Omnidirectional thermal anemometer for low airspeed and multi-point measurement applications

Abstract

<p>Current control strategies for livestock and poultry facilities need to improve their interpretation of the Thermal Environment (TE) that the animals are experiencing in order to provide an optimum TE that is uniformly distributed throughout the facility; hence, airspeed, a critical parameter influencing evaporative and convective heat exchange must be measured. An omnidirectional, constant temperature, Thermal Anemometer (TA) with ambient dry-bulb temperature (<em>tdb</em>) compensation was designed and developed for measuring airspeeds between 0 and 6.0 m s−1. An Arduino measured two analog voltages to determine the thermistor temperature and subsequently the power being dissipated from a near-spherical overheated thermistor in a bridge circuit with a transistor and operational amplifier. A custom wind tunnel featuring a 0.1 m diameter pipe with an access for TA insertion was constructed to calibrate the TA at different temperatures and airspeeds, at a constant relative humidity. The heat dissipation factor was calculated for a given airspeed at different ambient temperatures ranging from 18 °C to 34 °C and used in a unique fourth-order polynomial regression that compensates for temperature using the fluid properties evaluated at the film temperature. A detailed uncertainty analysis was performed on all key measurement inputs, such as the microcontroller analog to digital converter, TA and <em>tdb</em> thermistor regression statistics, and the calibration standard, that were propagated through the calibration regression. Absolute combined standard uncertainty associated with temperature corrected airspeed measurements ranged from 0.11 m s−1 (at 0.47 m s−1; 30.3% relative) to 0.71 m s−1 (at 5.52 m s−1; 12.8% relative). The TA system cost less than $35 USD in components and due to the simple hardware, this thermal anemometer is well-suited for integration into multi-point data acquisition systems analyzing spatial and temporal variability inside livestock and poultry housing.</p>