MEASNET Calibration of Anemometers according IEC 61400 12-1 standards
When calibrating anemometers, it is essential this is done properly to ensure the calibration procedures like ISO / IEC 61400 12-1 Ed.1 or Ed.2 standards and MEASNET recommendation are followed accurately. For the anemometer calibration, a wind tunnel and pitot tubes are used, among other equipment.
Necessary Equipment for the Calibration
One will need a wind tunnel when calibrating an anemometer and it must meet the required standards as highlighted in ISO / IEC 61400 12-1 (Ed. 1 and Ed.2) and in MEASNET recommendations to ensure high accuracy and acceptance. The wind tunnel needs to have turning vanes and a diffuser to ensure the airflow is smooth and laminar. The anemometer to be calibrated should be placed in the centre of the wind tunnel at a well-defined position and fastened well so that it can't move during the test. During calibration the anemometer in the wind tunnel shouldn't significantly impact the flow field, and the airflow across the area where the anemometer covers needs to be uttermost uniform.
The pitot tubes will be required to measure the reference wind flow velocity, as it were the “true” wind speed. A pitot tube is a narrow tube with two precisely positioned holes. A hole on the front measures the stagnation pressure and should be placed directly into the flow field. It should also have a hole on the side, which will measure the static pressure of the air flow. When it comes to measure the differences between these two different pressures, one can calculate the dynamic pressure following Bernoulli's formula and work out the precise speed of air.
The Procedure for Calibration
One needs to allow the anemometer to warm-up and run for around ten minutes before beginning the calibration. The reason for this is that significant variations in temperature may influence the mechanical friction of the bearings in the anemometer. One should perform the anemometer calibration under both, rising and falling wind speed. This is to identify any hysteresis effects present in the sample measuring equipment.
The frequency for sampling needs to be 1 Hz minimum with a minimum sampling interval of 30 seconds. One needs to allow adequate time between taking the readings at each wind speed so that stable flow conditions can be established after changing the wind speed level in the wind tunnel. Depending on the facility used this time may vary, but it would typically be around one minute.
During the calibration, one needs to measure air density and the mean flow wind speed using the appropriate equations and sensors. Facilities that calibrate anemometers in accordance to ISO / IEC 61400 12-1 standard and MEASNET recommendation will collect all relevant information appropriately and will be able to calculate everything very accurately.
Following the calibration of anemometers, one should carry out an uncertainty assessment and analysis. In this uncertainty analysis, one needs to take the following parameters into consideration at least:
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Uncertainty of the flow speed measurement (pitot tubes).
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Frequency measurements of the sample anemometer.
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The calibration and flow gradients of the wind tunnel being used.
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The flow variability in the vicinity of the anemometer.
To ensure accuracy, it's essential that the correct processes and procedures (ISO / IEC 61400 12-1 Ed.1 and/or Ed.2) are used for a MEASNET calibration of anemometers and/or wind vanes. One also undoubtedly needs to make sure that the exact equations are being used to calculate the different measurements and the process is following the relevant requirements in accordance to ISO / IEC 61400 12-1 and MEASNET.
ProfEC Ventus Laboratory offers anemometer calibration services and wind vane calibration services in our ISO 17025 accredited facilities, which include a MEASNET membership wind tunnel.