Drop Size Classifications for Agricultural Sprays Size Classifications for Agricultural ... spectra that will be produced by the atomization of aerial sprays ... Drop Size Classifications

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Drop Size Classifications for Agricultural Sprays

Andrew J. Hewitt*Stewart Agricultural Research Services, Inc.

Macon, MO 63552

David. L. Valcore*Dow AgroScience

Indianapolis, IN 46268

Milton E. Teske*Continuum Dynamics, Inc.

Princeton, NJ 08543

Rudolf J. Schick*Spraying Systems Co.Wheaton, IL 60187

Abstract

Schemes for classifying agricultural sprays according to droplet size and drift potential are described. These schemes have particular benefit for the comparison of spray data generated using different particle measurement techniques. Labels for agricultural chemical products can refer to the classification schemes for specification of the droplet size spectrum needed for effective application and drift minimization.

As presented at: ILASS–Americas, 11th Annual Conference on Liquid Atomization and Spray Systems, Sacramento, CA, May 1998

*Corresponding author

2 www.sprayconsultants.com


Introduction

The safe and efficient application of pesticides requires, among other things, the definition of an appropriate droplet size spectrum. A review of interests affecting the selection of parameters for spraying operations was given by Matthews (1979) and Hewitt (1997). The ideal spectrum will maximize spray efficiency for depositing and transferring a lethal dose to the target, while minimizing off-target losses such as spray drift and user exposure (Elliot and Wilson, 1983). Take et al (1996) showed that the droplet size spectrum of a spray application, and the factors that affect this spectrum, are the most important variables affecting spray deposition levels downwind of the application area.

Considering the importance of droplet size upon spray performance and drift minimization, and given the range of nozzles available for spray application, it is logical that sprays should be classified according to droplet size. Systems have been developed by the British Crop Protection Council (BCPC) and American Society of Agricultural Engineers (ASAE) for classifying agricultural sprays by droplet size. An additional component to the BCPC scheme, a drift potential factor, allows the further description of sprays, which have substantially different transport behavior from most conventional hydraulic nozzle.

Spray Classification By Droplet SizeBCPC Scheme

The BCPC nozzle classification scheme was devised in the mid 1980’s as a “spray classification system that divides the quality of spray produced by hydraulic nozzles and other atomizers into five categories” (Doble et 1985). The five categories considered relevant to ground spray applications in Europe in the 1980’s formed the basis of the scheme. These are as follows: Very Fine, Fine, Medium, Coarse and Very Coarse. The definition of an additional (Aerosol) category is being considered.

Although the BCPC scheme was originally developed for classifying ground hydraulic application nozzles, other atomizers are also encompassed.

The BCPC scheme recognized that different droplet size spectra are sometimes reported for similar sprays measured by different instruments and techniques (e.g. laser diffraction; optical imaging, phase–Doppler), and for different sampling techniques within an instrument type.

Four reference sprays (different flat fan nozzles and operating pressures) are used to establish the boundaries between the reference categories. Figure 1 shows the reference categories measured in a wind tunnel using a laser diffraction technique.

ASAE Scheme

To ensure harmonization of nozzle classification scheme efforts between the U.S. and Europe, the Power Machinery PM41 committee of the American Society for Agricultural Engineers (ASAE) has developed a standard, no. X572, based on the BCPC droplet size classification scheme. The ASAE standard, Spray Nozzle Classification by Drop Spectra, includes the basic BCPC size classes and an additional Extra Coarse class, to represent sprays that contain very large droplets. Issues affecting sampling in conjunction with the ASAE scheme were discussed by Maynard et al (1996).

Classification of Different Distribution Types

The BCPC scheme was originally developed for the classification of sprays produced by ground sprayers fitted with flat fan hydraulic nozzles. When operated within normal ranges of application variables, these nozzles typically produce the type of volumetric droplet size spectra profile shown by the reference nozzle curves on Figure 1. Flat fan nozzles were selected because these were widely used for arable crop spraying in the U.K. at the time of initiation of the BCPC spray classification scheme.

CLASSIFICATION SCHEMES



Figure 1. BCPC Cumulative Volumetric Droplet Size Reference Categories Measure Using Malvern 2600


After its original development, the BCPC scheme was adopted for the classification of nozzle types producing spray plumes with patterns different from those of flat fan nozzles. There are currently many different types of atomizer and nozzle available for the application of agricultural and biological sprays. The major types include the following: disc-core (swirl), solid stream, hollow cone, full cone, flat fan, deflector, rotary cage/drum/sleeve, spinning disc/cup, twin fluid and electrostatic. These nozzle types represent many different spray plume patterns. The droplet size spectra that are produced by some of these nozzle types have substantially different profiles from those of flat fan nozzles. In some cases, the cumulative volumetric droplet size spectrum curve can cross several BCPC reference categories, particularly for highly polydisperse sprays.

