Path: bloom-picayune.mit.edu!snorkelwacker.mit.edu!americast.com!americast.com\!americast-post Newsgroups: americast.mech From: americast-post@AmeriCast.Com Organization: American Cybercasting Approved: americast-post@AmeriCast.com Subject: Neither meteorological data nor the best combined efforts of Date: Wed, 4 Nov 92 10:28:41 EST Message-ID: Neither meteorological data nor the best combined efforts of local, state, and federal authorities could halt the fury of hurricane Andrew as it swept through Florida and other southern states in September. However, instruments such as anemometers and weather satellites, which monitored AndrewÕs wind gusts and tracked the stormÕs direction, assisted authorities in planning the evacuation of 750,000 residents from afflicted areas. Anemometers use moving parts such as spokes, propellers, and vanes connected to electronic sensors to measure wind speed and direction. Newer wind gauges such as hot-film anemometers collect wind data without moving parts. But weather instruments such as these were no match for the strong winds of Andrew. Still, anemometers play a big role in tracking these storms, said John Forsing, deputy director of the Eastern Region of the National Weather Service (Silver Springs, Md.). ÒIn the middle latitudes we rely more on pressure measurement derived from barometers to forecast weather, but we still need the additional resolution of wind data to get a more complete picture of the weather pattern.Ó Not Just for Weather Forecasting The use of anemometers has spread beyond meteorology to fields including aviation and pollution monitoring. Manufacturers such as the Scott Paper Co. (Muskegon, Mich.), use wind monitors to measure the pollution from their factories. Likewise, utilities such as Pacific Gas and Electric Co. (San Francisco) monitor emissions from power plants. The Illinois Department of Transportation (Springfield) uses anemometers on bridges to detect crosswinds that could create traffic hazards. In agriculture, the devices are used to check wind speeds in order to control the dispersion of herbicides and pesticides. One of the anemometers used in south central Florida during the course of hurricane Andrew was an R.M. Young Co. (Traverse City, Mich.) Wind Monitor located at the Archibald Biological Station in Lake Placid, Fla. This propeller-vane wind monitor was developed in 1980 for the National Data Buoy Center (Stennis Space Center, Miss.) for use on offshore weather buoys. So that it could withstand the rigors of offshore buoys, the Wind Monitor needed to be more durable and compact than conventional propeller vanes. R.M. Young engineers developed the body of the anemometer from injection-molded Rovel thermoplastic from Dow Chemical Co. (Midland, Mich.). This acrylonitrile butadiene styrene polymer alloy offered the necessary corrosion resistance and high-impact strength. Injection-molding major components of the Wind Monitor allowed the company to reduce the productÕs final cost by more than 30 percent over thermoforming and machining of parts. The Wind Monitor fuselage is mounted on stainless-steel bearings atop the central shaft, allowing it to rotate. A propeller and tail are located at opposite ends of the fuselage. The tail causes the fuselage to move in the direction the wind is blowing. Changes in the direction of the fuselage are signaled by a potentiometer. R.M. Young engineers used a precision conductive plastic potentiometer in place of the wire-wound potentiometer used in many wind gauges. The smooth plastic disk is sealed in a chamber within the housing. A known excitation voltage is applied to the potentiometer so that the output signal from the sensor is directly proportional to an azimuth angle. The plastic potentiometer eliminates the friction and wear found in wire-wound potentiometers in wind gauges. This gives the conductive plastic potentiometer a longer life than its wire-wound counterpart. ÒThe potentiometer in the Wind Monitor is rated at 50 million cycles, compared to the 10 to 20 million cycles typical of wire-wound potentiometers,Ó said Tom Young, a mechanical engineer and vice president of marketing at R.M. Young. The four-blade polypropylene propeller mounted at the end of the fuselage opposite the tail is used to measure wind speed. Besides drawing on the chemical and moisture resistance of polypropylene, the Wind Monitor propellers also contain ultraviolet inhibitors to prevent warping from sunlight. Like the wind-direction transducer, the wind-speed transducer of the Wind Monitor represents a departure from standard wind-sensor technology. The Wind Monitor uses a rotating magnet and coil transducer with an air gap between them. Since these parts do not touch, they will not wear as the propeller rotates. This contrasts with the generator-type wind- speed transducer used in many wind gauges that is coupled to wire brushes and thus subject to wear and corrosion, according to Young. Propeller rotation in the Wind Monitor produces an ac sine wave voltage signal with frequency directly proportional to air speed. This signal is carried by instrument cable, along with the wind- directional signal, to a signal conditioner to prepare them for visual display or computer storage of data. R.M. Young offers three types of wind monitors. The standard one was designed for permanent installation to monitor weather conditions. It becomes active in the presence of 2.2-mile-per-hour winds and can survive winds up to 220 miles per hour. The propeller is mounted on greased ball bearings that are sealed to prevent dust or moisture from affecting the instrument. The Wind Monitor AQ was designed for long-term installation to study pollution from industrial facilities and power plants. Its greased bearings are only shielded, not sealed, and the instrument can survive winds up to 100 miles per hour. It is more sensitive than the standard model, becoming active from 0.9-mile-per-hour winds. R.M. YoungÕs Wind Monitor RE was designed for temporary installation to conduct detailed meteorological research for applications such as dynamic thermal rating of power lines. The tail and propeller of this model are made of expanded polystyrene. Its light weight makes it sensitive enough to activate in the presence of 0.5-mile-per-hour winds. However, it is rugged enough to survive 80-mile-per-hour winds. The Argonne National Laboratory (Argonne, Ill.) is one user of this device. The Cup-and-Vane Anemometer Another anemometer that operated in South Florida sites during hurricane Andrew was the Digital Wind System Model 2000, developed by Belfort Instrument Co. (Baltimore). This device is used at the three Automated