My prep-chemistry classes do a lab every year in which one of the activities is to tape straws together and see how high students can suck water. Some students invariably are able to reach the roof, about 10 ft high. However, it is difficult to seal the straws tightly together and results are very questionable. This year I decided to build a "giant straw" with which we could actually test the stated column peak of 34 feet.
Giant Straw of Science
Background:
Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of air above that surface at any given point in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 14.7 pounds (65 N). The weight of a 1 m
2
(11 sq ft) column of air would be about 101 kN.
The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101,325 Pa or 101.325 kPa. The following units are equivalent, but only to the number of decimal places displayed: 760 mmHg (Torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One atmosphere (101.325 kPa or 14.7 psi) is the amount of pressure that can lift water approximately 10.3 m (34 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). This is also the maximum height to which a column of water can be drawn up by suction.
Materials:
-50 feet of 1/4-inch clear vinyl tubing (aquarium tubing from a pet store, or get at most hardware stores)
-butcher paper - 50 foot roll
-two 2-in dia plastic pipe sections, 2 feet long
-water with red food coloring
-vacuum pump (hand pump model)
Procedure: I rolled the butcher paper out on the floor, and taped the rubber tubing down the center of it. With poster paint I marked 1 foot increments from the bottom to the top. I added a few drops of red food coloring to the water so that the level would be more visible against the white paper background.
We took the apparatus out to the roof of our theater building which stands approximately 50 feet high along the roof wall. The plastic (ABS) sections were taped to the bottom and top of the butcher paper to hold it down and to facilitate rolling it up.
On initial experiments in which we sucked on the "straw" only with our mouth we were able to attain a column height of only about 16 feet. We then brought out a hand vacuum pump and attached it to the top of the straw. Pumping by hand with the device we were able to acheive a maximum height of just over 30 feet. Although the pump pressure guage dial went to a max of 30 inches Hg (-100 kPa), we were only able to physically reach about 25 inches Hg (-85 kPa).
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