Calibrating Cylindrical Hot-Film Anemometer Sensors

Abstract

We report the results of 82 separate calibrations of cylindrical, platinum hot-film anemometer sensors in air. The calibrations for each sensor involved a determination of its temperature-resistance characteristics, a study of its heat transfer in forced convection, and an investigation of its yaw response. Because most sensors were calibrated more than once during a 28 month period, the resistance-temperature data allowed us to look at sensor aging. We found no conclusive evidence that either the reference resistance, R0, or the temperature coefficient of resistance, α, changed with time (months to years). Hence, frequent, full calibrations of the sensors are probably unnecessary. It is, nevertheless, wise to measure R0 whenever possible. The convective heat transfer relation that we derive predicts the Nusselt number of the sensor as a linear function of R0.40, where R is the Reynolds number based on sensor diameter (1 ≤ R ≤ 43). For the 53-?m- diameter sensors that we used, this heat transfer relation applies to wind speeds typical of the atmospheric surface layer, 1 to 20 m s?1. From the heat transfer relation we also devise a method for determining hot-film operating characteristics at temperatures other than the calibration temperature. Hinze's relation is the best model for the yaw response of these sensors, being valid over virtually the entire range of yaw angles, 0° to 90°. Although the yaw parameter k does depend on the flow velocity, that dependence is so weak in the atmospheric surface layer that k can be assumed constant at 0.3.