eFunda: Introduction to Hot-Wire Anemometers
 Hot-Wire Anemometers: Introduction
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 Overview The Hot-Wire Anemometer is the most well known thermal anemometer, and measures a fluid velocity by noting the heat convected away by the fluid. The core of the anemometer is an exposed hot wire either heated up by a constant current or maintained at a constant temperature (refer to the schematic below). In either case, the heat lost to fluid convection is a function of the fluid velocity. Typical Hot-Wire Anemometer By measuring the change in wire temperature under constant current or the current required to maintain a constant wire temperature, the heat lost can be obtained. The heat lost can then be converted into a fluid velocity in accordance with convective theory.
 Further Information Typically, the anemometer wire is made of platinum or tungsten and is 4 ~ 10 µm (158 ~ 393 µin) in diameter and 1 mm (0.04 in) in length. Typical commercially available hot-wire anemometers have a flat frequency response (< 3 dB) up to 17 kHz at the average velocity of 9.1 m/s (30 ft/s), 30 kHz at 30.5 m/s (100 ft/s), or 50 kHz at 91 m/s (300 ft/s). Due to the tiny size of the wire, it is fragile and thus suitable only for clean gas flows. In liquid flow or rugged gas flow, a platinum hot-film coated on a 25 ~ 150 mm (1 ~ 6 in) diameter quartz fiber or hollow glass tube can be used instead, as shown in the schematic below. Another alternative is a pyrex glass wedge coated with a thin platinum hot-film at the edge tip, as shown schematically below.
Pros and Cons

 • Pros: - Excellent spatial resolution. - High frequency response, > 10 kHz (up to 400 kHz). • Cons: - Fragile, can be used only in clean gas flows. - Needs to be recalibrated frequently due to dust accumulation (unless the flow is very clean). - High cost.