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(Left to right) Matthew Bond and GTRI researchers Tom Perry and John Trostel discuss a field mill prototype at GTRI's Severe Storms Research Center. Photo: Gary Meek |
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On a recent blustery afternoon, scientists gathered on a rooftop at the Georgia
Tech Research Institute (GTRI) to observe two atmospheric electric field-mill
devices monitor the buildup of electrical charge in nearby clouds. The larger
device was a commercial model costing about $5,000. The smaller one, built by
five Georgia high-school students using a coffee can, electrical components,
and a motorcycle battery, cost about $200.
The accuracy of the field mills' readings of electrostatic charge—a critical
measure of lightning potential—was comparable.
"If we could put 25 of these low-cost field mills on high schools
around Georgia, the resulting array would let us measure charge buildup in
storm clouds and help answer important questions involving lightning
activity," said John Trostel, a senior research scientist who is director
of GTRI's Severe Storms Research Center. "We'd also be offering a unique
educational opportunity to Georgia
science students—the chance to build a precision scientific instrument and then
see it in operation."
Building an accurate field mill, Trostel explained, is not a trivial task.
The present device is the result of several semesters of work by four students
from Kennesaw Mountain
High School in Kennesaw,
Ga., and another student from The Walker
School in Marietta, Ga.
Under the guidance of Georgia Tech personnel, each student worked a semester
during his senior year as a research intern in the Severe Storms
Research Center
at GTRI's Cobb County Research Facility. For the Kennesaw
Mountain students, the work at GTRI
represented a capstone project completing four years in that school's Academy of Math and Science Magnet Program.
"We've been fortunate to have students working at GTRI for six years
now," said Kelly Ingle, a science teacher who supervises the Magnet
Research and Internship Program at the high school. "It's a real win-win
situation, because the students are gaining experience that will help launch
them on their college and professional careers, and they're also doing
something beneficial for the mentoring facility."
A team effort
The five students—David Brinkmann, Matthew Bond, Alex Hale, and Andrew
Brinkmann from Kennesaw Mountain and Stephen Pfohl from The Walker School—each
tackled part of the research and design effort required to build a working
field mill, explained Tom Perry, a GTRI electrical engineer who played a major
supervising role.
Starting in 2008, David Brinkmann spent a semester researching and building
a prototype. His task was to demonstrate that it was indeed possible for high
school students to build an inexpensive but accurate field mill.
David Brinkmann—now a student at the U.S. Coast Guard Academy—considered
several options before choosing a field-mill concept from a German website. He
developed a cylinder-shaped device, consisting of handmade circuit boards held
together by nuts and bolts that fit snugly into a coffee can. The total cost
was about $90.
In 2009, Bond spent his spring semester improving David Brinkmann's model by
adding electrical components, including a data-logging function for detecting
voltages and a voltage regulator that ensures uniform performance of a rotating
blade that spins above the sensing elements, inducing alternating positive and
negative charges.
He then tackled the challenge of calibrating the field mill, which involves
correlating voltage readings with field strengths in the atmosphere.
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(Left to right) John Trostel, Matthew Bond, and Tom Perry adjust a student-designed field mill operating on the roof of GTRI's Cobb County Research Facility. To the left is a commercially made field mill. Photo: Gary Meek |
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"I built a 10 ft by 10 ft structure—basically a capacitor—out of wood
and aluminum foil," recalled Bond, now a student in mechanical engineering
at Mississippi State University. The mill was then inserted into the gap between
the two capacitor plates. "We ran different field strengths across the
capacitor and then did a regression on the data. That gave us the equation we
needed to translate the voltages we were seeing to actual field strengths,
which are the useful data that we wanted."
In 2010, Hale used computer-aided design (CAD) design software to develop a
final circuit-board design that could be fabricated commercially. Now at the University of Alabama, Hale also built a field-mill
mockup to demonstrate that fabricated boards would fit correctly, and he
improved the device's electrical characteristics.
"We used several professional-level computer programs to improve the
layout and develop a final design that could be sent to a fabricator,"
Perry said. "Alex did all the CAD work that went into the final board
design. At each step I showed him partially how to do it. Then I would erase my
work, and he would go on to complete it."
The field mill currently being tested by GTRI employs a commercially
fabricated circuit board that was made using Hale's design. The robustness of
the $200 field mill is still under study. Device accuracy is excellent, but
issues include long-term calibration consistency and materials lifespan.
"Before we actually distribute any field mills to schools, we're
calibrating several of them and comparing their operation with commercial field
mills over time," Trostel said.
Continuing the project, during the summer of 2011 Pfohl was tasked with
constructing five copies of this successful design to be used in comparison and
for stability testing. Andrew Brinkmann, David’s younger brother, then spent
the fall semester of 2011 testing and comparing the outputs of the five
devices. This testing led to numerous improvements in the mill design that
should make them both more reliable and more robust.
Field mill kits
Trostel explained that with commercial circuit boards and other parts now
easily available, it may soon be possible to take the next step: distribution
of low-cost, standardized field mill kits and operational procedures to leading
Georgia
high schools. That step would allow students around the state to investigate
local electric field effects. Mounted on the school's roof, the device would
send lightning data via a wired connection to a computer in the school.
Having 25 or so field mills around the state would allow Georgia schools
to participate in an extensive field-mill array capable of making large-scale
electric field measurements. Such an array could make a real contribution to
lightning research by supporting investigation into such research problems as
the initiation and cessation of lightning during a storm.
"An array of field mills over an extensive area would give us more
insight into when lightning first begins during a storm, and when it
ends," Trostel said. "When we know more about when these points are
reached, it will help us establish dependable guidelines on lightning
dangers."
Also under study are the best methods for supplying power to the device.
Solar power shows promise, and its use would make the student field mill more
portable than commercial models, which require connection to power lines. But a
solar panel adds to the overall cost, and researchers must still solve the
problem of storing enough solar energy to outlast sunless periods in winter.
Many U.S.
high schools are already part of existing weather networks that integrate
weather data into classroom learning, Trostel said. The GTRI team wants to work
cooperatively with such a network, enabling a Georgia field-mill array to connect
to an existing Internet-based system.
"It would be really valuable to have high school students be part of
this kind of cutting-edge, real-world experiment," Trostel said. "It
would help get them excited about science."
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