The choked air flow lab is connected to an 80 psig supply line with a flow meter, valves, and two pressure sensors.

Choked flow may occur when a flowing fluid at a given pressure and temperature passes through a restriction into a lower pressure environment. At initially subsonic upstream conditions, the conservation of mass principle requires the fluid velocity to increase as it flows through the smaller crosssectional area of the restriction. At the same time, the Venturi effect causes the static pressure, and therefore the density, to decrease downstream beyond the restriction. Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment while upstream pressure is fixed. Note that the limited parameter is velocity, and thus mass flow can be increased with increased upstream pressure (increased fluid density). from Wikipedia