Abstract: A system and method for the automated measurement of properties related to a stack gas stream, flow velocity for example. A probe assembly (24) is introduced into a stack through a test port and operated by an automated mechanism mounted on the test port to position a sensing tip (28) at various locations within the gas stream at proper yaw angle. A framework (30, 32, 34, 36) of the automated mechanism provides bearing support for the probe assembly while allowing the probe assembly to translate on the framework along an axis but constraining the probe assembly from turning on the framework about the axis. With a gripper assembly (44) gripping the probe assembly and another gripper assembly (56) released, a linear actuator (40) can move the gripper assembly (44) along the axis to translate the probe assembly on the framework.