Anemometers: sonic, and not

Servizi Territorio srl sells three-dimension (and quality two-dimension) ultrasonic anemometers. We're quite specialised - and we even designed and build various kind of ultrasonic anemometer data acquisition, processing and transmission systems. From time to time we're asked about the differences between ultrasonic and conventional anemometers, and why ultrasonic anemometers cost more than quality conventional: that's a more than understandable curiosity. And, I'll try to answer - to both prospective users and institutional buyers.

• No moving parts. The design of conventional anemometers permits converting wind to some mechanical change, which is then converted to electrical signal. "Mechanical changes" of whatever kind can only be probed by moving mechanisms subject to friction, inertia and wearing. An ultrasonic anemometer has no moving parts.

• Thanks to friction, mechanical anemometers will stay locked in their position if wind is not strong enough, even at speeds which are common to existing and diffused sites (Po valley to name one). This is not the case with sonic anemometers: they can sense the wind at a speed of 1 cm/s, much smaller of wind velocities one can find in Nature. Said differently, ultrasonic anemometers are immune to wind calms.

• High sampling rate of sonic anemometers, combined with their high resolution and ability to sense wind whatever its velocity allow them to sense turbulence directly. On the contrary, friction and inertia impart mechanical anemometers the quality of a low-pass filter, restricting them to measuring the mean wind, or a smoothed version of it.

For these reasons, ultrasonic anemometers should be preferred in those applications where measuring turbulence has a value, or "wind calms" are not tolerable. For example, the modeling of pollutant dispersion, or air quality studies, or the management of incidents in chemically dangerous plants (or, what's after all almost the same, the active protection against the direct effects of chemical or biological weapons during was episodes or after terrorist attacks).

What I've said applies anyway to three-dimensional ultrasonic anemometers. A forest of two-dimensional ultrasonic anemometers is available on the market, but they are almost always designed to emulate conventional mechanical anemometers: their shape and firmware are built to reproduce the behaviour of "old-style sensors", ctivation threshold and low-pass filter effect included. It's a common error by buyers to get a two dimensional ultrasonic anemometer when actually a third dimensional was needed, out of their lower price tags. The intention is nice - saving some money - but the final effect catastrophic - wasting money to buy the wrong thing. Of course there exist exceptions in the two-dimensional anemometers panorama, one of them the uSonic-2 line by Metek GmbH, of whom Servizi Territorio srl is distributor for Italy. But they are just that: exceptions.