Glossary

The ideal gas law is the go-to equation that relates the four big “knobs” of a gas—pressure (P), volume (V), temperature (T), and amount of gas (n)—assuming the gas behaves ideally (i.e., the molecules are tiny, don’t attract each other, and collisions are perfectly elastic). It’s written as:

PV = nRT

Where P is absolute pressure, V is volume, n is the number of moles of gas, T is absolute temperature (Kelvin), and R is the universal gas constant (the value of R depends on the units you choose).

Why it matters in industrial/chemical work: it lets you calculate how a gas will expand, compress, heat up, or cool down in systems like pressure vessels, piping, compressors, cylinders, and purge/blanket systems. For example, at constant volume, increasing temperature increases pressure; at constant temperature, compressing volume increases pressure—exactly the kind of reality that makes pressure equipment design and safety such a big deal.

A couple of important notes:

- T must be in Kelvin and P must be absolute (psia, bara), not gauge pressure, or the math goes sideways.

- Real gases deviate from ideal behavior at high pressure and/or low temperature (and near condensation). In those cases you’ll often see a compressibility factor (Z) used: PV = ZnRT to correct for non-ideal behavior.