Determination of the Blockage Effect on a Thermal Anemometer using a Small Open Jet Wind Tunnel

When an object, specifically an anemometer, is presented before a small open jet wind tunnel the flow field will be altered deflecting the flow around the anemometer creating what is commonly known as the Blockage Effect. Directly comparing a thermal anemometer with a vane anemometer in the same flow field, the velocity measured by the thermal anemometer may be significantly different to that measured by the vane anemometer as a result of blockage. In this paper we consider the blockage created by a thermal anemometer. A simple mathematical model is derived to directly compare the thermal anemometer with a primary standard vane anemometer. The calibration results obtained are compared with those obtained by the manufacturer and an ISO/IEC 17025 accredited laboratory chosen as the Reference Laboratory for the purposes of the paper. We conclude with an analysis of the results, discussing the differences in the measured output and postulating how these results may be unified.