The main objective of this experimental investigation is to obtain and provide experimental data on the plenum to plenum natural circulation (NC) flows in a High Temperature Gas Reactor (HTGR) such as a General Atomics 350 Mat MHTGR. Experiments are performed using nitrogen gas and in the future will be performed using helium and a mixture of nitrogen and helium as well. The objective is to investigate a wide range of temperatures and pressures up to 750 C and 70 bar to simulate plenum to plenum gas dynamics under both Pressurized and Depressurized Conduction Cooldown (PCC/DCC). Gas circulation rates, and velocity and temperature distributions between upper and lower plane connected by multiple parallel vertical tubes with different linear heat generation rates will be measured using novel measurement methods, such as a thermal time-of-flight velocity sensor.