1999

     Though 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!

     It 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.

     To 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.

    How 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.

     Imagine 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?

    The 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.

     To 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.

     There 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!

     Since 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 scale.

     If 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.