The LI-550 TriSonica Mini and LI-560 TriSonica Sphere use the same principles of physics as other ultrasonic anemometers: that is, mathematically deriving atmospheric conditions from the detected difference in time-of-flight for sound transmitted bi-directionally along the same acoustic pathway. A single bi-directional acoustic pathway allows for deriving wind speed. Data from two bi-directional acoustic pathways can provide both wind speed and two-dimensional wind direction.

As explained by Dr. J. Chandran Kaimal, the originator of acoustic anemometry, a three-dimensional wind direction solution be calculated by using three or more bi-directional acoustic pathways. The challenge in obtaining three or more bi-directional acoustic pathways is creating a structure for the sound-generating transducers with minimal perturbation of the air flow to be measured. The “other 3D anemometers” your customer mentions are likely the Orthogonal-style (Applied Technologies: producer of the original commercially available 3D ultrasonic anemometer) and the Oncley Non-orthogonal-style (Applied Technologies, R.M. Young, Gill Instruments, Campbell Scientific, and others). These anemometers use six transducers creating three bi-directional acoustic pathways, and solve the problem of transducer support structure by placing the sound-generating transducers on long thin probe arms. Following careful wind-tunnel studies, these units rely on software-based correction of structure- and transducer-shadowing effects. The size of the probe arms on these devices means that these anemometers are generally heavy and bulky.

Stephen Osborn, the principal engineer behind the LI-550 TriSonica Mini, worked for many years with Applied Technologies in designing, developing, improving, and programming their historically significant line of ultrasonic anemometers. It became apparent to him that the growing field of UAV-based weather sensing would benefit from a smaller and lighter multi-dimensional ultrasonic anemometer. With permission from Applied Technologies, Mr. Osborn began independently working on a smaller design that would eventually become the patented LI-550 TriSonica Mini anemometer. The LI-550 is able to scale down size because it features fewer transducers to produce four bi-directional acoustic pathways arranged in a tetrahedral configuration, similar to an idea first put forth by researchers at Woods Hole Oceanographic Institute for underwater current turbulence sensing.

The LI-550’s four acoustic pathways cross the wind path generally outside the structure of the probe itself, allowing for wind detection in three dimensions; and ongoing wind-tunnel study allows for continued refinement of shadow-correction software.

However, in the LI-550 TriSonica Mini, detection of the third-dimension (up-down) component of wind is limited due to the effect of the top and bottom structures to about 30 degrees from the horizontal, making it imperfectly suited for use as a flux-measurement system. The LI-560 TriSonica Sphere is designed to overcome these limitations.

The LI-550 TriSonica Mini is constructed of DuPont™ Zytel^® nylon resin. Zytel 80G33HS1L is a 33% glass fiber reinforced heat stabilized polyamide 66 resin engineered for demanding applications where outstanding impact resistance, high mechanical strength, stiffness and extreme temperature performance are required.

The LI-560 TriSonica Sphere’s all-aluminum construction provides excellent durability while maintaining its size, weight, and power advantage over competitive sensors.

Both the LI-550 TriSonica Mini and the LI-560 TriSonica Sphere can operate in temperatures ranging from -20 to 72 °C.

The body of the LI-550 is injection molded of glass-infused nylon supported by carbon-fiber rods, well suited for exposure to extreme environmental conditions, and which weathers to a pleasant gray patina while retaining structural integrity. The LI-550 has been thermal chamber tested, confirming that the device can operate at conditions down to -20 °C. and up to 72 °C.

As for humidity, of course a thoroughly soaking heavy rain or snow (100% humidity) will block the acoustic pathway between transducers, but the interior electronics continue to function. The humidity sensor is located on the interior of the LI-550, and air flows to it through a Gore-tex vent positioned to prevent incursion of liquid water on the electronics. The device is capable of operating in high humidity environments; however if a hot, high humidity environment suddenly transitions to a cold environment, the water vapor inside the LI-550 may condense, resulting in a humidity sensor reading higher than 100% until the condensate evaporates and transpires through the vent.

Both the LI-550 and the LI-560 output an ASCII string that can easily be stored to a text file. You can use a program called TeraTerm to view data from the sensors. Tera Term also features a logging function that will write data to a text file, which can be viewed and processed in spreadsheets or custom programs.

The LI-550F supports RS-232 digital output, while the LI-550P and the LI-560 both support RS-232, RS-422, and UART-3V digital outputs.

The optimal height for the anemometer is dependent on what you are trying to study. We have seen some installations that mount the LI-550 TriSonica Mini a few inches above the beach and rocks, while others that put it on towers at heights from 3 - 10 meters.

If you intend to mount the LI-550 or LI-560 on a multi-rotor UAS or drone, we recommend a minimum height above the rotor plane of 500 mm or 1.5x the rotor diameter, whichever is greater.

Researchers at the International Conference on Unmanned Aircraft Systems (ICUAS) have reported that influences from rotors are negligible at around 400 to 450 mm above the rotor plane. Our customers have mounted their TriSonica Mini as high as 18 to 20 inches above the rotor plane to negate any impact the blades might have on the readings.

If you are interested in measuring the wind speed on an island, then we suggest that you put the LI-550 TriSonica Mini high to avoid the turbulence caused by the island topology. The size of landscape features will affect on how high you need to be. The higher you go, the less of an effect the island will have on the airflow.

ICUAS Research: T. S. Johnansen, A. Cristofaro and K. Sorosensen, “On estimation of wind velocity, angle-of-attack and sideslip angle of small UAVs using standard sensors,” 2015.