Hot Wire and Hot Film Anemometers

Hot wire and hot film sensors are used for measurements of fluc­ tuations in the wind. Such anemometers are in competition with both mechanical and sonic anemometers. At least some of the mech­ anical sensors are more stable in calibration, and less expensive. The high frequency resolution of the hot wire/hot film sensors, however, cannot be matched by any other anemometers. Also, they can be made small enough and fast enough to measure within the dis­ sipation range. Hot wire sensors have been used in atmospheric turbulence measurements since 1936; there are review articles by Bradshaw (1963), Comte Bellot (1976), Corrsin (1963), and Sandborn (1972). 2. SENSOR TYPES In wind measurements the cooling of a heated wire or film is depen­ dent on the velocity and density of the flow past the wire. Hot wire sensors can be made by welding thin wires (of order 10~ m) to supports, or by using Wollaston wire (a platinum wire core embedded in a silver mantle). The Wollaston wire is soldered to the support and the desired sensor length is obtained by etching the silver mantle away. In a similar way, the sensitive length of welded wires is determined by plating the ends with conducting material. Since the resistance of the core alone is much higher than of the mantle plus core, only the piece of bare core is heated and is wind sensitive. Typical dimensions are: 5 ~ m wire diameter, 1.25 mm sensitive length, 3 mm total length of wire. Commercially avail­ able sensors are generally used, since accurate production requires some skill. This is even truer for hot film sensors. These usual­ ly consist of acyl indrical quartz or glass core, covered with a