we live and work beneath a "sea" of air, the atmosphere is a
thousand times less dense than our own bodies, and feels charmingly insubstantial.
Yet oceans of air course above our heads every day, bringing with them
storms, fair weather and an occasional bout of bursitis. Still, the atmosphere,
though thin, is some 20 miles high- and if it could be compressed into
a liquid state like water, would form a column 33 feet tall. So every
day, we really are swimming in the deep end of the pool!
would be fascinating to watch the tidal waves of air collide, making the
insubstantial visible. Television stations report a "high or low"
pressure system as the reason for a change in the weather. Most days,
these changes are quite modest- usually running from 29.5 to 30 inches
of Hg (measured
by a mercury barometer). This is only a 1% or 2% variation, which
is the same pressure difference caused by small waves in a swimming pool.
make something so small visible we need something large. And what is more
visible than a car suspended on air 20 feet above your head? Thus the
VW Beetle Barometer1.
it works: The red disk is a piston that slides freely in the clear
cylinder shown above. On the VW side of the piston is air at atmospheric
pressure (about 14.7 psi or 30 in. Hg), and the other side a vacuum. The
air pushes up on the piston, while the VW tugs downward by gravity. The
vacuum, of course, is emptiness and does nothing.
the area of the piston was chosen to just balance a 2000 lb VW on a day
when atmospheric pressure was exactly 14.7 psi. In that case, the area
would be (2000 lb/14.7 psi =) 136 in 2, or a piston
about foot in diameter. Amazingly, a one foot piston, held up by air,
can hoist a car weighing a ton off the ground. On a clear day, brought
by a high pressure front, the air pressure would exceed 14.7 psi, and
the piston would slam into the top of the cylinder. On a slightly grayer
day, as soon as the pressure dropped below 14.7 psi, the VW would slam
into the ground. Dramatic, but not very useful. How can the VW be designed
to move in synchrony with the passing fronts?
solution is to hang a light chain (shown in blue) from the VW. In this
case, the piston is adjusted to just balance the weight of the VW PLUS
the dangling chain (the pile of chain on the ground is supported by the
floor, and does not tug on the Beetle). Now imagine the pressure drops
below 14.7 psi. First, the Beetle starts to fall. But, as it does, the
suspended part of the chain shortens, reducing the downward weight of
the car+chain. Eventually, the lower air pressure matches the reduced
weight of the VW+chain, and it stops dropping. The same process, in reverse,
occurs on a high pressure day.
detect a 2% change in air pressure, the chain should weigh about 2% of
the VW, or 40 lbs. Assuming the VW barometer is mounted in an atrium of
a science museum, permitting two stories of motion, the chain weighs (40
lbs /20 ft = ) 2 lbs/foot. Where I live, air pressure typically changes
by 0.01 in. Hg an hour, which means the bug barometer would move about
six inches an hour. Enough motion to be seen between a coffee break and
lunch. And of course, a computer would display a time lapse sequence of
images of the barometer rising and falling throughout the week. But on
a stormy day it might move six inches in five minutes. So the height of
a car suspended above the floor can be used to predict the weather. A
very practical Damoclean sword.
is one tricky part to this design- that frictionless piston is hard to
build in the real world. Even a well lubricated O-ring would lock the
piston in place until a large pressure change causes it to abruptly move,
by exceeding the "stiction" forces holding the piston to the
cylinder. Fortunately, the larger the piston, the smaller the effect of
friction. For example, the 136 in 2 piston above has a perimeter
of 41 inches. Assuming the O-ring contacts the cylinder over a narrow
contact area of (0.1 in x 41 in=) 4.1 in 2, and the
O-ring squeezes with at least 15 psi to remain leak free, it presses with
60 lbs of force on the cylinder. If the coefficient of friction is as
low as 0.1, then (60 lbs x 0.1=) 6 lbs of static frictional forces
must be overcome by atmospheric pressure before the piston moves. Not
too bad- this is equivalent to a sensitivity of (6 lb/2000 lb = )
0.3%- but a jumping one-ton barometer might be a little disconcerting!
the friction forces depends on the perimeter, while the piston air pressure
depends on the area, in a small piston the perimeter forces dominate.
For example, a similar calculation for a 500 lb motorcycle barometer gives
a sensitivity of 1.2% - which means (since the total measurement range
is 2%) the barometer might not move at all on some days. Fortunately,
there are a few possibilities (notably a liquid seal, and a rolling seal)
which are virtually frictionless and are easy to build on such a large
you are interested in a specific design for installation, please contact
Greg at the address below.
1 In 2006 Jeff Koons revealed plans to hang a full sized locomotive from a crane over the new LA County Museum of Art.