Drop Size Measurement
How methods and instruments have kept
pace with changing technology
There are many methods and instruments available for drop size data collection.
Since repeatable test results are essential in comparing nozzle performance
data, it is essential to use testing procedures that take into account
all of the potential variables in the sampling technique for both methods
and instrumentation.
By spraying water into a pan of oil and shuttering the spray, it is possible
to count and size individual drops using a microscope. This technique
is still used by some researchers. Problems with this method involve drop
coalescence, inadequate sample size and the fact that very small drops
will be deflected away from the oil by air currents at the surface due
to the spray velocity. Also, larger drops can and do break-up from impacting
the surface.
The same type of method is used when spraying a dye onto a stationary
card, or water onto liquid sensitive paper. Again, the small drops might
be deflected away from the target and the large drops can break-up from
the impact. Data collected by these "intrusive" methods depends on a number
of uncontrolled variables making such test results generally non-repeatable.
While drop size data was being collected in the early 1950's using methods
such as flash photography, probably the first real breakthrough in drop
sizing technology was the development in 1961 of an automated imaging
analyzer (Figure 5).1
1 The Electronic Imaging
Analyzer was developed at Spraying Systems Co. by Dr. Verne Dietrich and
built by the Dage Division of TRW, Michigan City, Indiana. The design
was awarded U.S. Patent 3275733 in September of 1966, and is currently
in its second generation.
Basically, the Electronic Imaging Analyzer incorporates the spatial measurement
technique using a strobe light to illuminate the spray and record the
image with a vidicon tube. The image is scanned, and the drops are sized
and separated into different classes. Resulting data can be mathematically
corrected using velocity data to give a flux distribution. Sources of
error early in the development of this device included blurring, depth
of field variations and vidicon tube saturation. These sources were recognized
and corrected.
The imaging type analyzer is still actively promoted by some nozzle manufacturers.
The limited availability of this type of instrument, however, prevents
independent researchers and other interested members of the drop size
analyzer community from verifying data arrived at from a particular test
or comparing performance from similar designs.
More recently the development of commercially available drop size analyzers
makes it feasible to verify drop size results by independent sources.
This new breed of analyzers incorporates lasers, special optics and digital
circuitry to minimize imaging error. Some of the more commonly recognized
manufacturers of laser measurement instruments include Malvern, Particle
Measuring Systems (PMS), and Aerometrics. The following is an analysis
of three of their instruments.
Malvern Particle Analyzer
The Malvern Analyzer, which is considered a spatial sampling device,
utilizes the fact that a spray drop will cause laser light to scatter
through an angle dependent on the diameter of the drop (see Figure 6).
The scattered light intensity is measured using a series of semicircular
photo diodes. Theoretically, the distance of the individual photo diodes
from the centerline of the laser and the intensity functions are all
that are needed to calculate the drop size distribution. A curve-fitting
program is used to convert the light intensity distribution into any
of several empirical drop size distribution functions. Since the Malvern
has some self-diagnostics, potential sources of error are easier to
identify. The instrument must be aligned and calibrated periodically
using reticle slides with known etched drop distributions.
Perhaps the biggest
source for error with this type of instrument is multiple light scattering.
If the spray is too dense, there is a possibility that the scattered
light from one drop might be scattered again by other drops further
down the beam axis. The Malvern is equipped with an "obscuration level"
indicator which can be used to determine if the spray is too dense,
but such a determination is often difficult. To circumvent this in the
lab, the technician typically moves the nozzle farther away or uses
special shielding to permit only a portion of the spray to enter the
sample area.
Particle Measuring Systems
Particle Measuring Systems, also know as PMS, produces instruments
known as Optical Array Probes. The PMS Optical Array Probe is a
flux sampling instrument (see Figure 7). As the drops pass through
the sampling plane, the drops are sized and counted providing information
which can be used to determine velocity. The two-dimensional grey
scale OAP can provide drop measurement in two ranges, 100 and 6200
microns and 200 to 12,400 microns, and is currently the most sophisticated
offered by PMS.
The PMS OAP Grey
Scale probes are extremely advanced and have extensive self-diagnostics.
These probes will reject drop images which are out of focus or which
do not meet a series of other acceptability tests automatically.
Problems with PMS units usually center on improper calibration or
maintenance. The optics tend to get wet easily and cleaning and
alignment require some skill. Also, dense sprays tend to overload
the circuitry and sample area reductions are often necessary. Sample
area correction factors and drop distribution curve fitting equations
are needed and left up to the operator to include in the analysis.
Aerometrics
The Aerometrics Phase Doppler Particle Analyzer, or PDPA, is a point
sampling device and a flux-sensitive instrument (see Figure 8).
Point sampling refers to an instrument that focuses on a portion
of the total spray pattern and requires targeting several test points
within the spray in order to obtain a composite sample of the spray
flux distribution.
The PDPA uses
a low power laser that is split into two beams by utilizing a beam
splitter and a frequency module. The two laser beams intersect again
into a single beam at the sample volume location. When a drop passes
through the intersection region of the two laser beams, an interference
fringe pattern is formed by the scattered light. Since the drop
is moving, the scatted interference pattern sweeps past the receiver
aperture at the Doppler difference frequency which is proportional
to the drop velocity. The spatial frequency is inversely proportional
to the drop diameter.
Aerometrics offers an optional fibre optic probe which isolates
the instrument from the spray and eliminates the potential for error
due to vibration caused by direct contact with larger capacity sprays.
Other drop sizing instruments which are commercially available,
generally use lasers and operate on principles which we have previously
discussed.
|
