Calibrating a Plot Sprayer for Airblast Crops

The calibration of handheld plot sprayers is an important part of agricultural research, and this article already covers all the bases… as long as you are spraying broadacre or row crops. But what happens when you are trying to emulate an airblast sprayer and treating a tree, bush, cane or vine?

Pro Tip: To maintain a consistent boom height without a wheel, coil a measured length of wire from a plot marker flag to guide you.

The key difference is that spraying a two dimensional area requires the operator to pass the boom over the target at a uniform height and pace to achieve consistent coverage. But, a three-dimensional target requires the operator to circle the target, or spray from both sides, until it has received the required dose (or volume).

In order to scale down a typical airblast carrier volume for small plot work, we need to know three things:

  1. The area you wish to treat (e.g. bush, grape panel, tree, etc.), including it’s share of the alley (in m2).
  2. The emission rate from the calibrated plot sprayer (in US gal./min.)
  3. The airblast carrier volume you wish to scale down (e.g. L/ha).

The illustration below shows two options for calculating the treated area. Option A requires you to measure from the outermost edges of the canopy (imagine if the canopy was wet and dripping – the dripline is that outermost point). It is less consistent than the preferred Option B, where the area is determined from row centres and planting distance.

Two options for scaling down an airblast carrier volume for small plot work. Both produce the same treated area, but Option B is the preferred method.
Use the average planting distance and row spacing in metres. For a panel of grapes, use the centre of each panel as the planting distance.

If you are using a CO2 powered hand wand (preferred over a manual pump) with one or more hydraulic nozzles, then you can calibrate it using the methods in this article. However, if you are using a backpack mistblower, which better approximates an airblast sprayer compared to a hydraulic hand boom (see this article), it requires a difference approach. Plus, you get to look like a Ghostbuster, which is a win in my book.

When transporting a mistblower, use a loop of nylon cord to secure the boom in an upright position.
For calibration, fill the completely empty sprayer with a known volume of water. If the boom is gravity-fed, be sure the feed valve is closed so the water doesn’t run out of the boom.
With the sprayer on the ground, brace it with your foot. Step on the metal frame, not the motor housing or tank. Follow the operating instructions to pull start the motor.
Being cautious of the hot exhaust, set the sprayer on a tailgate, or other elevated surface to facilitate strapping it on.

Be aware that most mistblowers use gravity to feed the spray mix from the tank to the boom. If the boom is pointed above level, spray will slow or even stop. This greatly diminishes your ability to reach high targets and achieve consistent coverage. If your target requires you to point the boom up, use a deflector before proceeding with the calibration.

Deflectors angle the spray upwards without having to lift the boom. This is easier on your shoulder and keeps the rate consistent.

Set the flow rate to the preferred setting (usually a dial at the end of the boom), and using a stopwatch, time how long it takes to spray the entire volume. Be sure to move the boom exactly as you would when spraying the target, either side-to-side or up-and-down, to capture possible rate changes from the gravity feed. Convert the output to US gal./min.

When timing output, move the boom as you would when spraying the target.

Now you know the area and the emission rate. You should have a target carrier volume in mind (e.g. L/ha). Using the following example, let’s determine how long you need to spray the target:

A sample calibration.

In this example, an ideal airblast Carrier Volume [C] for the orchard is 400 L/ha. We want to scale this down to determine the Volume for Treated Area [V]. First, divide [C] by 100 to convert it to 40 mL/m2. Then, because in Canada our nozzles are in US units, we do an ugly conversion: Since 1 mL = 0.000264 US gallons, [C] becomes 0.0106 US gal./m2.

The Treated Area [A] measures 3.5 m by 2 m = 7 m2.

The Emission Rate [R] is the rate the plot sprayer sprays. While we prefer using a mistblower, many still use a hand wand with no air assist. In this case let’s suppose we are using a hand wand with two 8002 flat fan nozzles operating at 40 psi. According to our calibration, we confirm it sprays 0.4 US gal./min.

  • [C](US gal./m2) × [A](m2) = [V] (US gal.)
  • 0.0106 US gal./m2 × 7 m2 = 0.074 US gal.

We know we want to spray the target with 0.074 US gal., and we also know [R] which says our boom emits 0.4 US gal./min. We convert this to seconds by dividing by 60, so [R] = 0.0067 US gal./sec. From this we can calculate how long [T] we must spray the target.

  • [V](US gal.) / [R](US gal./sec.) = [T](seconds).
  • 0.074 US gal. / 0.0067 US gal./sec. = approximately 11.0 seconds.

So, we know that to spray the target with an equivalent 400 L/ha, we must achieve consistent coverage from all sides by spraying it for a total of 11 seconds. Pro tip: Always mix a little more spray volume than you will need to account for priming.

This is only one way to calibrate a backpack sprayer for spot spraying. If it’s isn’t quite what you need, check out these resources:

  1. Calibrating a Knapsack Sprayer (www.weedfree.co.uk – 2008)
  2. Don’t Overlook Backpack Sprayers (John Grande, Rutgers)
  3. Hand Sprayer Calibration Steps Worksheet (Bob Wolf, Kansas State University – 2010)
  4. Sprayer Calibration Using the 1/128th Method for Motorized Backpack Mist Sprayer Systems (Jensen Uyeda et al., University of Hawai’i – 2015)

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