■ On the first run, both Hobo dataloggers recorded nearly
the same temperature changes. So, it appears that the MLI
is a waste of time and materials for near space experiments.
■ But then again, maybe it’s not a wasted effort. This chart
shows that the MLI covered cube is nine degrees warmer
after 35 minutes in the environmental test chamber.
temperatures are not quite as low. To
find out, I did a little experiment with
my new environmental test chamber.
For this experiment, I built two
identical cubes from 1/2 inch thick
Styrofoam. One cube has a wrapping
of green tape and the second a
wrapping of three layers of MLI.
The cubes were hollow, so a Hobo
datalogger could sit inside and record
the interior temperature. After
loading the Hobos, the cubes
were tightly taped shut. The big
environmental chamber was then
loaded up with dry ice and allowed
to chill in preparation for the test.
Both cubes were loaded inside the
chamber, the door was closed, and
the chamber pumped down. The test
ran for about 30 minutes before the
cubes were removed.
Because of the possibility that
the dataloggers wouldn’t record the
same temperature under identical
conditions, the cubes were allowed
to warm up and the dataloggers were
switched between cubes. The
process was run a second time for
about 30 minutes. Afterwards, the
Hobos were removed and their data
downloaded. You can see from the
charts that it was a good thing my
classroom ran the experiment a
second time.
Looking at these results, I suspect
the Hobos are not calibrated quite the
same. Perhaps it would be appropriate
to average the two tests and claim
the MLI covered cube remains four
to five degrees warmer than a cube
without MLI. I’d like to think this is
the case because of the amount of
time I spent covering my airframes in
MLI. Before I try this test again, I‘ll set
the Hobos out and verify that they
record the same temperatures.
The only problem I ran into during
this experiment is that the lid on the
large environmental chamber is loose
enough that air pressure outside can
shift it just enough to the side to break
its seal. Eventually, I’ll have to have a
new lid machined for the large chamber
that fits a bit more snuggly.
THE
HANDI-VAC
I was out grocery
shopping when I saw
■ Running a test on
the Handi-Vac vacuum
chamber. Inside the
sealed zip lock bag is the
Rubbermaid container
with a pressure sensor.
Reynolds (maker of aluminum foil)
developed what might be a
convenient vacuum pump. Handi-Vac is a food storage system. Food
is loaded into a zip-lock storage
bag and then a battery-operated
handheld pump evacuates the air
inside. So, I thought this would make
a great vacuum pump for science
experiments. I purchased one and
an extra set of bags in the name
of science.
The Handi-Vac pump has a
rubber seal in its nose (it’s called the
suction tip) that’s placed in contact
with a specific region marked on the
zip-lock bags. This area has a series
of tiny openings that allow air to be
pulled out, but seal when air tries to
flow back in.
Now, I’ve observed vacuum
bagging before. After removing the
air, the contents inside the bag are
crushed by atmospheric pressure.
This is good when you need to
apply pressure all around an object,
like when curing a composite set-up.
But the crush is a disaster for the
experiments I want to perform. To
prevent the crush, I purchased some
large Rubbermaid ™ cube shaped
plastic storage containers. The
vacuum bag can’t crush a storage
container inside of them, but
experiments inside the container
will still experience a vacuum if holes
are drilled through the container
and its lid. I selected roughly cube
shaped storage containers to
22
November 2009
