Abstract: An improved and accurate enthalpy tunnel is presented, designed to condition and control airflow patterns within the main tunnel and air sampling subsystem so as to present a homogenous volume immediately upon introduction into the enthalpy tunnel. The air volume velocity is slowed and the air volume is completely mixed in a settling chamber which also serves to maintain the static discharge pressure on the unit under test. The rectangular design in use in the industry is replaced with circular geometry, using a cylindrical tunnel shape as opposed to a rectangular shape. The circular geometry creates a flat, uniform velocity profile. This tunnel improves the design of current art enthalpy tunnels by using a single nozzle instead of a bank of nozzles, presenting to the sampling mechanism a smooth, stable and uniform flow profile. The sampling method used in the prior art is replaced with a sampling tunnel that also conditions its air flow profile, leading to consistent sampling.