Spray Droplet Sizing –Understanding the Basics

Brad FritzUSDA ARS

Aerial Application Technology

Research Unit

College Station, Texas

Droplet Diameter

DD = droplet diameterV = droplet volume

At equal volume with droplets of ½ diameter

8X the # Droplets

Droplet Volume in the Spray Cloud

One 400 µm drop

Droplet Volume in the Spray Cloud

8 - 200 µm drops

Droplet Volume in the Spray Cloud

64 - 100 µm drops

Total Spray Volume

Characteristics of total spray volume.

Number Distribution Or

Volume Distribution

Volume vs Number

1X 400µm

8X 200µm

64X 100µm

Total Spray Volume of:V400 + 8(V200)+ 64(V100)

V400 = 64 x V100= 8 x V200

We have a total Volume V.

V400 + 8*V200 + 64*V100 = V

Why Volume vs Count?Count Distribution

We have a total of 73 drops.

1 + 8 + 64

400 µm =

200 µm =

100 µm =

Volume Distribution

400 µm =

200 µm =

100 µm =

Amount of active product:Volume of spray droplets

NOT Number of droplets

Volume Distribution

• DV10 or DV0.1

• DV50 or DV0.5 or VMD

• DV90 or DV0.9

• %Vol<100µm• %Vol<200µm

• How do we get to the numbers we typically deal with?

• Using some measurement system, the percentage of the total spray volume that is in each droplet size range is determined.

300 µm10%

Review• DV10, DV50 or VMD, DV90

• The diameter for which 10% of the total spray volume is made up of droplets of equal or lesser diameter.

100% 10%D

This diameter is the DV10

Review

100%50%

This diameter is the VMD

D

150 µm 370 µm 634 µm

4.7%

17.2%

Relative Span

An indicator of the width of distribution.

RS = 0.67

RS = 1.4

RS = 2.2

VMD = 300µm

DV10

200 µm

DV90

400 µmDV10

165 µmDV90

435 µm

DV10

90 µm

DV90

510 µm

Take Home

• At equal volume:– Halving the diameter creates 8X droplets– Quartering creates 64X droplets

• The smaller the diameter, the greater the number of droplets, and the less control you have over them.

• Volume Distribution corresponds to available product and efficacy– DV10, DV50 (VMD), DV90, RS, %<100µm, etc…

Spray Measurement

Measurement

Groundspeed?

It’s All About How You Measure…

Likely Will Get Different Exact

Numbers

It’s All RelativeSlowest

Fastest

Same Concept

SMALL

MEDIUM

LARGE

While Numerical Diameters may differ, Relative Size will be consistent

Reference Sprays• Establish Reference Nozzles/Sprays

VF/F F/M M/C C/VC VC/XC

Reference Nozzle Curves

VERY FINE

FINE

MEDIUM

COARSE

VERY COARSE EXTREMELY

COARSE

MEDIUM SprayDV10: 80 to 110 um

DV50: 190 to 280 um

MEDIUM SprayDV10: 105 to 155 umDV50: 230 to 350 um

Be Cautious with NumbersASABE S572 Standard

Labels – Beware of Conflict

Better to go by Class than Size

Take Home

• Droplet Size Number can vary from lab to lab, company to company….

• BUT

• Droplet Size Classification is consistent.

Differences Within a ClassMEDIUM Spray – USDA ARS Current Nozzle Models

141

180

280

429

MEDIUM

2 MEDIUM SpraysMedium #18°, 60 psi and 150 mphDV10 = 184VMD = 322DV90 = 543%<100um = 2.7%

Medium #238°, 60 psi and 150 mphDV10 = 146VMD = 282DV90 = 433%<100um = 5.6%

CP11TT with 4015 FF

2 MEDIUM Sprays

141

180

280

429

MEDIUM

MED#1

MED#1

What Happens in the Field?

Downwind Drift

Medium #11.6%

Medium #2 2.4%

AGDISP

Take Home

Consider Size Class first.

Then

Optimize setups to decrease fines.

RememberNot all MEDIUM or COARSE sprays are equal