Anemometer and Wind Speed Calibration Services
Looking for anemometer calibration? Our laboratory is equipped with a state-of-the-art, computer-controlled wind tunnel which can achieve speeds of up to 30m/s.
Starting from as prices as low as £70, we can calibrate handheld, portable and fixed anemometers, wind speed indicators, wind meters and hot wire anemometers to traceable standards.
We can calibrate anemometers from all leading manufacturers, including Kimbo, Skymate, Extech, Dreadloze, Kestrel, Davis, Testo, Airflow, R.S Components, Latrow, ATP and Lutron.
Anemometer and Wind Meter Calibration – About Anemometers
An anemometer is an instrument that measures wind speed. The name of the instrument is derived from the Greek word “anemos” which means “wind”. Anemometers are mainly used in weather stations for meteorological purposes. But they are also used in industrial processes, in automotive industry, in aerodynamics and in various environmental applications. As any other instrument used for measurements, anemometers require a proper calibration in order to make sure that they measure correctly.
Before studying their calibration procedure, it is important to see the main types of anemometers and their principals of operation. These instruments are mainly classified into two major categories: those that measure the velocity (speed) of the wind and those that measure the pressure of the wind. Since there is a close connection between pressure and velocity, an anemometer of either of the two categories, can provide data for both wind speed and pressure.
Types of anemometers
One of the oldest types of anemometers is the cup or rotational anemometer. It has three or four cups, each mounted on one end of a horizontal arm. These arms (three or four) are attached to a vertical rod. As the wind blows it causes the cups to rotate, making the rod spin. The faster the cups rotate, the faster the wind speed is. Therefore, counting the turns of the rod over a set time period produces the average wind speed. Three cup anemometers are currently used as the industry standard for wind resource assessment studies.
This is also a mechanical anemometer measuring the velocity of the wind. It is very similar to the normal windmill in which the axis of rotation runs parallel to the direction of the wind thus making it horizontal. But the wind keeps on changing its direction. Therefore the axis has to change its direction; so an aerovane is also incorporated into the device. An aerovane consists of a propeller and a tail so that precise wind speed and direction measurements can be obtained.
Hot wire anemometers
This type of anemometer takes advantage of the fact that air cools a heated object when it flows over it. Hot wire anemometers use a very thin wire (in the order of some micrometers) electrically heated up to a temperature above the ambient. The amount of power needed to keep the wire hot is used to calculate the wind speed. The higher the wind speed, the more power is required to keep the wire at a constant temperature. Hot wire anemometers have extremely high frequency response and fine spatial resolution. This is why they are commonly used for detailed study of turbulent flows in which rapid velocity fluctuations are of interest.
Laser Doppler anemometer
They use a beam of light from a laser which is divided into two beams, with one propagated out of the anemometer. The velocity is determined on the basis of the amount of light that has been reflected off by the moving air particles. These types of anemometers are very accurate and they can measure even the slightest changes in airflow. They are also used in river hydrology.
In this type of anemometer, ultrasonic sound waves are used to measure the wind velocity. The speed of wind is obtained by sending sound waves between a pair of transducers and calculating the way their speed is affected. This type is quite popular in aircraft, scientific wind turbines, ship navigation and weather stations. Since they also do not have many moving parts they can as well be used in the automated weather stations.
They belong to the second category of anemometers which measure the wind pressure. A tube anemometer measures the air pressure inside a glass tube that is closed at one end. By comparing the air pressure inside the tube to the air pressure outside the tube, wind speed can be calculated.
Anemometers are calibrated in accordance to Annex F of IEC 61400-12-1 “Wind turbines – Part 12-1: Power performance measurements of electricity producing wind turbines”. Although the calibration procedure refers to cup anemometers, it is widely accepted for the calibration of any type of anemometer.
The equipment used for the calibration is:
- A wind tunnel: Wind tunnels are large tubes with air moving inside. They have a fan which moves the air inside. The fan must have straightening vanes to smooth the airflow. The instrument under test is placed in the middle of the tunnel and it is fastened so it does not move.
