Choosing the right wind sensor technology for any automatic weather station or wind measurement system is a very important topic. Whether you choose ultrasonic or mechanical cups and vane wind sensors will depend on various aspects that need to be taken into consideration.
The aim of this article is to provide an in-depth technical guide that will be useful when comparing different types of sonic wind sensors and cups and vane sensors, in order to make your choice in an informed manner, in line with your needs and specifications.
Choice based on mechanical strength
Mechanical strength is an important factor to consider when choosing a wind sensor. Sonic ones are generally stronger, as they have no moving parts; but this does not mean that cups and vane sensors are fragile.
Let’s analyze some aspects of these technologies:
- Rotor and vane breakage is a very rare event, which occurs in extremely strong wind situations, exceeding 65 m/s (234 km/h). These wind speed events occur only during strong hurricanes, when the mechanical support structure of the sensors and the sensors themselves are at risk of destruction, also due to flying objects dragged by the force of the wind that may hit the sensors.
- One of the problems of cups and vane wind sensors is the possible breakage of the rotor and vane due to the weight of accumulated snow and/or ice in particularly cold climates. To avoid this, there are different models of heated wind sensors, however, these models consume considerably higher levels of energy and are therefore not suitable for systems powered by batteries or solar energy.
- The shafts and bearings designed for the latest generation of cups and vane sensors have, in most cases, no limits to their operation for their entire life. Problems of “seizing” of the bearings or misalignment of the shafts are events that can only occur in situations where the sensor is installed on slopes subject to strong and constant winds coming from below compared to the axis of the sensor.
Choice based on the quality of the measurement
The quality of the measurement is a crucial factor in choosing a wind sensor. Sonic wind sensors are potentially better then cups and vane types due to their measurement principle; however, it is important to take some factors into consideration:
- Sonic wind sensors are particularly recommended in situations where extremely low wind speeds, less than 0.2 m/s, need to be assessed. This is due to their better specification in terms of measurement threshold compared to cups and vane wind sensors.
- Sonic wind sensors are particularly suitable to assess the intensity and duration of wind gusts, as they have virtually no delay distance, hence an ability to respond rapidly to sudden wind speed changes.
- When it comes to total measurement accuracy, however, it is important to make a distinction. Sonic wind sensors with a top sonic sound reflecting plate, due to their geometry, have generally worse accuracy, especially at high wind speeds, when turbulence phenomena occur between the two sides of the measuring chamber. These phenomena are difficult to correct with the adjustment performed within wind tunnels during the calibration process, as the wind in nature has a much bigger turbulent behavior, with a larger variable origin angle in terms of amplitude and frequency, compared to the air flow generated inside a wind tunnel, which tends to be laminar. These turbulences increase as the wind speed increases and it is for this reason that at high speeds the general accuracy of the sonic sensor can deteriorates significantly.
- In operating conditions in an external environment, the sonic wind sensor receives the wind in its reflecting plate chamber from angles of incidence that are not necessarily horizontal. In wind conditions that are more angled than the horizontal plane, the “roof” and the “floor” of the reflecting plate chamber determine a shielding action to the air flow; these flows are instead more easily appreciable using cups speed sensors. Overall, a sonic wind sensor (i.e. top sound reflecting plate chamber type), has a potential underestimation of the wind speed measurement, which does not follow, as it should, the response curve with respect to the cosine of the flow angle with respect to the horizontal plane.
- In conditions of heavy rainfall and strong wind, the presence of water drops moving between the sonic transmitters and receivers can alter the transit time of the ultrasonic wave. The transit time is the fundamental element that allows the measurement of both wind speed and direction. Therefore, during heavy rains, the effectiveness of the measurement with sonic sensors can worsen, a problem that occurs to a lesser extent in cups and vane sensors.
- Cups wind sensors also have problems with turbulence created by the geometry, but generally have a better linearity response. This facilitates the adjustment of the measurement through correction formulas programmed into the data acquisition system.
