8.2. MEASUREMENT TOOLS AND TECHNIQUES
487
Hot-Wire and Hot-Film Anemometers
Hot-wire and hot-film anemometers are based on the principle that a heated
surface is cooled by fluid moving past the surface. Hot-wire anemometers
consist of a small wire held between two supporting legs, whereas hotfilm anemometers have a thin layer of conducting material deposited on a
nonconducting support. The conducting wire or film is heated by electrical
current, and flow velocity is related to the current necessary to maintain
the conductor at a constant temperature (Wessels, et al. 1990). Some
hot-wire/film anemometers are equipped with temperature compensation
circuits to handle variations in fluid temperature; however, the instruments
cannot be operated successfully with unknown variations in salinity (Sharp
1981).
Hot-wire and hot-film anemometers both measure the magnitude of the
fluid velocity vector, and two nearby sensors are needed to resolve a twodimensional flow vector into velocity components. Wessels, et al. (1990)
described a split-film sensor that works on the same principle as the hotfilm, but with the capability to determine two components of velocity.
Hot-wire anemometers are fragile, and they are intended for use in air
rather than water. If hot-wire anemometers are used in water, the wire
must be coated to prevent electrolysis. Hot-film anemometers are somewhat more robust because they are usually coated to protect the conducting
film against dirt and abrasion (Sharp 1981). The coating electrically insulates the film from the water while still maintaining thermal conductivity.
However, frequency response will decrease as coating thickness increases,
so there is a trade-off for increased robustness. Nevertheless, hot-films are
still delicate instruments and subject to fouling by air bubbles, oils, etc.
Therefore, successful operation of hot-films in water necessitates frequent
and careful cleaning of the probe.
Hot-film anemometers are quite small, they measure velocity over a
small volume of the flow, and they have a high frequency response. For
these reasons, they have been used for laboratory studies of fluid turbulence.
Although it can be argued that the laser Doppler velocimeter is more suited
to turbulence measurements, hot-film anemometers cost a fraction of what
an LDV system costs. Hot-wire and hot-film anemometers are commercially
available.
Electromagnetic Current Meters
Electromagnetic (EM) current meters are used extensively for field mea
surements, and miniaturized versions have been developed for la ora y
use. The EM current meter is based on the Faraday principle ° e ec ,
magnetic induction. Fluid flow perpendicular to a magnetic e
487
Hot-Wire and Hot-Film Anemometers
Hot-wire and hot-film anemometers are based on the principle that a heated
surface is cooled by fluid moving past the surface. Hot-wire anemometers
consist of a small wire held between two supporting legs, whereas hotfilm anemometers have a thin layer of conducting material deposited on a
nonconducting support. The conducting wire or film is heated by electrical
current, and flow velocity is related to the current necessary to maintain
the conductor at a constant temperature (Wessels, et al. 1990). Some
hot-wire/film anemometers are equipped with temperature compensation
circuits to handle variations in fluid temperature; however, the instruments
cannot be operated successfully with unknown variations in salinity (Sharp
1981).
Hot-wire and hot-film anemometers both measure the magnitude of the
fluid velocity vector, and two nearby sensors are needed to resolve a twodimensional flow vector into velocity components. Wessels, et al. (1990)
described a split-film sensor that works on the same principle as the hotfilm, but with the capability to determine two components of velocity.
Hot-wire anemometers are fragile, and they are intended for use in air
rather than water. If hot-wire anemometers are used in water, the wire
must be coated to prevent electrolysis. Hot-film anemometers are somewhat more robust because they are usually coated to protect the conducting
film against dirt and abrasion (Sharp 1981). The coating electrically insulates the film from the water while still maintaining thermal conductivity.
However, frequency response will decrease as coating thickness increases,
so there is a trade-off for increased robustness. Nevertheless, hot-films are
still delicate instruments and subject to fouling by air bubbles, oils, etc.
Therefore, successful operation of hot-films in water necessitates frequent
and careful cleaning of the probe.
Hot-film anemometers are quite small, they measure velocity over a
small volume of the flow, and they have a high frequency response. For
these reasons, they have been used for laboratory studies of fluid turbulence.
Although it can be argued that the laser Doppler velocimeter is more suited
to turbulence measurements, hot-film anemometers cost a fraction of what
an LDV system costs. Hot-wire and hot-film anemometers are commercially
available.
Electromagnetic Current Meters
Electromagnetic (EM) current meters are used extensively for field mea
surements, and miniaturized versions have been developed for la ora y
use. The EM current meter is based on the Faraday principle ° e ec ,
magnetic induction. Fluid flow perpendicular to a magnetic e