Entonox is not 100% nitrous oxide. N2O becomes a liquid somewhere over about 700 psi (sorry - working from memory). So a full nitrous tank is actually liquid, and the gauge reads ~700 psi until all the liquid has vaporized and there is less than 700 psi of nitrous in the tank.I know when an entonox (laughing gas) cylinder sits in the cold it separates, not sure about the science.
I would guess that the nitrous in Entonox (N2O and air at pressures over ~700 psi) would begin to return to a liquid phase over time, especially at low temperatures.