What makes a spray "fine", "very fine" or not fine at all?
Erdal Ozkan
  

Pesticide labels today are likely to include a recommendation such as this one: "apply this product with nozzles producing a fine spray quality."

What is the difference between "fine", "medium" or "very fine" spray qualities, and why is is important to you? Preparing to manage Soybean Rust disease is a prime example of the importance. You may have already read that nozzles producing "fine to medium quality spray are more likely to provide better coverage on leaves", and for that reason, should provide better protection against Soybean Rust.

Spray Quality
Perhaps Soybean Rust made spray quality a household name, but the work of classifying sprays based on droplet size was started because we wanted to rate nozzles based on their drift potential. After wind speed and direction, spray droplet size is the next most important factor affecting drift. Each class of nozzles, even the same type of nozzle with different orifice sizes (flow rates), or the same nozzle operating under different conditions all will have different droplet size distributions, consisting of driplets ranging from very small to very large at different proportions.

Almost all major agricultural nozzle manufacturers now have a version of "Low Drift" nozzles. These nozzles are designed to create larger droplets at the same flow rate and operating pressure compared to conventional flat fan nozzles. However, if operated at low pressures, some conventional nozzles can be as effective in reducing drift as the low drift nozzles operating at higher pressures. Mostly for this reason, pesticide manufacturers were not able to recommend one particular type of a nozzle for drift control. On the other hand, the US EPA has been requesting some type of information on labels which applicators can use to determine nozzle size, type and operating parameters to reduce drift.

Nozzle classification by droplet size
To help manufacturers of pesticides and regulatory agencies such as the EPA, the American Society of Agricultural Engineers has developed a standard called Spray Nozzle Classification by Droplet Spectra (ASAE S-572). This Standard defines droplet spectrum categories for the classification of spray nozzles relative to specified reference nozzles discharging spray into static air. It defines a means for relative nozzle comparisons based on droplet size only. 

This standard identified 6 spray quality categories: very fine, fine, medium, coarse, very coarse, and extremely coarse, as defined in the table. Also, a unique color is assigned to each class. (This color should not be confused with the color coding for flow rates). Sprays are classified by approximate VMD (Volume Median Diameter) droplet sizes. The droplet sizes given are for comparison purposed only and should not be used for all nozzles, and operating conditions.

Nozzle flow rate, spray pressure, and physical changes to nozzle geometry and operation can affect nozzle classification. In other words, a given nozzle can be classified into one or more droplet size categories, depending on the selection of flow rate, operating pressure, and other operational conditions.

To determine the exact drop size classification of a nozzle under a given set of operating conditions, check the data given in the spray quality charts in the manufacturer's catalog. For controlling soybean rust, select a nozzle and operating pressure that will give a fine to medium spray quality. For example, if you have 0.04 gallons per minute flat-fan nozzles (red tips on your boom, you need to operate these nozzles at a pressure higher that 25 psi to achieve a spray quality that falls in the "medium" category. Below 25 psi the same nozzle will produce a spray quality that is considered "coarse", and will not give the good leaf coverage and canopy penetration needed for soybean rust control.

TABLE

Spray Classes

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Classification category                Symbol                   Color code                Approximate Droplet Size (VMD)*

Very fine                                     VF                         Red                          < 150 micron

Fine                                            F                           Orange                        150-250

Medium                                                    M                          Yellow                          250-350

Coarse                                                     C                           Blue                            350-425

Very coarse                                            VC                         Green                          425-500

Extremely Coarse                                 XC                          White                       > 500 micron

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* VMD, Volume Median Diameter droplet size, is the size of the droplet which divides the spray in two equal parts by volume. Half of the spray volume is contained in droplets smaller than the VMD, the other half in droplets larger than the VMD. (For reference: 125,000 microns equals one inch; a human hair is about 100 micron thick).

 

 

Erdal Ozkan, Professor and Extension agricultural engineer, can be reached at 614-292-3006 or ozkan.2@osu.edu.