Low-Pressure Generator Makes
Cleanrooms Cleaner
Industrial Productivity/Manufacturing Technology
Originating Technology/ NASA Contribution
Scientists at NASA’s Kennedy Space Center work in cleanrooms:
laboratories with high degrees of cleanliness provided
by strict control of particles such as dust, lint,
or human skin. They are contaminant-free facilities,
where the air is repeatedly filtered, and surfaces
are smooth to prevent particles from getting lodged.
Technicians working in these environments wear specially
designed cleanroom “bunny suits” and booties over their
street clothes, as well as gloves and face masks to
avoid any contamination that may be imparted from the
outside world. Even normal paper is not allowed in
cleanrooms—only cleanroom low-particulate paper. These
are sensitive environments where precision work, like
the production of silicon chips or hard disk drives,
is performed.
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Setra Systems,
Inc., has produced the first portable, low-pressure
calibration system capable of performing in
situ calibrations of high accuracy, very low-differential
pressure transducers. |
Often in cleanrooms, positive air pressure is used
to force particles outside of the isolated area. The
air pressure in the Kennedy cleanrooms is monitored
using high-accuracy, low-differential pressure transducers
that require periodic calibration. Calibration of the
transducers is a tricky business. In prior years, the
analysis was performed by sending the transducers to
the Kennedy Standards Laboratory, where a very expensive
cross-floated, labor- intensive, dead-weight test was
conducted.
In the early 1990s, scientists at Kennedy determined
to develop a technique and find equipment to perform
qualification testing on new low-differential pressure
transducers in an accurate, cost-effective manner onsite,
without requiring an environmentally controlled room.
They decided to use the highly accurate, cost-effective
Setra Model C264 as the test transducer.
For qualification testing of the Setra, though, a portable,
lower-cost calibrator was needed that could control
the differential pressure to a high degree of resolution
and transfer the accuracy of the Standards Laboratory
testing to the qualification testing. The researchers
decided that, to generate the low-differential pressure
setpoints needed for qualification testing, very small
gas volume changes could be made against the test article,
and a corresponding pressure change would be detected
by a pressure standard. This allowed the researchers
to recreate cleanroom air pressure settings without
the use of a cleanroom.
Thus was born the low-differential pressure generator.
In 1993, a prototype was developed using a pair of
PVC tanks, a volume controller, and a 1-pound-per-square-inch
pressure standard. By 1995, the prototype was
perfected into the unit that is still used today.
Partnership
As with so many NASA-inspired inventions, the scientists
were in need of a new piece of equipment, so they built
it themselves. Stephen Stout and Richard Deyoe of Kennedy
were the two principal researchers on this project
and they, with Greg Hall, patented it as the Low Differential
Pressure Generator in 1997.
Ironically, personnel at Setra
Systems, Inc., the Boxborough,
Massachusetts company whose original Model C264 pressure
transducer had been tested, came across the new technology
in 2002, while conducting a search for the same type
of equipment that the staff at Kennedy had wanted.
The new low-pressure generator was described in an
article in NASA Tech Briefs. Setra was seeking a pressure-generation
method that would isolate the differential pressure
sensors from environmental noise during the calibration
procedure, a problem that was discovered while working
with pharmaceutical manufacturers attempting to certify
critical air handling processes.
Setra then contacted Kennedy’s Technology Transfer
Office to obtain rights to the patent, and the NASA
office facilitated the paperwork and provided them
exclusive rights to the technology.
Product Outcome
Setra, known for its simplicity of design, high accuracy,
exceptional long-term stability, and competitive pricing,
incorporated the NASA technology into its Micro-Cal
Low Pressure Calibrator, and now offers this unit among
its product line.
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The unit is battery
powered, with compact size and a lightweight
case that can be carried to cramped, remote
locations, even up stairs or ladders. |
Technicians have just one portable unit, instead of
having multiple components, like the pressure indicator,
pressure generator, and data logger to carry to calibration
sites. The unit is battery powered, with compact size
and a lightweight case that can be carried to cramped,
remote locations, even up stairs or ladders. It is
a significant improvement over the expensive primary
pressure standards that the transducers had to be tested
against. Additionally, it boasts superb pressure-reading
accuracy, as well as fast, stable, repeatable, and
accurate pressure generation. The user can also select
or configure pressure generation profiles.
It offers calibration data storage with download capability,
data and process security, dual reference pressure
sensors to cover a wide range of test pressures at
the highest accuracy possible, and calibration times
as fast
as 5 minutes per unit, which generate significant laboratory
cost savings. It also has a simple personal digital
assistant user interface. Most attractive, even with
all of these added features, it is almost half the
cost of a laboratory benchtop calibrator.
Most manufacturers of low-pressure calibrators do not
use true low-range reference pressure sensors. Instead,
they use higher-range sensors and attempt to achieve
high accuracy at lower pressures through intricate
microprocessor correction. The resulting higher levels
of noise and instability limit the ultimate accuracy
available. Setra uses patented stretched diaphragm,
capacitive sensor technology for highest output at
the lowest pressures.
Another area where typical low-pressure calibrators
fall short is generating a stable, repetitive, and
accurate test pressure. Most companies use micro-solenoid
pressure generation and regulation, a technique that
applies small pressure pulses to the positive and negative
pressure test volumes to regulate the test pressure.
During active pressure regulation, this system generates
pneumatic noise. Setra uses NASA’s low-differential
pressure-generation technology that produces maximum
pressure-setting sensitivity with minimum noise.
The pressure generation is accomplished using a piston/cylinder
arrangement, whereby the differential pressure sensor
under test has both high- and low-pressure ports connected
to the cylinder in a push/pull configuration. As the
stepper motor-driven piston advances in the cylinder,
it applies positive pressure to the high port of the
test pressure sensor and negative pressure to the low
port. The resulting pressure-generation system is sealed
and immune to the outside environmental noise and has
twice the sensitivity as a single-sided piston and
cylinder.
Setra has automated the patented NASA Low Pressure
Generator using micro-stepping motors and true low-pressure
reference transducer feedback. This combination has
produced the first portable, low-pressure calibration
system capable of performing in situ calibrations of
high accuracy, very low-differential pressure transducers.
Micro-CalTM is a trademark
of Setra Systems, Inc.
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