Anemometer Technology – About to Change for the Better

Traditional mechanical anemometers (wind measuring devices) seen on thousands of modern day boats were invented in 1846 by an Irish researcher, Thomas Romney Robinson. The hemispherical cups were designed to spin at a measurable speed proportionate to wind velocity. These devices have been in use for more than one and a half centuries, but new technology promises a favorable shift in wind speed and direction measurement.

Mechanical anemometers have several drawbacks, but most importantly, they are mechanical, which means they wear out. They also get entangled in the rigging and sometimes the cups and vane get broken or bent. Mechanical anemometers also have a problem with accurately measuring wind speed and direction. Consider, for example, when the vessel heels and the cups are no longer parallel to the earth’s surface. Now the wind must overcome the gravity of the cup, causing an error in the indicated wind speed. Likewise, the wind vane wants to hang heavy side down, causing an error in the indicated wind direction. The cups and vane also have a certain amount of mass and inertia that resists change in speed and direction, creating a lag in actual wind speed and direction during gusty conditions.

Ultrasonic anemometer technology overcomes these problems, and since there are no moving parts, they won’t wear out. Fragile moving parts are removed so there’s nothing to get caught in the rigging and break off. The accuracy of the wind speed and direction is improved since there is no mass or inertia to overcome. Ultrasonic anemometers work by measuring the amount of time it takes for sound to travel a known distance. Usually, there are three or four sensors that act as both sound transmitters (speakers) and sound receivers (microphones). One sensor transmits while the others measure the time it takes for the sound to arrive. This process continues with the next sensor transmitting while the others listen until each sensor has played the role of transmitter and speaker. Once the flight times are known for each path (sound traveling with the wind arrives earlier than the sound traveling against the wind), simple trigonometry is used to compute the resultant wind vector, which includes both the wind speed and direction. And since there are no cups and vanes that are affected by gravity, nor inertia that takes time to speed up or slow down the mechanical apparatus, ultrasonic anemometers are extremely accurate.

It almost sounds too good to be true. So why haven’t ultrasonic anemometers been introduced sooner to the marine industry, especially since the technology was developed in the 1970’s? The reason is that it takes sophisticated, and in the past, expensive electronic circuits to accurately measure the changes in the speed of sound caused by the wind. With the rapid advancements of chip technology, it is now possible for ultrasonic technology to compete and even improve on the price of older mechanical cups and vane anemometer technology. Eventually, ultrasonic anemometers will proliferate as marine electronic manufacturers begin to introduce new products based on this technology.

Although ultrasonic anemometers solve many of the problems associated with mechanical systems, they must be carefully designed for use in the marine environment. Many commercially available shore based units are being adopted for marine applications without concern for the extraordinary conditions that exist on the water. For example, shore based ultrasonic anemometers remain parallel to the earth and the wind. When these units are tilted, a significant amount of turbulence occurs making accurate wind speed and direction measurement impossible. Careful consideration to the aerodynamic design is important since anemometers used in the marine environment don’t remain parallel to the wind. Figure 1 shows a product specifically designed for marine applications.

Figure 1 – Aerodynamically Designed for Tilt

Notice that the same amount of air traverses the sensor regardless of the tilt (at least up to 30 degrees) without significant distortion of the airflow. This aerodynamic design yields wind speed and direction measurements more accurately than traditional shore based designs that exhibit turbulence as the vessel heels over.

Others areas to watch are claims of accurate north-referenced wind direction through the use of integrated compasses. Imagine if you will, how the fluid of a compass would slosh around at the top of the mast even with the smallest of waves. Even electronic compasses experience this sloshing effect unless they use a rate gyro, which means the computed wind direction will not be accurate since the heading is in error. A better solution is to use the vessel’s compass mounted down low that doesn’t experience as much centrifugal force as a compass at the top of the mast. Better yet would be the use of a rate gyro-stabilized compass where the effects of centrifugal force can be mitigated, resulting in accurate north-referenced wind direction.

Maretron has developed an ultrasonic anemometer (Figure 2 shows the WSO100) from the ground up specifically for marine use. This new product offers all the benefits of ultrasonic technology including no moving parts to wear out or break while providing unprecedented accuracy, even while tilted up to 30 degrees. It includes a humidity sensor, an outside air temperature sensor, and a barometric pressure sensor for monitoring local weather conditions.

Figure 2 – Maretron Ultrasonic Wind and Weather Station (WSO100)

The WSO100 also contains the latest NMEA 2000 ® interface so it is compatible with any NMEA 2000 ® equipment including displays such as the Maretron DSM200 (Figure 3 shows typical wind and weather DSM200 screen shots). The networking aspect is important since an anemometer only measures apparent wind speed and direction. If the anemometer is networked with boat speed (paddle wheel for speed through water or GPS for speed over ground), then wind speed and direction referenced to the vessel can be indicated. Taking it a step further, vessel heading (compass or gyrocompass) could be networked together with vessel speed and apparent wind to indicate north-referenced wind speed and direction.

Figure 3 – WSO100 Wind and Weather Screen Shots

In summary, the introduction of ultrasonic anemometer technology into the marine industry promises to provide reliable, accurate, and affordable wind measurement while over coming the short falls of older mechanical cups and vane anemometers.