Apr 6, 2015

Upgrade and calibrate your backpack sprayer

Note: information for this article is drawn from “Don't Overlook Backpack Sprayers for Small-Scale Farms” by John Grande and Jack Rabin, Rutgers.

Backpack sprayers are very useful tools for crop farmers to have on hand. Whether your farm is large or small, newly established or centuries old, certified organic or conventional, there is a spot for a backpack sprayer or two on your farm. However, to make the most of a backpack sprayer, we recommend that you make some upgrades to the sprayer wand assembly and of course, keep your sprayer calibrated.

There are a many advantages to using backpack sprayers on the farm. Commonly used for spot-spraying herbicides, backpack sprayers can also be used to apply fungicides, insecticides, foliar fertilizers, and many other products very efficiently. Backpack sprayers are inexpensive, so you can have multiple sprayers set up for specific uses. Backpack sprayers are simply designed, so are easy to fill, clean, repair and maintain. Unlike larger sprayers, backpack sprayers are connected directly to the operator's arm and brain, allowing higher precision applications.

John Grande and Jack Rabin of Rutgers did some really great work on backpack sprayers a few years ago, and have posted a comprehensive set of resources for growers on selecting the best models for their needs, upgrading standard parts to improve function, calibrating backpack sprayers, and measuring small amounts of products as used in backpack sprayers. In this article, we'll concentrate on just a couple of these.

Spray wand conversion: Most backpack sprayers are generally well designed and built, with the exception of the spray wand. The spray wand consists of a flexible hose connected to the tank pump, a stiff wand with a trigger handle, and a simple flood nozzle at the end. These parts are usually made of plastic, with little to no ability to make adjustments. John Grande has developed a method to dramatically improve the functionality of a backpack sprayer by replacing the plastic spray wand with one custom assembled using compatible, off-the-shelf components from a sprayer supply company. The total cost of retrofitting a backpack sprayer with a new wand is around $200.

The new wand includes several key components. First, it includes a CF valve, which solves a key problem of calibrating a backpack sprayer with a typical wand. In most backpack sprayers, the output from the nozzle increases and decreases as the pressure in the tank varies. You can't really calibrate a sprayer unless the flow rate is uniform over time. With a CF valve built into the spray wand, the sprayer will only spray when the tank pressure is high enough to maintain output pressure and hence, flow rate. When the pressure drops below the CF valve's working pressure, flow stops completely, a clear sign to the operator that they need to pump more air into the tank in order to continue spraying.

Another important component of the retrofitted wand is the inclusion of a standard nozzle body and cap. This allows you to change nozzles depending on the product, application, or conditions. It is a simple thing to switch between flat fan, hollow cone and flood nozzles, or nozzles of different droplet sizes. Finally, adding a barbed swivel to the wand so that it can be easily pointed and positioned makes operating the sprayer a lot more comfortable.

Sprayer calibration: Now that you have the backpack sprayer retrofitted, it is important to calibrate it so that you can apply products accurately and according to labelled rates. Three factors are required to be determined for calibration:

A constant spray output or volume. The retrofitted wand will give us a constant output, and the manufacturer of the nozzle will provide the output rate for each nozzle at a given pressure. But you can check the output rate by following these steps: Half fill the sprayer with water, pump the sprayer, point the tip into a container, and squeeze the trigger handle for one minute (timed). Determine the volume collected per minute and convert the flow rate to gallons per minute by dividing by 128 (since there are 128 ounces in one gallon). Now you have nozzle output in gallons per minute (GPM).

A constant walking travel speed. It is very important to practice your walking speed, and to do this on the actual ground you will be walking on when operating the sprayer. Mark off 100 feet on the uneven ground, and time how long it takes to walk it. Do this several times so that the time is consistent. Most people's comfortable walking speeds fall in the range of 2.0 MPH (34 seconds to walk 100 feet) to 2.5 MPH (27 seconds) to 3.0 MPH (23 seconds). Now you have your walking speed in miles per hour (MPH).

Knowing and maintaining the spray width. Each nozzle type is manufactured with a spray angle and a recommended spray height which provides a known width listed in the parts catalog. Make sure you hold the boom at a constant height when spraying. 20″ and 30″ widths are common for 110 degree nozzles. Alternatively, you can spray water on an asphalt surface and measure the effective band width.

You now have the measurements you need to calculate the output of your sprayer, as you will be operating it, in gallons per acre (GPA).

GPA = (Nozzle GPM X 5,940) / (MPH X spray width inches)

If gallons per 1,000 square feet is more useful for your purposes, substitute 136 for 5,940 in the formula above.

– Lee Stivers, Penn State Extension