So, you’re calibrating your sprayer. You’ve confirmed your ground speed, operating pressure (airblast here, field sprayer here) and selected nozzles (airblast here, field sprayer here). That means your sprayer should spray the predicted rate, right? Well, perhaps… but quite often when we calibrate a sprayer with math, reality has other ideas. You have to test the sprayer output to be sure.
Test Area Method
One way to check is to fill the sprayer with enough water to spray one hectare (or acre) and then go spray the hectare (or acre) – if the tank is empty, the sprayer output is as expected. This isn’t a common method because most operators don’t have an accurate test area marked off. Further, this method may give a false positive if some nozzles are partially blocked while others are worn. You may think that’s improbable, but We’ve seen it. We’ve also seen brand new nozzles out-of-spec. by +/- 10%.
Dip Stick Method
Another method is to fill the sprayer to a known volume using the sight tube or a graduated dip stick. Then, spray for a given amount of time and determine the difference in the volume remaining in the tank. There’s the volume/time rate. This method can be defeated by parking the sprayer on a grade, or setting the dipstick incorrectly in a tank with a rounded bottom. Further, it does not account for ground speed or variation nozzle-to-nozzle. This is not a recommended method.
Timed Output Method
The most accurate (and traditionally time-consuming) method is to measure the actual output-per-minute of each nozzle. Admittedly, this method does not capture ground speed issues, but it does confirm nozzle accuracy. While wearing appropriate personal protective equipment, perform this “timed output test”:
- Clean the sprayer, fill it half-full with clean water and park it on a level surface.
- If airblast, turn fan off. Bring the sprayer up to operating pressure. Start spraying with all nozzles open. Prevailing winds shift so you’re going to get wet.
- Place a collection vessel under the nozzle to be tested. Use a 2 foot length of 1” diameter braided hose to direct the spray into the vessel and to reduce foaming. This size hose is stiff enough to allow the operator to hold it securely against the nozzle body with one hand, while standing out of the spray and holding the collection vessel in the other.
- Collect spray for one minute, or if the collection vessel overflows, collect for thirty seconds. One minute is preferable because it improves the accuracy. Remember to double the output you collect if only measuring for thirty seconds.
- Determine the nozzle output either by looking at the graduations on the side of the collection vessel, or preferably, weighing the output on a kitchen scale. If using a scale, one millilitre of clean water weighs one gram. Remember to “tare”, which means subtract the weight of the collection vessel.
- Convert the findings to either U.S. gallons per minute or litres per minute; whichever corresponds to the ratings in the nozzle manufacturer’s catalogue.
- Replace any nozzles that are 10% more or less than the rated output; 5% is a preferable limit, if possible. If two or more nozzles are worn, replace all the nozzles at the same time.
Anyone that has tried the timed output test in Canada knows that it sounds easy until you try it. The problem is converting between Metric and US Imperial units. Canadian sprayer operators tend to use both because label rates are in metric, but most sprayer equipment is rated in US Imperial. You can find conversion tables here. Here are the most common calculations required for the timed output test:
Many sprayer operators loathe sprayer math. Fortunately, there are now electronic calibration vessels like the ATI Agritronics AppliMax Spray Nozzle Calibrator, or the Innoquest SpotOn. Both collect the spray and give a fast and accurate digital result in US Gallons per minute, ounces per minute or litres per minute. You need a hose both to direct the spray into the vessel and to slow it down to prevent splashing. Further, the spray has to be very clean because foaming can wreak havoc with the readings.
When calibrating airblast nozzles it can be challenging to hold a braided hose over a nozzle body while juggling a pitcher or collection vessel. The hose can pop off, affecting the reading and increasing the chance of spraying the person performing the calibration. Dr. Heping Zhu, (ARS USDA) passed on a tip to use dairy inflation tubes that snug over the entire nozzle, freeing a hand. This can work, but the hose is short, can crimp when bent, and doesn’t fit over check valves.
A better choice is a nozzle clamp, like the aams-salvarani version pictured below. Some operators use passive flow meters, but they are difficult to source in North America because they aren’t very accurate. They’re fine for comparing relative flow from tip to tip, but the act of suffocating the exit orifice with the unit affects the rate. This is true of any calibration device that could restrict flow from the nozzle, and it has an exaggerated affect on air induction nozzles. Avoid smothering the air inlets.
No matter your preferred method, confirming your sprayer output is a critical part of calibration and it should be performed regularly – not just at the start of the season.