Barometers

A barometer is basically a giant straw. For that reason it is essential that you understand how drinking straws work BEFORE you read this section.

As discussed before, when you drink through a straw you allow the atmosphere to push the liquid up the straw by decreasing the pressure in the straw. The question that can be asked then, is how high can the atmosphere push water?

The answer to that questions depends on two things: how hard the atmosphere is pushing and how low the pressure inside the straw can become. (In case you were thinking that the width of the straw also mattered, here is an explanation of why it doesn't.)

The second of those two is easiest to control. If we put a perfect vacuum at the top of the straw, then the atmospheric pressure is all that determines how high the water can be pushed. This is actually quite easy to do. In stead of using a regular straw, use one that is sealed at one end. Place the sealed end down and FILL the straw with water. Then, invert (turn over) the straw without letting any of the water out and place it in a glass (or tub or pool, etc) of water. If the water in the straw drops there will be NOTHING above it (since that end is sealed).

If you have ever blocked a straw with your tongue, you have seen a similar situation. In that case, the water did not drop and the straw stayed filled. In fact the straw will stay filled (the water won't drop) with a straw that is taller than you are. 

In fact to see the water drop you would need a straw that was more than thirty feet tall.

Now, here's the weird part. If you had a 35 foot straw, a 50 foot straw and a 100 foot straw, then filled and inverted them all, the water in each would drop. But, it would drop much more in the longer straws so that the height of the water remaining in the straws was the same (no matter how much empty space was above it.

That height (roughly 32 feet on a normal day, at sea level) is how high the atmosphere can push water. Remember, the atmosphere is fighting the pull of gravity on the water, so that has to be a limit as to how high the atmosphere can push (or hold) water.

Now, here's the important part: if a mass of air moved in (the weather was changing, say) that wasn't pushing as hard...the water would drop a little. Conversely, if a mass of air moved in that was pushing harder, the water would be pushed higher. That means that the height of the water in our straw (with a vacuum at the top) is directly (and measurably) related to the pressure of the atmosphere.

There are two problems with this scenario, both are about water in particular. The first is that water evaporates. That means that it is actually impossible to have NOTHING at the top of the straw. 

The other problem is that it is very difficult to find a place to put a 40 foot straw.

We can solve both of those problems by changing liquids. Instead of water, we use mercury (Hg). Mercury is 13.6 times denser than water, so the atmosphere can not push it nearly as high (greater force of gravity to fight) and it does not (measurably) evaporate, so we can maintain a vacuum at the top of the straw. This is called a barometer.

Since we can now determine the atmospheric pressure by measuring the height of the column of mercury, one of the more common units of pressure is millimeters of mercury (mm Hg). This unit is also called a Torr, named after Evangelista Torricelli (the inventor of the mercury barometer). So 23 mm Hg is the same thing as 23 Torr.

In the United States, where we still haven't caught on the the metric system, the unit is inches of mercury. So, a weather forecasters might be heard to say something like “the barometer is a 30.5 and falling.” Translated, that means that the pressure of the atmosphere, as measured by a barometer, is 30.5 inches of mercury and that it is getting lower (because an air mass that isn't pushing as hard is moving in).

Torr (or mm Hg) are useful, because the are related to the way we measure pressure, but the numbers can be annoying. For instance, during the course of the year the pressure in one area may vary from 720 – 830 mm Hg. It could be substantially lower if you live in the mountains. 

When we do math with numbers, we often prefer simple numbers that are easy to work with. For that reason, chemists invented another unit – the atmosphere.

The atmosphere is a simple unit to understand. If you took a barometer to a point that was at sea level and then measured the atmospheric pressure every hour for a year, the average pressure would be about 760 Torr. So, we (the chemistry community) decided that an atmosphere would be defined as 760 Torr. 

This means that when we do math with pressure, under most circumstances the pressure is close to 1 atm which is easier to deal with that 760 Torr.

There is another unit which is also used for pressure. This unit is based on the metric system and the definition of pressure (force/area). The standard metric unit for force is a Newton. (A Newton is roughly the force of gravity (the weight) of a stick of butter. The standard unit for area is the square meter. So, pressure is measured in Newtons/m². This unit is called a Pascal (Pa). 

In simplest terms then, a Pascal is roughly the pressure exerted by a stick of butter spread out over a square meter – not a lot of pressure. 

The atmosphere exerts much more pressure than this. In fact 1 atm = 101.325 kPa. In other words, the atmosphere normally pushes as hard as 101,325 sticks of butter spread out over a square meter. Although the kPa is somewhat disconnected from actual gas pressure measurements, it is used often enough, simply because it is a standard metric unit, while the others (mm Hg, Torr and atm) are not.

There are, of course, other units of pressure like psi (pounds per square inch) and millibars, but you don't really need to understand either of these to understand gases.