Pressure
Pressure is a measure of the force exerted on a surface per unit area. It is expressed in units such as psi (pounds per square inch), kPa (kilo Pascal), millibar and many others.
Pressure can be exerted by all states of matter. The atmosphere exerts a pressure of 101 kPa on everything at the surface of the earth. Pumping blood exerts a pressure of 120 mmHg against the walls of your arteries.
The pressure exerted by solids is mainly of interest to mechanical or structural engineers and has to be calculated. You cannot simply “sample a bit” of a building to understand the pressure it is exerting on it’s foundation. I’ll be focusing on fluid pressure in this article since that’s what I understand and deal with in my profession.
Why measure pressure?
Pressure is hugely important when dealing with gases. A gas is compressible — it’s volume is a function of temperature and pressure. As pressure increases the volume of a gas is reduced. To put it another way, the relationship between volume and mass is not a constant, i.e. density is a function of pressure. What this means is that “how much” of a gas is contained in a given container (volume) is dependent on temperature and pressure! So if you are buying or selling a gas, it is critical to know the temperature and pressure of the gas.
