In a plasma, the Boltzmann relation describes the number density of an isothermal charged particle fluid when the thermal and the electrostatic forces acting on the fluid have reached equilibrium.
James Joule tried to measure the internal pressure of air in his expansion experiment by isothermally pumping high pressure air from one metal vessel into another evacuated one.
The effective volume increase can be as much as 40% and is due primarily to a reduction in the speed of sound, and not to the popular misconception of a change in operating conditions from adiabatic to isothermal.