|From Ocean Navigator #85 |
However, the latest solution to the problem of knowing how hard the wind is blowing involves using electronic devices with no moving parts. Because these units don’t rely on mechanical elements that rotate, among other things, they have seemingly endless life spans. And they aren’t in danger of being damaged by extremely high winds. (Of course, if you’re in winds that high, having your anemometer carried away may be the least of your problems.)
One of the simplest of wind sensors is a clear plastic tube with a white plastic ball resting at the bottom and two holes at the top. You cover one hole with your finger, and as the wind blows across the other hole it creates a partial vacuum that draws the ball up the tube. The height of the ball in the tube is a factor of the low pressure produced by the wind blowing past the hole. And the pressure is directly related to the speed of the wind. Along the inside of the tube are markings for various wind speeds. This system has the obvious advantage of simplicity, even if it is fairly inaccurate and labor intensive to use.
Simplicity is also a valued attribute to the meteorologists at the Mt. Washington Observatory in New Hampshire. Their gear must be simple and robust to survive. In addition to the pervasive formation of ice on their buildings and equipment, the researchers atop the 6,288-foot peak also have fairly serious winds to measurethe fastest wind speed ever observed, 231 mph in an April 1934 storm, was recorded on Washington’s summit. “There are a number of mountains around the world that have very high surface winds,” said Dave Thurlow, host and executive producer of “The Weather Notebook,” a radio program that is an educational effort of the Observatory. “But Mt. Washington just happens to have a continuously manned weather station.” Measuring the wind at the Observatory is done using a pitot tube anemometer. This device uses pressure differential to measure wind speed. At the nose of the airplane fuselage-shaped device is a small hole. As the device weathercocks into the wind, this hole in the nose receives the full brunt of the wind pressure. Located on the side of the device is another hole. This side hole is always aligned perpendicular to the wind, so it feels none of the effects of the wind pressure. The only pressure it senses is static atmospheric pressure. Atmospheric pressure is subtracted from the pressure reading obtained from the forward-facing pitot hole, and the remainder is the pressure of the wind. This pressure can then be correlated to a wind speed. Of course, to make sure that the unit doesn’t become encrusted with the ever-present rime ice that forms at the summit, the pitot anemometer is heated by up to 2,000 watts. What about a pitot tube anemometer for boats? This pitot anemometer approach works well for a stationary weather station, but it wouldn’t work on a pitching, rolling sailboat.
The tried-and-true method for sailors when it comes to measuring the wind is to place an apparent wind sensor at the top of the mast. There a variety of different designs, but they all involve some sort of rotating vane for measuring direction along with a set of three or four rotating plastic cups for measuring wind speed. The vane has an angle encoder that sends a signal to the control head. This is done using a “Hall effect” sensor (see below) or a potentiometer. Since most non-racers don’t need to know the wind direction with pinpoint accuracy, most direction sensors have an accuracy in the vicinity of ± 3° or so.
Most anemometers with rotating cups measure wind speed using Hall effect sensors. (The Hall effect was discovered by Edwin H. Hall in 1879 at Johns Hopkins University.) The anemometer version of the Hall effect involves embedding a magnet in the cup assembly. The stationary support post, on the other hand, has a strip of conductive material through which a current is passed. Every time the cups go through one revolution, the magnet passes the conductive strip. This affects the flow of current in the conductor. This current change is detected and then converted to wind speed. This technology is well known and very reliablethe average wind sensor can last for many years with little or no maintenance.
Companies like Autohelm, Brookes & Gatehouse, Datamarine, Simrad, KVH, Navico, and others make wind sensors using the tried-and-true windvane and wind cup approach.
One interesting spin on the vane and cup method is offered by Autohelm. Its RotaVecta system uses three anemometer cups, but there is no vane for determining direction. Instead, one of the cups has a small vane element attached to one of the cups. This “drag plate” causes the three-cup assembly to accelerate and decelerate as it spins through the wind. This variation in speed is analyzed by an onboard microprocessor, and from it wind direction is derived. Speed is determined by the average speed of the cups. This ingenious system eliminates the need for a windvane.
Some companies have units that do away not only with the windvane but with the cups as well. One of these devices is being offered on the U.S. market by Vetus Denouden in Baltimore. Originally developed by a Dutch company called Mierij Meteo, the WSD 833 unit has no moving parts, yet can reportedly measure both wind speed and direction. This unit is an electronic version of a simple device that has been around for years called a “hot wire anemometer.” An electric current is passed through a wire, causing it to heat up. When wind blows across the wire, it cools the wire and makes it contract. This contraction is directly proportional to the wind speed. As the wire contracts, an attached pointer indicates the wind speed on a scale.
The Vetus product performs this same process but does so using a microchip and some signal processing. On the chip are four sets of thermocouple elements, one set on each side of the chip. Protecting the sensing element is a ceramic coating. An electric current heats the thermocouples in a manner similar to the hot wire anemometer. When the wind blows across the sensor it cools the thermocouples on the windward edge of the chip. The location of the cooling tells wind direction and the extent of the cooling tells wind speed. The WSD 833 will operate in wind speeds up to 116 knots and will reportedly show wind speed with an accuracy ±3% and wind direction to better than 2°.
Enclosing the sensor are two plastic disks. In between the disks are louvers for channeling air properly across the sensing element. The entire sensing assembly is 4.72 inches high, weighs about half a pound, and comes with 50 feet of cable for connecting the unit to wind direction and speed display heads. According to Leo Van Hemert, president of Vetus, the unit will have a suggested retail price of $925this includes the sensor and an analog apparent wind direction display as well as an analog wind speed display.
Another unusual wind sensor is the 425A/AH Ultrasonic Wind Sensor from Handar, Inc., of Sunnyvale, Calif. This device has a central aluminum cylinder and three upward-extending aluminum armssomewhat like the legs of an overturned chair. At the end of each arm is a ultrasonic transducer. The unit measures the wind by transmitting ultrasonic signals between the three arms. The time of travel between the transducers is either increased or decreased depending on the angle of the wind to the sensors. By processing the time of travel of the signals, the 425A/AH can determine the wind speed and direction. According to Handar, the 425A/AH has a confirmed operating range of up to 125 knots and with an speed accuracy of ± of a knot and a directional accuracy of ±2°. The 425 costs roughly $1,500.
These solid-state units aren’t about to make the windvane and spinning cups of the traditional apparent wind indicator obsolete, but they might be the future of wind sensors. Marine electronics manufacturers are no doubt working on solid-state devices. “We’ll probably all go to solid-state down the road,” said Talbot Pratt, National Sales Manager for Autohelm. “It will become cheaper as these units are mass-produced.”