Pascals, pounds per square inch, millimetres of mercury, inches of water, bar, atmospheres (or Atm), Torr—you might recognize some of these, but they all essentially measure the same thing. Air pressure. But why are there so many different units to measure air pressure?
Air pressure is what keeps Farley domes up, so here are some interesting facts behind why there are so many names for the same thing.
Pressure is a relatively simple physical property. Essentially, it is measured by the amount of force acting on a given amount of area. For an example of this, you’ve likely seen—or even experienced—a bed of nails. If you were to lie on one nail, you could seriously hurt yourself, right? But lying on a bed of nails doesn’t hurt because the force of your body mass is distributed across a larger area. The same amount of total force exists, but the pressure at any point decreases as the surface area increases. Simple, right?
Air pressure does essentially the same thing. The air in a system exerts a force on everything it touches, and this creates air pressure. Follow this link for a more in-depth explanation of air pressure and how it helps domes stay inflated.
Getting back to the units of air pressure: the fact that there are so many units of air pressure have confused nearly everyone—even those that deal with it all the time. Misunderstandings can easily happen, so why are there so many different units?
Well, some measures of air pressure were decided upon by how much force the air is able to exert on a given parameter. For example, how much water is being displaced by the force of air pushing down on it. That’s where the obvious measurements like inches of water and millimeters of mercury come from. The unit Torr, was derived from the millimetre mercury measurement and was the first unit to describe the atmospheric pressure.
After the Torr, the bar (and millibar) was introduced by a meteorologist named William Napier Shaw in 1909. A bar is roughly the pressure of the atmosphere. This is actually where the word barometer comes from.
An interesting thing about the atmospheric pressure, though, is that it’s constantly changing. Which is why another measurement exists, called an “atmosphere.” This measurement is also meant to be the pressure of the atmosphere, but confusingly 1 bar is roughly 0.987 atmosphere. Yeesh!
Luckily, some units make a bit more sense.
You probably know of the pressure unit of pounds per square inch, or psi. This is a common measurement used when inflating things like soccer balls, tires, or air supported structures. Since air pressure is force over area, pounds per square inch is a simple translation. Pounds is the measurement of force, and square inches is the measurement for area. Easy!
And the last major unit of measurement for air pressure is the Pascal. If you follow meteorology, you’ve likely heard of the Pascal or the kilopascal (kPa)—which is simply 1000 pascals. The pascal is now the standard unit for measuring air pressure, so most units get converted to kPa, for simplicity sake. There’s actually a cool history to the Pascal unit, but we’ll talk about that in a follow up blog.
So as you can see, the multiple units each have a unique history based on when, how, and by whom they were discovered. And this is why there are so many. A lot of work has been done for meteorology, fluid physics, and gas chemistry, and all those scientists were quantifying and measuring (almost) the same thing! Pressure is an incredibly important thing—and not just for air domes. It’s no wonder it’s such a popular field of study.