What Does an Anemometer Measure and How Does It Work
When you are in a specific situation and you need to gage the speed & direction of the wind, this is going to be your number 1 instrument. Whether your situation involves controlling pollution, accounting for the safety of tall structures, controlling of wind turbines, manoeuvring ships and aircrafts ready for landing or studying the effects of winds on crops – you can get more significant benefits from an anemometer.
What is an anemometer?
Anemometer is an instrument that is used to measure wind speed, and it’s one of the fundamental parts of a weather station. It looks like a weather vane. But instead of pointers that shows wind direction, it has four cups that measures the speed of the wind. Each cup is attached to the end of a horizontal arm, each of which is mounted on a central axis, like spokes on a wheel. When wind pushes into the cups, they rotate the axis. The faster the wind, the faster the cups spin the axis.
Who Invented the Anemometer?
The person who first invented the mechanical anemometer is Leon Battista Alberti. The Italian art architect made his instrument in 1450 consisting of a disk placed perpendicular to the wind. It would rotate by the force of the wind, and by the angle of inclination of the disk the wind force momentary showed itself. Since the first invention, people and now using various types of anemometer that come in the market.
So choosing an anemometer perfect for your needs can be complicated. You can find lots of different types of anemometers, each with unique style and functionality. To select the right one, you must consider specific factors and aspects. Here are some of the most critical factors that you need to contemplate while choosing an anemometer.
First of all, you should decide the kind of anemometer you require and are willing to purchase. Here are the most common types of anemometers that would suit your weather station;
1. Cup Anemometer
Cup anemometers are the most popular ones. A cup anemometer consists of 3 or more cups that rotate about a vertical axis. The cup turns when the wind blows, and an integrated read switch or Hall Effect sensor measures the rotation, which is then converted into wind velocity reading.
- This type of anemometer doesn’t need to point towards the source to sense the wind speed.
- Cup anemometers are highly reliable and rugged.
- The anemometer will sometimes have very high starting velocity.
- Low measurements are not very well studied.
- In turbulent conditions, the cup anemometers can underestimate the wind speeds.
- The rotating parts wear out with use over time.
- Mostly fail due to icing effect.
There are two types of cup anemometers;
- 3-cup anemometer
- 4-cup anemometer
3-cup is recommended over 4-cup anemometer as the errors are lesser, and it also has more constant torque and responds faster to gusts than the 4-cup anemometer.
2. Vane Anemometers
Vane anemometers have a mechanism similar to the cup anemometers, but the sensors of a vane anemometer are shaped like windmill or turbine. Unlike the bowl anemometers, this type of anemometer has to be parallel to the wind and rotate about a horizontal axis.
- They can handle harshest and dirtiest environment and are more rugged and durable than other anemometers like a hot wire.
- Vane anemometers can handle higher wind speeds.
- To achieve the best result or accurate readings, the unit must be pointed towards the source.
- Anemometers with smaller vanes often have higher starting speeds.
- Like in cup anemometers, the rotating parts of vane anemometers also wear out with use over time.
3. Hot Wire Anemometers
Hotwire anemometers work in a completely different mechanism. In a heated wire anemometer, a skinny wire within the equipment is electrically heated, which cools down as the wind flows. The current required in keeping the wire temperature constant or the change in the temperature in a continuous current is then measured. The air velocity is finally achieved by converting the heat loss.
- It offers low starting threshold.
- No rotating parts, so this equipment lasts longer when compared to a cup or vane anemometer.
- It provides high accuracy reading, with quick response.
- The sensor is comparatively much smaller in size.
- Large particles in the air can damage the unit, so this type of anemometer is not recommended for the dirty environment.
- Not appropriate for locations that have rapid fluctuations in the temperature.
4. Pitot Tube Anemometers
Pitot tube anemometers measure the pressure of the wind, which is then converted into wind speed reading. In this type of anemometer, the pitot tube weighs the pressure of the incoming air and the static tube measures the static pressure in the wind flow. The air velocity is then calculated by the meter, using Bernoulli’s equation.
- Can usually handle very high air temperatures and velocity.
- Pitot tubes are quite small and can be easily inserted into ductwork.
- Have no rotating parts, so this doesn’t wear out easily.
- Offers quick response.
- Equipment must be pointed towards the flow of the wind.
- Not recommended for locations with highly fluctuating wind flow.
- Not suitable for areas with low air velocity.
5. Sonic Anemometers
Sonic anemometers are the most advanced type. Ultrasonic sound waves are used by a sonic anemometer to measure the wind velocity. Wind speed is measured by the time of flight taken by sonic pulses between the transducers.
- Can capture measurements with profoundly excellent temporal resolutions of 20Hz or more.
- Weather resistant and very less likely to fail due to icing effect.
- Have no rotating parts, so this type of anemometer has better durability than cup and vane.
- Very rare and more expensive than other types of anemometers.
- Prone to birds perching on the probes, which can cause problems, resulting in abnormally high false wind gusts.
When assessing the quality of an anemometer, accuracy rating is, in fact, the most crucial factor to be considered. Accuracy rating of the unit signifies how much accurate the reading provided by a particular anemometer is. For better results, always look for an anemometer that has accuracy ranging from ± 5%, or better.
Response and Wind Speed
The wind speed reported by the equipment is a significant aspect that you must consider. A good anemometer will usually have a wind speed reporting that ranges from 0 to 100 mph or more. Responses provided by the anemometer should be quick. The wind speed can fluctuate fast, so the equipment must be able to serve smart and updated wind speed reading.
Size and Design
Different anemometers come with varieties sizes and designs. For example, you’ll find some vane anemometers with a larger vane and some with smaller, or cup anemometers with a different number of cups. Vane anemometers with huge vanes are better than the smaller ones as mentioned earlier. Similarly, the 3-cup anemometer is more accurate than the 4-cup. The equipment with longer cables is better, as the higher is the location of the wind sensor, the more accurate reading can be achieved. Certain little things have to be kept in mind while choosing an anemometer.
The material and construction of the anemometer are necessary factors as the durability of the equipment depends on these aspects. The anemometer will constantly be exposed to the outdoor environment, and face the harshest weather conditions, so make sure that the unit you select is rugged and durable that can withstand the worst climates.
-Usually, cup and vane anemometers can’t withstand ice and fail due to icing effect, but nowadays some high-end heated version cup anemometers are available that can also work even during freezing climate. Again, with advanced technology, some cup and vane anemometers with nonfiction rotating parts are available, which can last much longer than the normal ones.
The color of the anemometer somewhat matters for locations that are often affected by ice. Anemometers are available in two colors, black and white. Both types are susceptible to icing, but the black anemometers would thaw out quicker than the white ones.
There are much more other features that you can look for, such as electrical and mechanical components, accessibility for clean-up and maintenance, and digital pulse data that help prevent electrical interference from various sources like a radio transmission.
Choosing an anemometer is not an easy task, and can be very much time-consuming. Although expensive units are the best, there are also some low-end anemometers that are no less in functionality and durability. No matter what your budget is; if you consider all the factors mentioned above, you can quickly select the most suitable one. Going through the reviews provided by other experienced users could also help you in some way. Take your time, and choose the best.