Surface Observation System (ASOS) platforms located on Key West, north of Miami, and on the Gulf. The computerized ASOS platforms are being built at airports around the country to provide up-to-the-minute weather information to pilots as part of a joint project between the National Weather Service and the Federal Aviation Administration (Washington, D.C.). The Digital Wind System 2000 is a cup-and-vane-type anemometer. The mast and arms of the T-shaped wind sensor are made of aluminum. ÒWe chose aluminum to reduce the deviceÕs inertia, making the wind gauge more sensitive to lighter winds,Ó said Fred Sebly, a mechanical engineer at Belfort. The metal is also resistant to the electromagnetic interference common to the air bases and airports where the system is used. A wind-direction assembly and wind-speed assembly are mounted on opposite crossarms of the Model 2000. Both the directional and speed measurement parts are comprised of a stainless-steel cylinder approximately 3 inches in diameter. A 5-inch-long rotating shaft, also made of stainless steel, is welded to a brass hub on the crossarm of the anemometer. On the wind-direction finder, the rotating shaft is connected to a vane that has a weight on one end and a rectangular tail on the other. Belfort engineers connected a potentiometer to the shaft within the housing that supports the vane. ÒThe potentiometer is configured as a variable resistor that acts as a voltage divider. As the direction of the vane changes due to changes in the direction of the wind, it is reflected in the voltage of the potentiometer,Ó explained Cliff Greenblatt, an electrical engineer at Belfort. The shaft of the wind-speed assembly is topped by three spokes with cups on their ends. An optical disk is connected on the rotating shaft of this assembly. As the shaft spins, the disk acts as an interrupter. Since a certain number of interruptions corresponds to a single rotation of the shaft, the frequency of interruptions put out by this sensor corresponds to a particular wind speed. Signals from both the direction and speed indicators are carried by shielded cable to a signal converter box that will enhance them for display on a computer. A conductive rubber gasket was affixed within the door of the Model 2000 as an added precaution against electromagnetic interference affecting the accuracy of the systemÕs electronics. The Model 2000 has operated in wind velocities as high as 145 miles per hour. It also functions at temperatures as low as 140¡C at weather stations in Antarctica. Belfort designed a heater system to keep rime off the gauges. (Rime is the accumulation of ice on the windward side of exposed objects formed from cooled fog or clouds.) BelfortÕs heater system is similar to the rear-window defroster used on automobiles. Copper wires with a resistive element are run on the exterior of the Model 2000, including the shaft and cups, as well as inside the cylinders. They are electrically heated to prevent formation of rime. Gauging Wind Sans Moving Parts Hot-film anemometers, which use no moving parts to measure wind speed and direction, represent the next generation of anemometers. They are sensitive yet rugged enough to withstand stronger winds than the vane-type gauges. Sutron Corp. (Herndon, Va.) developed its microprocessor-controlled hot-film anemometer in the late 1980s. It is now being deployed at U.S. Air Force and U.S. Army bases worldwide. It is currently in use at bases including Scott Air Force Base (Belleville, Ill.), Patrick Air Force Base (Cocoa Beach, Fla.), Keesler Air Force Base (Biloxi, Miss.), and the U.S. Army base at Ft. Rucker (Ala.). Sutron mounted its deviceÕs sensor head on a 10-inch-long aluminum tube that feeds into a rectangular box containing the system transformer and power supply. Some system electronics also feed into the box. Another aluminum tube beneath the box connects the entire assembly to a mast. The sensor head is enclosed in a stainless-steel cage that allows air to flow over the sensing elements of the anemometer while protecting them from debris. The sensing elements are two pairs of coated ceramic tubes placed at right angles to each other without contact. Each element in a pair is separated by an adhesive and connected to a platinum wire. All four resistive elements are coated in quartz glass. The transformer within the anemometer provides the electricity needed to heat the resistive elements to 100¡C above the ambient temperature. Just below the sensor head of the hot-film anemometer is the measurement and control electrical mechanism, which is controlled by an Intel Corp. (Santa Clara, Calif.) 8088 microprocessor that monitors the power required to maintain the temperature of each of the four resistive elements. Dave Goodman, an electrical engineer at Sutron, said that the area of the elements registering the greatest heat loss will indicate the direction from which the wind is blowing. ÒThe amount of heat lost from the resistive elements will tell us how fast the wind is going,Ó Goodman added. The microprocessor of the hot-film anemometer samples the present wind data once per second, computing the wind speed and direction, while compensating for air density and temperature to provide accurate wind measurements. Up to four field sensor assemblies can be incorporated into the Sutron hot-film anemometer system. The anemometers are commonly located at key locations along a runway. Twisted-pair communication wire will transmit the data compiled by the sensor assemblies to indicator and recorder assemblies that are typically located in the control tower and flight operations center. SutronÕs hot-film anemometer is sensitive enough to detect a wind lighter than 0.4 mile per hour and can survive wind speeds of 230 miles per hour. During cold conditions, the heat of the resistive elements prevents rime formation within the sensor head. The exterior of the anemometer head is heated by separate heating coils to prevent rime from forming there. · Whether they use moving parts or hot film, tomorrowÕs anemometers must be engineered to meet new applications, according to Tom Young of R.M. Young. ÒWe must also reduce the cost of our devices, for example, by injection-molding parts, while meeting our clientsÕ needs,Ó he added. Newer applications for anemometers, however, will not overshadow their primary role in weather forecasting. As John Forsing of the National Weather Service explained, wind data are, and will remain, a critical component of storm warnings and weather forecasts. Copyright 1992, Mechanical Engineering. For more information, send-email to American Cybercasting Corporation (usa@AmeriCast.COM)