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can’t see gas. Some gases smell. The air around us is invisible.
It’s very hard to imagine air’s gas molecules pressing on
anything – unless the wind blows, of course. Go back 459 years. In the year 1643, an Italian scientist, Evangelista Torricelli, became curious about the air around him. He hypothesized that the air’s molecules created a pressure on the Earth’s surface. But how much? He wanted to find out. So, he took a tube, closed at one end, and filled it full of mercury (symbol Hg). He chose mercury because it is a heavy element, a metal, which also flows like a liquid. If his guesses were correct, he would need the weight of the mercury to balance the weight, or pressure, of the air. Water, he reasoned, might work, but probably would not be heavy enough, or would require too long a tube. He put his thumb over the open end of his tube full of mercury, turned it over, and lowered it into a bowl, which was also full of mercury.* Amazingly, the mercury in the tube didn’t flow out into the bowl. Most of it stayed in the tube. When he measured the height of the column of mercury in the tube, it measured almost exactly 760 millimeters (mm) from the top of the mercury in the bowl. If the mercury stays in the tube, he reasoned, it must mean that something, and not just the vacuum in the closed end of the tube, was keeping the mercury in the tube. The gasses of the air pressing down on the mercury in the bowl were keeping the mercury in the tube. AIR creates pressure! Torricelli experimented with his tube of mercury at different altitudes – by the sea and in the mountains. He found that at sea level, on a clear, dry day, the column of Hg in the tube was always the highest. • Torricelli was not aware that Mercury is poisonous. Do not do this experiment without taking proper precautions and using the proper equipment, gloves, etc. The tube has become a standard. After 459 years, weathermen use an instrument comparable to Torricelli’s “tube” every day. The tube has become our barometer. When the mercury in a barometer goes up or down, it means the pressure of the air at that spot is changing. This usually means the weather is changing. It might also mean that the barometer is being read at higher or lower altitudes. Modern scientists needed to define a standard to which they could compare all other air pressures. So they invented a standard called the “standard temperature and pressure” (STP). They decided that the perfect temperature would be 59( Fahrenheit and the perfect pressure would be 760 mm Hg. Every pressure reading is now compared to this standard. The pressure of 760 mm Hg has become known as one atmosphere. For his curiosity and ingenuity, Torricelli has been honored with a scientific unit all his own: 1 mm Hg = 1 Torr. Back to Space Station Alpha To make sure that they are all talking about the same thing – air pressure, NASA scientists and astronauts use mmHg as the scientific unit to describe the atmosphere and the gases on board the Space Station Alpha. Sinking into these ideas... In case you were wondering, Torricelli would have needed a tube 10.334 meter long to hold enough seawater to balance the air pressure at sea level! Put another way, 760 mm of mercury in a tube creates the same pressure as a 10.34 meters, or 33 feet of water in a tube. Deep sea diving Let’s dive a little further. If you like to swim and dive, every 10.34 meters you go down under water the pressure on your body increases by 1 atmosphere. So at about 33 feet under water, your body is experiencing 2 atmospheres of pressure – one atmosphere of water and one atmosphere of air. Get it? Divers and astronauts have to know this so they can make sure their bodies gradually get used to the different pressures. Acclimation to lower pressure is required to avoid the physical pain caused by stuffed-up sinuses and the bends that occur when the body goes from high pressure to low pressure in a very short period of time (like when a plane lands or when you drive quickly down a steep, high mountain). Divers and Astronauts beware If you dive deep, let’s say down to 3 or 4 atmospheres, you are in danger if you come back up too quickly. If this happens, what happens in your blood is like what happens when you take the top off a bottle of soda water. At great depths, the gasses you are breathing with the help of your diving apparatus get compressed in your blood. As you return to the surface the pressure drops, the gases in your blood that had been under pressure, turn into bubbles, and they fizz-up. The bubbles go through your veins to your joints and create no end of pain when you move – this is the bends. Astronauts would experience the same thing if there were a big leak on the space station and the atmospheric pressure dropped too quickly, or if they went from the safe pressures of the Space Station into their Space Suits without taking the necessary precautions. Tying some ideas together... You have read the Story of the Cube of Air. Now we can build some even more complicated ideas. Such as – if, where we are holding our cubic centimeter of air, the atmospheric pressure drops from 760 mmHg to 700mmHgHH, would the number of molecules of gas in our cube go down also? Yes. Air pressure is actually a measure of the number of molecules that we find in the air in a given weather situation or at a given altitude. In this way, we can say that the number of molecules in the air at any one spot depends upon the altitude or the weather – it’s the molecules of air that create the air’s pressure. In Space Station Alpha, we have power over the air pressure and the number of molecules of each kind of gas. Is there anything else? Yes, one confusing idea that’s basically simple! There are many different units scientists can use to talk about air pressure. The most widely used are “millimeters of mercury (mmHg)”, “pounds per square inch” (psi), “kilopascals” (kPa), and “inches of mercury” (inHg). NASA and Mission Control use mmHg in Space Station Alpha. Some scientists may use psi or kPa, depending upon the experiments they are conducting and the situation. Weathermen on US TV stations use “inches of Mercury” when they describe atmospheric pressure changes due to weather. Here’s how they compare. STP = 760 mmHg = 14.696 psi = 101.325kPa. = 29.9213 inHg. In Space Station Alpha missions, you won’t have to convert anything. Phew! We’ll go with NASA “mercury” talk. |
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