Validate airblast output – nozzle calibration

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About Jason Deveau (Spray_Guy)

Dr. Jason Deveau (@spray_guy) has been the OMAFRA Application Technology Specialist since '08. He researches and teaches methods to improve the safe, effective and efficient application of agricultural inputs in specialty crops, field crops and controlled environments. He is the co-administrator of Sprayers101, co-author of the Airblast101 Textbook, a slow cyclist and a slower runner.

See all posts by Jason Deveau (Spray_Guy).

Sprayer math is important. It ensures the operator applies the correct product rate and has enough to complete the job. But, it assumes the airblast sprayer is behaving as expected… and it often doesn’t. After confirming the airblast travel speed, use one of the following methods to assess sprayer output. There are pros and cons to each.

The area method

Operators that claim the sprayer empties in the same place every time assume everything’s alright. They are performing a variation on the area method.

Essentially, you fill the sprayer with enough water to spray one hectare (or acre) and then spray that area. If the tank empties where expected, you know your output rate (i.e. volume / area). But, there are a few problems with this method:

  • Most operators don’t have an accurate test area marked off, and even when they think they know the area, measurements prove otherwise. They’re always amazed when this happens.
  • The area method has poor resolution. It reveals the total output but does not assess individual nozzles. For example, partially-blocked nozzles and worn nozzles average out (we’ve seen it). Rate controllers provide whatever pressure is required to match the desired output, masking individual nozzle problems.

The dip stick method

Another method is to fill the sprayer to a known volume using a flow meter, while observing a sight level or a graduated dip stick. Then, while parked, the operator sprays for a given amount of time and determines the difference in the volume remaining in the tank.

This method can be defeated if volume is misread. It’s an easy error to make if the sprayer is parked on a grade, or the dipstick shifts in a tank with a rounded bottom. And, of course, it also masks individual nozzle problems.

Sight levels can be misleading when the sprayer is parked on a grade. They are often opaque and hard to read.

The timed output method

The preferred method is to measure the output of each nozzle individually. It can be messy and time consuming, but it’s accurate. Appropriate personal protective equipment is required to perform the timed output method – expect to get wet.

1. Fill the rinsed sprayer half-full with clean water and park it on a level surface.

2. With the fan(s) off, bring the sprayer up to operating pressure. Start spraying with all nozzles open (closing any will change the pressure).

3. You will need 1 meter (3 feet) of 2.5 cm (1″) diameter braided hose (have a second, longer hose to reach the top of a tower sprayer). It should be stiff enough that you can slip it over a nozzle body while holding the other end. Use it to guide flow into a collection vessel, held with your other hand. The hose not only reaches the top nozzle on towers, but it lets foam dissipate before it gets to the vessel.

4. When the flow from the hose is steady, direct it into the collection vessel for 30 seconds (a partner with a stopwatch is very helpful). It is preferable to collect for a minute because it improves the accuracy.

5. Determine and record the nozzle output per minute. Graduations on plastic collection vessels are unreliable. It’s preferable to weigh the output on a cheap, digital kitchen scale. One milliliter of clean water weighs one gram. Don’t forget to subtract the weight of the vessel (this is called taring) and double the output if you only collected for 30 seconds.

Interpreting the results

Once you have recorded all the outputs, you will have to convert the output to U.S. gallons or liters per minute, depending on units in the nozzle manufacturer’s catalogue (see common conversions below).

Replace any nozzles that are 10% (or preferably 5%) more or less than the rated output. This not only indicates a rate problem, but likely a problem with droplet size as well. If enough nozzles are worn, consider replacing all of them. Nozzles should go on as a set, and come off as a set (unless replacing a broken tip, of course). This can be an expensive proposition for large airblast sprayers, but it is part of operational costs.

Don’t assume new nozzles are accurate. We’ve found +/- 5% flow variation right off the shelf. Keep your receipts.

Testing and replacing nozzles is an important part of sprayer operation, no matter how many there are. This Air-O-Fan is nozzled for Australian almonds.

Helpful conversions

Anyone that has tried the timed output method in Canada knows the pain of our Metric-esque (Mocktric?) units. We’re an odd hybrid because our label rates are in metric, but our nozzles and many of our sprayers are US Imperial. You can find a complete collection of conversion tables here, but the most common calculations are reproduced below:

If collecting in ounces, converting to U.S. Gallons per minute:


If collecting in millilitres or grams converting to U.S. Gallons per minute:


If collecting in ounces, converting to litres per minute:


If collecting in millilitres or grams converting to litres per minute:


If collecting in ounces, converting to Imperial gallons per minute:


If collecting in millilitres or grams converting to Imperial gallons per minute:


A more sophisticated option

The timed output method is slow and requires math. You can avoid both problems by using electronic calibration vessels like the Innoquest SpotOn SC-4. We’ve tested both, and they are as accurate as weighing the output – but much faster.

They can, however, be fooled by foam. We’ve had good results using a length of braided hose to direct the flow and dissipate most of the foam. Typically, foaming means the sprayer wasn’t rinsed enough.

The SpotOn SC-4 calibration vessel is much easier, faster and more accurate than the classic pitcher-and-stopwatch approach to timed output tests.
The SpotOn calibration vessel is easier, faster and more accurate than the classic pitcher-and-stopwatch approach to timed output tests. The SC-4 (pictured) is for airblast and SC-1 is for field sprayers.

Another approach is to hose-clamp multiple hoses over nozzle bodies and spray all at once. This is tricky and takes time. Plus, if you suffocate the nozzle’s exit orifice (creating back pressure) or block the air inlets on AI nozzles, you will get a false reading.

Be careful not to plug air inlets on air induction nozzles – you may get a false reading.

We prefer nozzle clamps over hose clamps (see the AAMS-Salvarani nozzle clamp pictured below). There are pincers designed to latch behind the nut of the nozzle body, but compatibility can sometimes be an issue (e.g. with Turbomist sprayers).

Passive flow meters (also pictured below) remove the need for a collection vessel, but they’re a better fit for field sprayers since they have to be held in place manually. They are difficult to source in North America because their accuracy is questionable, but they are fine for comparing relative flow from tip to tip.

Nozzle clamp or flow meter, avoid suffocating the nozzle exit orifice or AI nozzle air inlets.

Left: Nozzle body hose clamp. Right: Passive flow meter.

Some grower groups, or professional consultants, spring for very sophisticated and accurate units, such as AAMS-Salvarani flow measurement system pictured below.

AAMS-Salvarani flow measurement system. We used these on a pumpkin sprayer in New Hampshire, but they work with airblast too.

No matter your preferred method, take the time to confirm your sprayer output at the beginning of the season and whenever you make repairs or significant changes to your sprayer.