For cases where the classification could be made into more than one reference category, it is most logical to indicate all such categories contained between the DV0.5 and DV0.9. For example, if a spray has a classification of “Fine” by DV0.1 and “Medium” by DV0.5, it could be described as a “Medium–Fine” spray. From a drift perspective, the spray volume contained in droplets with diameter below 100 to 200 µm is more

important than parameters indicative of larger droplet size classes. The ASAE X572 standard suggests putting emphasis on the spray volume in droplets with diameter < 150 µm for cases where the boundary curves are crossed.

Drift Potential Factor

The classification of sprays according only to droplets size does not take account of the velocity, density, air-entrainment or trajectory of the droplets comprising the spray clouds. For a given type of nozzle, these factors may fall within a sufficiently narrow range to support the use of a classifications based only on droplet size. However, many researchers have shown that such classifications do not accurately describe the performance of many ground spray drift-reduction nozzle systems with regard to near field drift and/or coverage. For example, Walklate et al (1994) showed that relative drift predictions using the BCPC percent spray volume with diameter less than 100 µm did not give a fair representation for a dual–orifice flat fan nozzle, a twin fluid nozzle producing a “Fine” BCPC spray, was 60% lower than that from a conventional “Fine” BCPC flat fan nozzle.



A technique has been developed and used for assessing airborne spray volumes from various agricultural nozzles in wind tunnels (Western et 1989; Milk et al 1993; Walklate et al 1994). —Ibis technique involves the use of collectors such as tubing to measure the vertical spray profile at one or more distances downwind of the nozzles, in a low speed wind tunnel. Typically, an array of collectors would be positioned at a distances of 2 m/s from the nozzle; and the wind would be 2 m/s. Collectors such as polythene tubing and cotton strings have been widely used for assessing airborne spray volumes in field and laboratory studies for many years. More recently, phase Doppler analyses (PDA) instruments have been used in lieu of cylindrical collectors. The PDA back–scattering technique has the advantage that collection efficiency and intrusive sampling factors are eliminated. The PDA technique involves emitting a non-intrusive laser beam through the air, and assessing airborne spray from analysis of the associated signal. From measurements using eight vertical collectors or PDA sampling, a Drift Potential Factor can be derived as follows (Helch et al 1977). A determination is made of the spray volume and the height above the ground of the center of gravity for the drift profile. A comparative scale is then established based on a calculation of the first moment of the airborne drift profile measured at the sampling distance downwind of the nozzle. The Drift Potential Factor (DPF) is calculated as follows:

∑=∑∑

∑= nnn

nnn hV

VhVVDPF

Where Vn is the volume of airborne spray collected at heights hn. The value derived will be related to the percentage reduction I drift to a defined reference spray. The drift reduction, or lack of it, will be described using appropriate terminology. Proposed technology includes “Higher”, “Normal”, “Low”, etc. drift potential (Southcombe et al 1997). These might correspond respectively to < 0, 0 – 25% and 25 – 50% reduction in drift potential. Additional terms might address even higher reductions in drift potential.

Systems which might show different drift potentials than their hydraulic flat fan nozzle counterparts include sprays from rotary, twin fluid, electrostatic and air-assist atomizers, and droplets containing air inclusions.

Application of Tee Scheme

The droplet size and drift potential factor classification schemes were developed with several anticipated end users. There was particular interest in the field of spray drift modeling. Therefore, the options for selecting droplet size input terms of BCPC category have been incorporated into the spray deposition model, AgDRIFT (Teske et al). If input is selected using BCPC categories, the associated worst-case droplet size spectrum is utilized for the model analysis. For example, if a “Medium” spray is selected, the model assumes the droplet size spectrum that divides the “Medium” and “Fine” categories. Many noble manufacturers have included the categories in their nozzle catalogues, for specific operational parameters. A booklet has been produced that contains tables showing the BCPC size categories associated with different nozzle and spray pressure. In the U.S., the National Agricultural Aviation Association Research and Education Foundation (NAAAREF) with SDTF help is finalizing a manual containing similar information for the major parameters affecting atomization for aerial applications. The SDTF atomization/physical Property model, DROPKICK, can also predict the droplet size spectra that will be produced by the atomization of aerial sprays through a wide range of nozzles, with a wide range of application and liquid Bid property variables. This model was described by Hewitt et al (1997). The data generated from any of these sources can be related to BCPC and ASAE classifications, if so desired. In Europe, systems are being developed for classifying the potential hazard of different types of spraying equipment (Parkin et al 1994). Spray classification will form an important component of such schemes (Southcombe et al 1997).




Limitations

The issue of classifying droplet size spectra that cross boundary curves has been discussed. The droplet size schemes may not be appropriate for classifying sprays that cross several boundary curves. Additional reference categories may be needed for specific applications. For example, given that a large proportion of agricultural sprays produce “Medium” sprays, it might be useful to sub-divide the “Medium” category.

