middle and 25 percent in the bottom third.

Measuring the flow rate comes next. Steinke said that can be done by measuring the sprayer’s total flow rate, or by measuring the rate from each nozzle tip.

"With an orchard sprayer, the only way to capture flow from each nozzle is to get really wet," Steinke said. "The other way to do it is to fill the sprayer with water to overflowing, run it for two minutes, then measure how much water it takes to fill it back to overflowing. That gives you the total flow rate, then by taking the different sizes of the nozzles you can figure out how much is going into each section of your spray arcs."

To measure speed in the walnut orchard with trees planted on a 30-by-30-foot spacing, Steinke set up a 90-foot course using traffic cones. He recommended running the sprayer at half full during the speed check to get the best approximate speed.

"It’s not important to have the sprayer running when you do this unless it’s a PTO-driven," he said. "Then it’s absolutely essential because you’re going to change the loading on the PTO drive, the throttle setting, and it will definitely affect your overall speed." While he was on the subject of speed, Steinke took the opportunity to caution his audience about exceeding optimum application speeds.

"Speed will kill your deposition because you won’t get material into the top of the tree if you’re pushing it," he said. "The sprayer has to displace all of the air inside the tree canopy, and unless you can find a way to change the laws of physics it just won’t work if you’re going too fast."

From there on it’s all math, first converting the sprayer’s time through the course to speed in miles per hour, then to acres per minute. Dividing the previously calculated flow rate by the speed in acres per minute determines gallons per acre.

"Some guys will do these calculations at the beginning of the year and then remember that it takes so many minutes to spray out a tank load as a way of keeping a constant check on the sprayer’s operation while they’re out working," Steinke said. "I would also suggest re-calibrating every time you put on a new set of nozzles, buy a new sprayer or even a new tractor to pull your old one with."

The last step is to rig a PVC pole with water-sensitive paper at different heights to check the sprayer’s output capability and adjust the nozzles for maximum effect.

"If you’re going to be spending money on making an application, then it’s really worth your time to make sure you’re doing it correctly and getting your money’s worth while you’re at it," Steinke said.