Calibration Methodology

There are different approaches available for hotwire calibration. Two important steps involved in any approach are data generation and curve fitting. Calibration data are generated by measuring the output of the anemometer when the probe is subjected to a flow with a known velocity and low turbulence levels. In principle, specially designed apparatus are used to generate high quality flow with uniform velocity, temperature and very low turbulence level However, a test cell that has clean approach flow conditions can also be used to carry out an in situ calibration. In the present study, calibration has been performed in the test cell itself, very close to the inflow plane. A much wider range of expected velocity variation was covered during calibration. Air velocity was directly measured using a pitot-static tube with an external diameter of 3 mm, mounted along the centerline of the test section. Calibration data are generated by measuring the output of the anemometer when the probe is subjected to uniform known velocity.

Small changes in room temperature can occur over the day of the experiment. For changes up to the recorded data can be compensated through the use of a correction formula that assumes a constant heat transfer coefficient over this small temperature range . The anemometer output voltage is corrected as

	(9)

Here is the acquired voltage, the sensor hot temperature, the ambient reference temperature at which calibration data is available, and is the ambient temperature during the experiment. In the present work, the probe was re-calibrated for larger changes in room temperature.