Sonic / Ultrasonic Anemometers The sonic anemometer measures the change in the speed of sound due to motion of the air. The time between transmission and reception of a sound wave pulse traveling a known distance is measured from which the wind speed is determined.

C = local speed of sound. a = an angle of wind with respect to sound wave. u = component of wind. speed parallel to axis. v = component of wind speed perpendicular to axis. t1 = time to travel from transmitter to receiver with wind V. B = distance sound travels in time t1 with no wind

Then,. Similarly,. and,. so,. which gives us the. wind component Then, Similarly, and, so, which gives us the wind component along one axis.

A pulse along an axis 90o horizontally to the previous one gives the component along its axis. Combining the two components gives the horizontal wind speed and direction.

Consider: t1 = 305 ms t2 = 295 ms d = 100 mm

Thermal Wind Devices Hot-Wire or Hot-Film Anemometer Type 1: Current applied to small wire. Wire warms due to resistance of the wire. Airflow across wire removes heat changing the temperaure of the wire. Changing the temperature causes a change in the resistance of the wire. Changing the resistance causes a change in the current in the wire.

Changing current causes a voltage imbalance in a bridge circuit. The voltage imbalance becomes a measure of the wind speed. Type 2: Everything is the same except the voltage imbalance is used to increase the current flow in the sensor wire until its temperature (resistance) is at a specified amount (Rs) greater than the resistance at ambient temperature (Ra), The overheat ratio (Rs/Ra) is usually expressed as a percentage, typically 50%.

The relationship between current and the wind speed is given by: The relationship between current and the wind speed is given by: where A and B are usually determined during calibration of the instrument and are related to heat losses due to convection, radiation, wire support conduction. “A” is the current in the wire when V = 0. Sometimes the equation is written in terms of temperature as: where Tw = wire temp. and Ta = ambient temp.

Wire used is typically platinum about. 01 to Wire used is typically platinum about .01 to .1 mm thick and about 1 mm long. Tungsten is also used.

Advantages: Disadvantages High accuracy Not good in rain. Very sensitive Radio interference Rapid time response Orientation ~1 sec. Fragile Can detect low speeds Expensive ~0.1 m/s Requires frequent maintenance High power consumption

Hot-film anemometers use a thin film of platinum. Usually more durable. More inaccurate at low wind speeds than hot-wire anemometer.

Kata Thermometer A spirit-in-glass thermometer with a large bulb. Two marks on the stem at 35oC and 38oC. The thermometer is warmed to over 40oC, placed in the wind and the time taken for the spirit column to fall from 38oC to 35oC is measured. The wind speed can be derived from: where F is oF and t is in seconds. A and B are constants determined from calibration.

Quartz Crystal Anemometer Can measure wind speed, direction, temperature, humidity, solar radiation, particulate deposition. The resonance frequency of oscillation of the current in a circuit in which the quartz crystal is mounted will change as the temperature of the quartz crystal changes.

Resonance frequency. is determined with. heater off. Crystals Resonance frequency is determined with heater off. Crystals at air temp. Heater turned on and raised to temp. above air temp. Each crystal’s frequency changes dependent on its temperature. (warmed by heater, cooled by air blowing across them.) Wind speed is determined by amount of frequency change for each crystal.

For wind direction: Wind flow blows heat from heater across crystals. Amount of frequency shift of each crystal is a measure of amount of heat received. Comparison is made between crystals to determine wind direction.

Vortex Anemometer Frequency of vortex formation is a function of wind speed. Ultrasonic signal (150 KHz) is beamed through region of vortex formation. Vortices scatter some of ultrasonic signal resulting in modulation of amplitude of ultrasonic signal with a frequency equal to the frequency of vortex formation.

150 KHz stripped from signal at receiver leaving modulation signal. Wind speed related to frequency of modulation by: where, fv = Frequency of Modulation S = Strouhal number, typically about 0.2 d = Diameter of obstruction V = Wind velocity

Comparison of several anemometers