The proposed design for an anemometer to determine the velocity of an airstream in a wind tunnel is comprised of a thin metallic strip whose ends are supported by stiff rods serving as electrodes for passage of current used to heat the strip. A fine-wire thermocouple is attached to the trailing edge of the strip and serves as the sensor for a system that controls the power to maintain the strip at a constant operating temperature for variable airstream velocities. Design conditions pertain to an airstream at T∞ = 25°C and 1

(a) Determine the relationship between the electrical power dissipation per unit width of the strip in the transverse direction, P' (mW/mm), and the airstream velocity. Show this relationship graphically for the specified range of u∞.

(b) If the accuracy with which the temperature of the operating strip can be measured and maintained constant is ±0.2°C, what is the uncertainty in the airstream velocity?

(c) The proposed design operates in a strip constant temperature mode for which the airstream velocity is related to the measured power. Consider now an alternative mode wherein the strip is provided with a constant power, say, 30mW/mm, and the airstream velocity is related to the measured strip temperature Ts. For this mode of operation, show the graphical relationship between the strip temperature and airstream velocity. If the temperature can be measured with an uncertainty of ±0.2°C, what is the uncertainty in the airstream velocity?

(d) Compare the features associated with each of the anemometer operating modes.

, Air stream -L= 20 mm Strip Support rod and current lead