The mini vane is for in duct only. It is too small for a register, it would not be as accurate as a large vane. All airflow instruments, hot wire, vane, and pitot require somewhat laminar airflow to get a good measurement. In more turbulent airflow a vane is going to be more accurate then a hot wire simply due to design characteristics. If you get stuck on a box plenum, you have to measure the airflow at each of the return registers and total the airflow. The air is to turbulent to get any kind of close to accurate measurement otherwise.