- Sonic sensors are potentially more accurate in measuring wind direction as well. However, their geometry provokes wind reflection at different angles; this means that, to ensure data accuracy, calibration must be performed at each potential angle of attack. Wind vane sensors may also have delay-problems due to the waving of the wind vane with respect to the direction axis; in this case, the calibration in the tunnel fails to reproduce the specific outdoor wind conditions. In fact, in nature, the direction of the wind, as its speed increases, does not remain within a laminar flow, but has a flag proportional to the speed. However, in meteorology, wind direction is frequently evaluated over 16 or 18 sectors, so it is not important to try to get very high wind direction resolutions.
Choice based on energy consumption
- In order to function effectively and maintain measurement accuracy, sonic wind sensors must be equipped with suitable heating elements, to avoid the formation of ice in the measurement top sound reflecting plate chamber and to remove any water (e.g. rain, condensation, frost) from the sonic transducers and the roof of the reflection plate.
- On the contrary, for cups and vane sensors, even in the presence of rain, condensation, frost, ice and even without a heating system, the operation is less critical. Firstly, because rain and condensation cause little disturbance to the measurement and, secondly, because frost and ice tend to be partially removed by the motion of the rotor and vane. In the event of a mechanical lockout, the cups and vane sensors would resume functioning as soon as the temperature returned above freezing, unlike the sonic sensor, which would need to dry out completely to fully resume functioning.
- However, ice can also represent a problem for cups and vane sensors in climatic conditions when no wind, simultaneous snowfall and subsequent drop in temperature: these conditions can determine the formation of frozen masses on the body of the sensor, compromising the operation and the mechanical integrity of the rotor and/or vane.
- Overall, the cups and vane sensors are much simpler sensors, equipped with electronics optimized for low-power consumption. Therefore, if the measuring system is to be powered by battery and solar panels, the choice of cup and vane sensor is the most advisable.
Choice based on maintenance
- While on the one hand the absence of moving parts is considered an advantage, on the other hand it allows for easier deposition of dirt on the sonic sensors.
- Sonic sensors experience problems when their sonic pulse emitting/receiving elements are clogged, or when the upper surface of the measuring chamber accumulates dirt. This can occur in the presence of dust, sand, or other substances deposited on the sensor surface. Furthermore, the top sound reflecting plate chamber is an ideal place for insect or small bird nests. If the sonic sensors are installed in areas characterized by a strong presence of dust, sand, dense vegetation, or at industrial sites, more frequent cleaning of the sensor will be necessary.
- The cleaning of the cup and wind vane sensors, installed in the sites described above, will be in any case necessary, albeit less often, in order to avoid the deposit of debris or contaminating substances which could, in the long run, compromise the integrity of the materials of which these sensors are made.
Choice based on installation
- Sonic sensors are easy to install as they do not require to be perfectly horizontal. Therefore, the structures that support them (poles and towers) do not necessarily have to be perfectly horizontal with respect to the ground plane. Conversely, for cups and vane wind sensors it is advisable to observe a good horizontality of the rotor, to prevent the internal shaft from working non-horizontally, and of the vane, to prevent it from being unable to orient itself, due to the weight of its extremes, correctly at very low wind speeds.
- If the choice falls on sensors with separate cups and wind vanes, it is necessary to consider the use of a fixing structure for the two sensors (coupling bar) bearing in mind that there will necessarily be two electric cables directed to the acquisition system. Instead, sonic sensors do not require special mounting accessories and have a single connection cable to the acquisition unit.
- Some models of sonic sensors are equipped with an internal magnetic compass which serves to avoid having to manually orientate the sensor with respect to the North. However, these compasses suffer from two problems: they point to the magnetic north and not the geographic north, which is required for correct measurement of wind direction; they are also subject to systematic errors due to the possible presence of ferrous materials nearby. These two problems often lead to having to deactivate the compass for automatic orientation and use, during the operations of installation, the classic method of orientation of the sensor through the use of an external compass.
When comparing sonic and mechanical cups and vane wind sensors, it can be said that there is no absolute best technology: the choice must be made by carefully considering the installation site, the required quality of the measurement results and the maintenance resources available.
When choosing, it is necessary to take into account the intended use and objectives; when this information is defined, the previously described variables can be evaluated, such as mechanical strength, measurement quality, energy consumption, maintenance and ease of installation.
In conclusion, the evaluation is effective when it takes into account all the different factors previously mentioned. If the technology has been chosen according to the right criteria, it will positively affect the effectiveness and efficiency of the measuring system.