Although the droplet size schemes help reduce the effects of differences in actual droplet size data being generated using different particle measurement techniques and sampling procedures, further work is needed to develop standards that can be adopted by different laboratories measuring agricultural spray. The current BCPC and ASAE schemes provide a series of reference sprays (specific nozzles and pressures) that should be measured by each laboratory producing data for the scheme. Data

measured by those laboratories for different nozzles can then be related to the reference size classes applicable to the same laboratory. Through the efforts of BCPC, the American Society for Testing and Materials (ASTM) and ASAE, standards are being developed for the representative sampling of liquid sprays using various measurement techniques. Using laser diffraction sampling techniques described by Hewitt and Valcore (1995), the reference boundary data from Figure 1 were similar to those measured at Spraying Systems Company using PDPA technique (Figure 2).

The BCPC and ASAE spray classification schemes continue to offer a valuable method of describing sprays for purposes such as pesticide labeling. As the schemes are more widely used, further improvements may occur to ensure that all the applicable variables are considered.


Figure 2. Cumulative Volumetric Spray Fractions Measured With Malvern & PDPA Systems



References1. S.J. Doble, G.A. Matthews, I. Rutherford and E.S.E Southcombe, A System for Classifying Hydraulic Nozzles

and Other Atomizers into Categories of Spray Quality/Pro. 1985 Bit. Crop Porf. Conf. – Weeds, Vol. 9A – 6, pp. 112.533, 1985.

2. J.G. Elliot and BJ. Wilson, The Drift of Herbicides, Occasional Publication No. 2, British Crop Protection Council, Thornton Heath, U.K., 1983.

3. C. Helck, A. Herbest and H. Ganzelmeier, New Approaches to Determine Drift Potential from Nozzles, presented at workshop of British Crop Protection Council Spray Nozzle Classification Scheme, Silsoe Research Institute, Silsoe, U.K., 1987.

4. AJ. Hewitt, The Importance of Droplet Size in Agricultural Spraying, Atomization & Spray Vol. 7(3), pp. 235 – 244. 1997.

5. AJ. Hewitt, C. Hermansky, D.L. Valcore and J.E. Bryant, Modeling Atomization and Deposition of Agricultural Sprays/Proc. ILASS–Americas 97. pp. 178 – 182. Ottawa, Canada, 1997.

6. G.A. Matthews, Pesticide Application Methods Longman, London and New York, pp. 336, 1979.

7. R.A. Maynard, A.R. Womac and I.W. Kirk, Nozzle Classification Factors for Ground Applications, Paper No. 961074, ASAE Annual Meeting: Phoenix, AZ, 1996.

8. P.C.H. Miller, C.R. Tuck, AJ. Gilbert and GJ. Bell, The Performance Characteristics of A Twin–Fluid Nozzle Sprayer, BCPC Mono. No. 46, AL-Assisted spraying in crop Prot., pp. 97 – 106, 1993.

9. P.C.H. Miller, E.C. Hislop, C.S. Parkin, G.A. Matthews and AJ. Gilbert, The Classification of Spray Generator Performance Based on Wind Tunnel Assessment of Spray Drift/Pmt. A. N.P.P. – B, C.RC. 2nd In Symp. Pestic. Apple Tech., pp. 109, 116, 1993.

10. C.S. Parkin, AJ. Gilbert, E.S.E. Southcombe and CJ. Marshall, The British Crop Protection Council Scheme for the Classification of Pesticide Application Equipment by Hazard, Crop Protection Vol. 13, No. 4, pp. 2X-5, 1994.

11. E.S.E. Southcombe, P.C.H. Miller, H. Ganzelmeier, J.C. Van De Zande, A. Miralles and AJ. Hewitt, The International (BCPC) Spray Classification System Including a Drift Potential Factor/Pro. Br. Crop Prot Conj., Vol. 5A – 1, pp. 371 – 380, 1997.

12. M.E. Take, J.W. Barry and B. Richardson, An FSCBG Sensitivity Study for Decision Support Systems, Paper No. 961037, ASAE Annual Meeting: Phoenix, AZ, 1996.

13. M.E. Teske, S.L. Bird, D.M. Esterly, S.L. Ray and S.G. Perry, A User’s Guide for AgDRIFT™ 1.0: A Tiered Approach for the Assessment of Spray Drift of Pesticides, Technical Note No. 95 – 10, CDI, Princeton, NJ, U.S.A., 1997.

14. P.J. Walklate, P.C.H. Miller, M. Rubbis and C.R. Tuck, Agricultural Nozzle Design for Spray Drift Reduction/Proc. ICLASS–94, Rouen, France, pp. 851 – 858, 1994.

15. N.M. Western, E.C. Hislop, P.J. Herrington and E.I. Jones, Comparative Drift Measurements for BCPC Reference Hydraulic Nozzles and for an Airttec Twin–Fluid Nozzle Under Controlled Conditions/Proc. BCPC Conf. – Weeds, pp. 641 – 8, 1989.

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Drop Size Classifications for Agricultural Sprays Size Classifications for Agricultural ... spectra that will be produced by the atomization of aerial sprays ... Drop Size Classifications