- Pitot tubes: They are pressure measuring instruments used to measure fluid flow velocity. The pilot tube is a slim tube which has two holes on it. The front hole is placed directly into the fluid flow and it measures the stagnation pressure. The side hole measures the static pressure. By measuring the difference between these pressures, the dynamic pressure can be obtained according to Bernoulli’s formula, which can be used to calculate airspeed.
There are several requirements for anemometers’ calibration that must be taken into account:
- All transducers and measuring equipment shall have traceable calibrations. Calibration certificates and reports shall contain all relevant traceability information.
- The pitot tubes used must be calibrated for appropriate wind speed ranges and be documented.
- Prior to every calibration, the setup must be verified by means of comparative calibration of a reference anemometer.
- The repeatability of the calibration shall be verified.
- An assessment of measurement uncertainty shall be carried out in accordance with ISO guidelines.
There are also special requirements for the wind tunnel. The presence of the anemometer shall not substantially affect the flow field in the wind tunnel. The flow across the area covered by the anemometer shall be uniform. Flow uniformity can be estimated using velocity sensing devices, i.e. pitot tubes, hot wires or Laser Doppler Velocimetry. The flow shall be uniform to 0.2 %. The wind tunnel calibration factor, which gives the relation between the conditions at the reference measurement position and those at the anemometer position, shall be appraised using pitot tubes. In order to assure the repeatability of the facility, five calibrations of a reference anemometer shall be performed. The maximum difference between these calibrations should be less that 0.5% at 10 m/s wind speed.
During calibration the anemometer shall be mounted on top of a tube in order to minimise flow distortion. This tube shall be of the same dimensions as the one on which the anemometer will be mounted in service in the free atmosphere. Mounting arrangements can have dramatic effects on instrument sensitivity, particularly if the ratio of tube diameter to rotor diameter is high.
It is important to ensure that the anemometer is not influenced by the presence of any reference wind speed measurement equipment.
The pitot tubes shall be positioned at the test section perpendicularly to the flow field of the wind tunnel as accurate as possible. The maximum declination allowed is 1o. The anemometer shall be positioned at the test section perpendicularly to the flow field of the wind tunnel as accurate as possible. The maximum deviation allowed is 1 o.
Before the calibration procedure begins, the anemometer shall run for about 5 minutes in order to avoid the fact that large temperature variations may influence the mechanical friction of the anemometer bearings.
Calibration shall be performed under both rising and falling wind speed in the range of 4 m/s to 16 m/s with steps of 1 m/s or less. By measuring both increasing and decreasing steps, it is possible to identify whether hysteresis effects are present in the measuring equipment.
The sampling frequency shall be at least 1 Hz and the sampling interval at least 30 s. Before taking readings at each wind speed, adequate time has to be allowed in order to establish stable flow conditions. This typically takes 1 minute, but it varies from facility to facility.
The air density (ρ) shall be calculated according to the following formula:
B is the barometric pressure (Pa)
T is the absolute temperature (K)
φ is the relative humidity (range 0 to 1)
Ro is the gas constant of dry air (287.05 J/kgK)
Rw is the gas constant of water vapour (461.5 J/kgK)
pw is the vapour pressure (Pa)
Where vapour pressure pw depends on the mean air temperature.
The mean flow speed () at anemometer position is calculated from mean differential pressure Δpref at reference position according to the following equation:
Ch is the pitot tube head coefficient
kc is the wind tunnel calibration factor
kb is the blockage correction factor
n is the number of samples within the sampling interval
After the calibration, an uncertainty analysis shall be performed. The uncertainty of measurement comprises of both type A and type B uncertainties, as described in EA-04/02 “Expression of the Uncertainty of Measurement in Calibration”. The following parameters shall be taken into account:
- Flow speed measurement uncertainty (pitot tubes, transducers, air density evaluation, etc.)
- Frequency measurements
- Wind tunnel calibration
- Flow variability in the vicinity if the anemometer.
Written by Sofia