Airblast Sprayer Calibration

Dr. Douglas G. Pfeiffer, 205C Price Hall; e-mail

I. General

A. How the sprayer works: Airblast sprayers operate by using a relatively low pressure pump to deliver the spray mixture into an airstream. This airstream is produced by a large fan and serves to carry the spray to the target. Advantages of this technology are that spray is delivered rapidly, and the entire air volume of the orchard may be treated with a pesticide-laden mist. One of the main disadvantages is the problem of drift; much of the mist is dispersed into the air either before hitting the target, or entirely missing the target. There is research and development underway on ways to reduce such drift, such as the tunnel sprayer at West Virginia University, as well as "smart sprayer" technology, where sensors determine missing trees, etc. and interrupt spray delivery accordingly.

B. Nozzles: Nozzles are of several types; the most common type has several parts through which the mix passes. As the spray passes from the supply line, it passes through a filter. This is to remove debris and other particulate matter before entering the nozzle. The next part is the swirl plate (or simply swirl). This plate has two or three beveled holes of variable size. As the pressurized liquid is forced through these beveled openings, the plate turns, breaking the spray into droplets, the size of which varies with the pressure and the swirl size. The spray then passes through the outer disc. The disc also has a hole of variable size. The entire assembly is held in place by a flange. These parts are shown here, with the inner parts on the right, and outward parts to the left.

The combination of disc and swirl sizes is critical to determining the output of the sprayer. Different combinations produce droplets of varying size. Small droplets have greater surface area and evaporation may be a problem. Therefore such combinations are not used on tall trees, or in upper parts of the spray pattern, because droplets may never contact the target surface. Large droplets carry farther. Treating upper parts of the tree is generally more difficult than treating the lower parts, especially in older, larger trees. The following illustration carries two lessons: (1) ensure that calibration will allow treatment of upper parts of the canopy, and (2) do not put phytotoxic materials in an airblast sprayer tank!

Nozzle parts are produced of several materials. The least expensive is brass; brass fittings are often supplied with the sprayer. These fittings are relatively soft and wear out the fastest. Other, harder materials include ceramic and carbide steel. Although these are more expensive, they are more economical because of increased durability.

C. Dilute sprays versus concentrate, low-volume sprays: Sprays are often applied at a recommended gallons per acre, read from the pesticide label. This is often referred to as dilute spraying. In order to reduce spray time, fuel, and water use, the recommended amount of pesticide is often applied in a reduced volume of water. This is referred to as concentrate or low-volume spraying. A spray may be described using multiplication factor of the mix used, e.g. a 3X spray. There are some advantages associated with concentrate spraying (mentioned above); there are disadvantages as well, that increase in severity with the degree of concentration. Droplet size tends to become smaller with highly concentrated sprays, so evaporation rate increases. Any other error factor (such as wind speed, etc) may become more important.

II. Sprayer selection

The first calculation that must be made is involved in sprayer selection: How large a sprayer is needed. The air capacity needed depends upon the tree volume to be sprayed. The tree volume is estimated by the equation:

tree volume (ft3) = pi r2 h,

where r is the radius of the tree and h is tree height. For example, for trees 12 ft in diameter and 15 feet tall, the tree volume will be (3.14)(6 ft)2(15 ft) = 1,696 ft3.

The air capacity may now be calculated, using the formula:

air capacity (ft3/min) = [speed (ft/min)/tree spacing (ft)] X tree volume (ft3) X 2 sides)

The speed is usually described in MPH; 1 MPH is equivalent to 88 ft/min. If the above trees are planted to a 25 by 25 foot spacing, and the sprayer will be driven at a speed of 2.5 MPH, then the air capacity will be:

air capacity (ft3/min) = [(2.5 X 88 ft/min)/25 ft] X 1,696 ft3) X 2 = 29,850 ft3/min

III. Calibration

Determining Gallons/Acre (GPA): Calibration is the adjusting of sprayer output to a desired level. The first step is determining the desired output, or gallons per acre (GPA). An old standard was 400 gallons per acre. This is rarely used now, except for fully dilute sprays on large trees, including dormant sprays. A more usual approach is to determine desired output based on tree row volume (TRV). A discussion of the relative efficacy of TRV spraying is available from West Virginia. A graph is published in the Virginia-West Virginia-Maryland Spray Bulletin for Commercial Tree Fruit Growers (Va. Coop. Ext. Pub. 456-419) that simplifies this task (click on the PDF for "growth regulators"). The following information is required to use the graph:

(a) Distance between rows,
(b) Tree height X width (ft2)

After these values are found on the axes of the graph, the desired value for gallons per acre are read. Furthermore, since less spray mix is wasted by runoff from the sprayed foliage, a reduced amount of pesticide per acre may be used, also determined from the graph. In our example orchard, the distance between rows is 24 ft. Tree size is 12 ft X 15 ft - 180 ft2. A dilute spray for this block would require 230 GPA.

New Jersey has posted information on tree row volume as well. New Zealand is also using the tree row volume method.

Determining Gallons/Minute (GPM): Once desired GPA is known, the output rate, or gallons per minute (GPM), must be calculated. The equation for this is:

GPM = (GPA X MPH X tree spacing X 2 sides) / 1000

In our example orchard, with a tree spacing of 16 ft within a row, and assuming a sprayer speed of 2 mph, this equates to:

GPM = (230 X 2 X 16 X 2 sides) / 1000 = 14.72 GPM

(Note the effect of travel speed - New Jersey Link)

An airblast sprayer has two manifolds, one on each side. Divide the calculated figure by 2 to determine to allocation of nozzle combinations on a manifold. (GPM per side = 7.36)

With the distributional pattern of output of 50% in the top third, 35% in the middle third and 15% in the bottom third, the following outputs should be allowed in the respective thirds: 3.68, 2.58, and 1.10. Some discretion may be required, depending on the number of nozzles on the manifold, and output values for specific disc/swirl combinations.

The following are output values (GPM) for a selection of nozzle settings at 200 psi:

.................................Disc size......................
Swirl size
D2 D3 D4 D5 D6 D7 D8
0.55 0.75 1.23 1.69 2.46 3.40 4.32
0.34 0.40 0.62 0.75 0.97 1.18 1.36

For the 8 nozzles on the manifold, a suitable distribution would be:
1 2 3 4 5 6 7 8
1.69 1.69 1.23 1.23 0.40 0.40 0.34 0.34
D5/56 D5/56 D4/56 D4/56 D3/25 D3/25 D2/25 D2/25

The calculated output was 7.36 GPM, comparable with the 7.32 GPM obtained.

Checking the calibration: The calibration should be checked both immediately following renozzling the sprayer, as well as periodically during the season. Immediately after adjusting, put a measured volume of water in the sprayer and determine the time trequired to spray out the water. Compare this with your calculated gallons per minute value.

Spray a known area of orchard (with known trees per acre calculated from tree spacing). How much water was required to spray this area?

To enable checking the calibration during the season, note where the sprayer runs out in the first spray after calibrating the sprayer. Mark this area. As the season progresses, note how soon the sprayer runs out. When the sprayer runs out significantly earlier than when freshly calibrated (about 10%?), it is time to recalibrate. The nozzles have become worn and output has consequently increased.

See also the New England recommendations for sprayer calibration, with comments on concentrate sprays, adjuvants, and alternate row sprays, and Penn State and New Jersey comments. 

Virginia Fruit Homepage Pesticide Usage Page

Updated 11 October 2002