# How do gas flow regulators avoid choking?

According to compressible flow theory, a downstream pressure of 0.528 times upstream pressure is sufficient to choke a flow device.

So, how do industrial grade pressure regulators throttle a wide range of pressures (say 150 bar to 20 bar)?

Update: if you're worried about energy losses in the gas stream, due to viscous dissipation: don't. If a perfect gas (that's an ideal gas with an additional condition for the specific heat) enters a thermally insulated throttle device at low speed and exits it at low speed (through a pipe with a large diameter), then its temperature will be the same as when it entered. One way is to look at the enthalpy of the gas, $H=m C_V T+ pV$, which must be conserved if no energy is added or removed along the way. Because $pV=nRT$, this condition can only be met if the gas has the same temperature before and after. Note that I mentioned "low speed", which is what you typically want in a pressure regulator. In the nozzle, where the speeds are sonic, the energy balance will also include kinetic energy of the gas jet.