Units of pressure

Lina needs to pump up her bike tyres. To avoid overinflating them, Lina checks the markings on her tyres, and uses a pressure measuring device — a pressure gauge — attached to her bike pump. I’m confused! The markings say I should pump it up to 80, but the scale on the gauge only goes up to 7! Look again, Lina.

The markings read “max. 80 psi” and the scale is marked “bar”. Psi - which stands for pounds per square inch - and bar are both units of pressure. Throughout history, people have measured pressure in many different ways. As a result, there are several different units we use to describe pressure measurements today!

Wait, before we discuss all these different units of pressure – what is pressure again? Pressure is a force distributed over an area. Such as the force the air exerts on the inner surface of the tyre. We can express it as an equation where pressure equals force divided by area. Based on the International System of Units — the SI system — force is expressed in newtons, and area is expressed in square meters.

So, the unit of pressure is newton per square meter — or pascal. 1 pascal is a very small unit. To express larger values of pressure, we often add a prefix such as hecto or kilo. One hectopascal is equal to 100 pascals, and one kilopascal is equal to 1000 pascals. Hecto- or kilopascals are often used to describe atmospheric pressure in weather reports.

One of the first devices used to measure atmospheric pressure was a mercury barometer. In this device, liquid mercury moved inside a glass tube in response to atmospheric pressure. The movement of mercury was measured in millimetres. So, the pressure indicated by those barometers was expressed using millimetres of mercury. With time, the unit became standardised, and so 1 millimetre of mercury is now equal to about 133 pascals.

Mercury barometers are not as popular anymore, but the unit is still used, especially in medicine — for instance, when measuring blood pressure. There is one more unit related to atmospheric pressure — the standard atmosphere. This is used mostly by physicists and engineers. 1 standard atmosphere is equal to the mean atmospheric pressure at sea level, which is 101,325 pascals. Lina’s pressure gauge uses yet another unit — bar.

Bar is a relatively large unit — that’s why Lina’s gauge only goes to 7. 1 bar is equal to 100 kilopascals. That’s just a little less than the pressure of 1 standard atmosphere, which is about 1.013 bar. Bar, like the pascal, is based on metres, grams, and seconds. It is a metric unit.

The unit used for the markings on Lina’s tyres — pound per square inch, or psi — is based on the imperial system of units. 1 psi represents the pressure the force of 1 pound exerts when applied to an area of 1 square inch. 1 psi equals about 6895 pascals. When you know how different units of pressure relate to each other, you can convert them. If Lina’s tyres need to be pumped up to 80 psi, and the unit on her gauge is bar, she can convert psi to pascal to bar.

Let’s see! 1 psi is about 6895 pascals. So, 80 psi is equal to 80 times 6895 pascals, which is 551600 pascals. That is 551.6 kilopascals. 1 bar equals 100 kilopascals, so now Lina needs to divide her result by 100.

That is 5.516… – approximately 5.5 bar pressure!