Whether you are the Captain of an aircraft carrier with orders to launch an air strike, a meteorologist tracking the development and progress of an hurricane in the Caribbean, or a power company technician monitoring a natural gas pipeline, some of the information critical to your performing your duties effectively will be supplied by an instrument called an anemometer. Anemometers show or measure aspects of fluid motion, velocity and direction, in a gaseous substance; most commonly air. They range from relatively simplistic devices that have been around for hundreds of years to the latest varieties using ultrasonics or pulsed lasers.
Vane anemometers are more commonly know as weather vanes or weathercocks and are often seen on church steeples in rural areas, particularly in Europe. For centuries, the wind direction shown by these anemometers, combined with visual inspection of the sky, the taste and feel of the air, and local experience often going back generations, allowed illiterate peasants to make highly accurate weather forecasts for the day or days ahead. Thus enabling them to plan and implement appropriate agricultural activities. Nowadays, vane anemometers are primarily considered decorative or even works of art, and may be seen on the roofs of houses and garages in suburban areas, in addition to their rural origins. However, some backyard meteorologists still use them in combination with barometers that measure air pressure and thermometers measuring local temperature to make their own forecasts of the day’s weather.
Propeller anemometers, usually abbreviated to prop and sometimes called windmill or spinner anemometers, primarily measure wind speed, determined by the speed of the propeller spinning. Rear or reverse prop anemometers have a spike that faces into the wind with the propeller blades at the rear; mounted on a swivel this device indicates both direction and velocity. Modern versions have electronic sensors that can be computer monitored to give reasonably accurate compass directions in the degree range and wind speeds to fractions of a knot. Sufficient for use at farm-based landing strips or even small, local airports, as guidance for controlling takeoffs and landings. They are more reliable at low air speeds, under five knots, than the cup anemometers discussed in the next paragraph. The keener backyard meteorologist might also have one of these instead of a vane anemometer.
Cup anemometers were first invented in the 19th century when hot air balloons were the state of the art in air travel technology. The original design, invented by Dr. John Robinson of Armagh Observatory in Northern Ireland in 1846, had four hollow hemispherical “cups” on equal length arms attached to a rotating swivel on a mast. It was used to measure wind speed in conjunction with a flag, or later a windsock, indicating direction. It was later found, in 1926 by Canadian John Patterson, that greater accuracy was possible using three cups on arms that vary in length in specific ratios. With powered, heavier-than-air vehicles now predominating, greater accuracy for evaluating the viability of take-offs and landings was necessary. Later improvements include grooving on the edges of the cups; patent applications for new designs are still being submitted to patent offices to this day. They are used to determine wind velocity and work best in the 5 to 100 knot range. Modern versions may be linked to an electric generator, the voltage being generated indicating air speed. They are most commonly mounted on masts at airfields and on ships.
Pitot-tube anemometers, also called pressure-tube anemometers, are used on aircraft because they require no moving parts. Based on the pivot-tube invented by Henri Pitot (16951771) to measure flow rates in rivers and streams, they consist of a tube with a vertical bend. Air speed is determined by the pressure occurring on the rear surface at the bend. While accurate under steady, strong conditions, they are decidedly less reliable in rapidly varying situations, such as the turbulence prevalent during storms. Hollywood actually has it right in their movies, when they display the needle of the airspeed gauge vibrating manically between extremes in the cockpit of an aircraft being tossed about in a storm. Pitot-tube anemometers are also the standard type used in wind tunnels, where models or prototypes of new aircraft are tested.
Thermal anemometers include hot-wire, hot-film and ovenized semiconductor devices. They determine the velocity of gaseous movement, usually within a constrained area such as a pipe, by heating a wire or surface the gas flows over or around. The latest varieties are called micro gust thermal anemometers; they are able to measure minute velocities. Velocity is measured by the voltage required to maintain the wire/surface at a predetermined temperature, the resistance generated in the heated wire or surface, or the temperature change that is induced by the gas flow; the effect commonly referred to as “wind chill” when related to meteorological effects. These are the type of anemometers predominantly used within gas-piping systems to determine flow rates. Even when the gas is flammable, as long as the piping system is airtight and therefore excludes oxygen, the minimal amount of heating required will not result in combusting the gas being measured. A break in the piping system close to such an anemometer may possibly result in either a flare through the break, or in extremely rare circumstances, an explosion. The hot-wire variety is particularly capable of monitoring changes when the gas flow rates vary significantly or dramatically.
Sonic or ultrasonic anemometers were first devised in the 1970s. They use ultrasonic sound waves transmitted between transducers; the path length between transducers, which typically varies between 10 to 20 centimeters, determines wind velocity to a very fine degree. Temporal resolution is as high as 20 hertz or more, making these anemometers very effective in highly turbulent weather conditions. As they have no moving parts, sonic anemometers are the instruments of choice for small, remotely located, automated weather stations. While their measuring capabilities are distorted by the supporting structure that the four to six transducers are held in, these distortions can be calibrated within wind tunnels prior to deployment, so that accurate measurements will still be obtained.
Laser Doppler anemometers generate a laser beam that is then split in two, one of which is propagated outside the anemometer. That beam impacts on the air molecules external to the device, resulting in a reflection or “backscatter” received by a detector. The reflection is compared to the twin that is contained within the anemometer. The external beam is modified by doppler shift determined by the motion of the air molecules, and the comparison between the two is used to calculate the average speed of the air molecules and therefore the air speed in general. This results in extremely accurate measurements of gas velocity; it is typically used in experimentation when such measurements are highly desirable in determining the results of the experiment.
Anemometers have come a long way since their earliest beginnings. The type used for a specific purpose may depend on the budget of the organization requiring them; their ability to perform the desired function varies based on the type selected.
