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FAQ - Frequently Asked Questions

When was the cup anemometer invented?

It is not certain who actually devised the instrument. But is was first described in 1846 by the Irish astronomer T. R. Robinson. The first cup anemometers had 4 cups. Patterson (1926) found that if the rotor had only 3 cups the torque is more uniform through the entire revolution.

Why is the calibration linear?

The short answer is that it is an empirical fact. In all wind tunnel calibrations of modern cup anemometers it is not possible to detect any deviation from linearity. Brazier (1914) found that the ratio of the cup diameter to that of the circle described by the centers of the cups is an important parameter and that the best linearity is obtained when this ratio is about 0.5.

How many pulses per one full rotor rotation is required to obtain a reliable signal?

In principle one pulse suffices. It is not possible to obtain a better velocity measurement corresponding to just a fraction of a revolution. The instrument averages over one full revolution, which will then be the resolution. For each revolution a column of air with a fixed length \(\mathit\ell\) passes through the rotor. This length is called the calibration length. Summing all these columns of length \(\mathit\ell\) over a particular period, often 10 minutes, we obtain the mean-wind speed in this period. For practical purposes we have chosen 2 pulses for each revolution, but it is really overkill.

How fast is the response of a cup anemometer?

The reaction time is determined by the so-called distance constant \(\ell_\circ\).The response time is obtained by \(\ell_\circ\) and the mean-wind speed \(U\) as \(\tau_\circ=\ell_\circ/U\). Let us consider a situation where we have a constant wind speed \(V\) and then have it changed instantaneously to \(V+\Delta{v}\). Then, after the time \(\tau_\circ\), the anemometer will show 63 percent (=1-\(\mbox{e}^{-1}\)) of the final value \(V+\Delta{v}\). We can also express this by stating that a column of air of length \(\ell_\circ\) must pass through the anemometer to get to the same result. The distance constant \(\ell_\circ\) is an instrument constant. However, the time constant is not an instrument constant, since it is inversely proportional to the mean-wind speed \(U\). The instrument has a slower response to changes in the wind speed the larger the distance constant.

What is overspeeding anyway? Is there a simple explanation?

Overspeeding is a positive bias on the measured mean wind speed \(U\). There are two basic reasons, the atmospheric turbulence and the asymmetric construction of the rotor. This asymmetry exists simply due to the fact that the wind forces are larger with the wind blowing into the cups than with the wind blowing on their back sides. This is essential for the working of the cup anemometer because without this difference the anemometer rotor would never start moving. The reason that the turbulence plays an important role is that the anemometer is calibrated in a wind tunnel with no, or very little, turbulence. So when exposed to turbulence in the atmosphere the cup anemometer will react to the wind fluctuations from a whole spectrum of eddy sizes. An anemometer with a small distance constant \(\ell_\circ\) can easily follow the larger eddies, but when eddies are of sizes equal to or smaller than the distance constant the anemometer cannot follow the fluctuations. However, these will cause the rotor to spend more time on the plus side than on the minus side of the mean wind speed \(U\) than it would have been measured in the wind tunnel with no turbulence. A special, but quite common, case is the signal from an anemometer in a windy surface layer. The overspeeding relative to the mean-wind speed is \(C(\sigma^2/U^2)(\ell_\circ/z)^{2/3}\). Here \(\sigma^2\) is the variance of the wind speed or the square of the standard deviation, \(z\) the height and \(C\) a dimensionless constant of the order of one.

It is possible to use the cup anemometer to measure the variance and higher-order moments. But these moments will show no bias.

What is the relation between IEC 61400-12-1, ISO 17025 and MEASNET?

Here is how it works:

IEC 61400-12-1 Power performance measurements of electricity producing wind turbines
This IEC standard specifies the procedures for measuring the power curve. A key element of power performance testing is the measurement of wind speed for which the standard prescribes the use of cup anemometers. Hence, it also specifies the procedure for cup anemometer calibration. http://www.iec.ch

ISO/IEC 17025 General requirements for the competence of testing and calibration laboratories
This is the main ISO standard used by testing and calibration laboratories. In most major countries, ISO/IEC 17025 is the standard for which most labs must hold accreditation in order to be deemed technically competent. In many cases, suppliers and regulatory authorities will not accept test or calibration results from a lab that is not accredited. ISO 17025 does not relate to the anemometer but to the requirements for the competence of the laboratories calibrating it and utilizing it in a power performance test, respectively. http://www.iso.org

MEASNET is a co-operation of companies which are engaged in the field of wind energy and want to ensure high quality measurements, uniform interpretation of standards and recommendations as well as interchangeability of results. http://www.measnet.com

Who is doing what?
WindSensor manufactures anemometers (BTW, the best on the market). WindSensor anemometers are classified according to IEC 61400-12-1:2005 Annex I.

WindSensor anemometers are calibrated in the SOH wind tunnel. The SOH wind tunnel is accredited according to ISO 17025 to calibrate anemometers according to the IEC 61400-12-1:2005 Annex F (BTW, with the lowest uncertainty on the market).

A number of testing institutes are accredited according to ISO 17025 to carry out power performance testing according to IEC 61400-12-1 using calibrated anemometers.

What is the service life of WindSensor anemometers?

There are basically two parameters which can influence the calibration of a cup anemometer, the geometry and the bearing friction. Assuming that the geometry, and hence the aerodynamics, is constant over the lifetime of an anemometer the critical parameter is bearing friction.

Bearing friction may among others be affected by lightning, mechanical impact, wear and corrosion. By design, WindSensor anemometers are tolerant of lightning due to the non-conducting property of the cup rotor, which tends to direct the lightning current along the surface to the body of the anemometer rather than through the bearings. Furthermore the bearings utilize a special steel type which is several times more resistant to wear and corrosion than steel types otherwise used in the industry. Combined with the most durable lubrication in the industry WindSensor anemometers have proven to operate in excess of 10 years under ideal conditions.

However, as cup anemometers are operated under different conditions all over the world in all types of environment it is not possible to specify rigid service intervals.

We specify a service interval of 1 to 2 years depending on the operating conditions. For example, it is common practice to replace cup anemometers on offshore towers at 1-year intervals due to the extremely high costs of accessing the site. We recommend that our customers should define a maximum service interval depending on the conditions on the specific site.

Given the fact that the bearings might be damaged even shortly after the installation or even worse by incorrect handling during installation, the most important precaution in operating a meteorological tower is to continuously monitor the performance by comparing measurements with anemometers at both the same level and at other levels.

We cannot recommend using different models of anemometers on a meteorological tower as different types of anemometers have different response to air temperature, air density, inflow angle and turbulence resulting in different measurements, even if installed at same level.