In physics, a gas is a state of matter, consisting of a collection of particles (molecules, atoms, ions, electrons, etc.) without a definite shape or volume that are in more or less random motion. Gases are compressible and change volume when placed under pressure or are heated or cooled. A volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions. References will be made to "actual" flow rate through a meter and "standard" or "base" flow rate through a meter with units such as acm/h (actual cubic meters per hour), Kscm/h (Kilo standard cubic meters per hour), or MSCFD (thousands of standard cubic feet per day).