A Calibration Facility for Hot-Wire Anemometers in Extremely Low Speed with Air Temperature and Humidity Variable and Controllable

Aimed at addressing the difficult problems existing in extremely low speed calibration facilities for hot-wire anemometers, where calibration accuracy is often insufficient and vulnerable to the contamination from temperature and humidity discrepancies between the calibration environment and the application environment, a calibration rig with a velocity range from 0.10 m/s to 1.0 m/s, an air temperature range of ambient temperature to 60 °C, and a humidity range from 20%RH to 80%RH was designed, developed, and constructed. The overall layout arrangement and the mechanical structure of the facility are illustrated. The master control system, the motion control system, the temperature and humidity control system are designed, tested and adjusted. The adjustment results are demonstrated and discussed. The analysis of the results reveals that the maximum velocity control error is 0.000989 m/s, satisfying the design target of 0.003 m/s; the corresponding maximum relative error is 0.241%, which is less than the design target of 0.4%; the maximum temperature control error is 0.9 °C, meeting the design target accuracy of 1 °C; the maximum humidity control error is 2.9%RH, which is below the design target of 4%RH. When the facility is applied to the calibration of a hot-wire anemometer, the maximum error of the fitting curves in modified King’s law is 0.02236 m/s, while that in Van der Hegge Zijnen’s formula is 0.023217 m/s, both of which satisfy the design target accuracy of 0.03 m/s. The maximum relative errors of fitting curves using the two formulas are 5.214% and 8.527%, respectively. Analysis of calibration data reveals that the discrepancy in temperature and humidity between application site and calibration site may bring errors that can reach up to 1.2676% per unit relative humidity discrepancy, and 5.672% per degree Celsius of temperature deviation, respectively.