You may have some anemometers that have been stored on a shelf for an extended period of time and you are wondering whether or not the 6 or 12 month old calibration is still valid. In addition, you are wondering whether or not you should recalibrate the anemometer after a measurement campaign.
Let's look at this question from both the perspective of a calibration wind tunnel such as the Ammonit Wind Tunnel (AWT) and from the PCM (Power Curve Measurement) and WRA (Measurement) campaign management perspective.
One should carry out power curve measurements (PCM) and wind resource assessments (WRA) with the aim of always having proof that the anemometer has remained stable during the campaign (proof of stability).
The complete calibration procedure of an anemometer for a measurement campaign is ideally:
a) Calibration before the measurement campaign in a MEASNET wind tunnel
b) Calibration after storage
(If the anemometer has been stored for 12 months, it should ideally be recalibrated.)
2) In-situ comparison
Very thorough In situ comparison monitoring of the two anemometers anemometers (according to Annex K of the IEC 61400-12-1, see also Annex S). The two top anemometers must be on the free flow for the dominant wind direction.
3) Post-calibration after the measurement campaign in a MEASNET wind tunnel, preferably in the same wind tunnel as during the pre-calibration.
If you are based in Europe, you can follow the ideal procedure.
If you are located outside Europe, post-calibration of the anemometer in a MEASNET wind tunnel is difficult to perform for logistical reasons. Therefore you can probably only do 1) a) Pre-calibration and 2) In-situ comparison and possibly Post-calibration in a non-MEASNET wind tunnel.
The validity of the calibration has been discussed several times in the IEC 61400-12-1 ed. 2 2017 community. Ultimately, the community decided not to set a maximum storage period. This means that the decision about (re-)calibration must be made by the end customer.
Companies that recalibrate anemometers after 6 or 12 months of storage almost never see a difference between 1 a) calibration before storage and 1 b) calibration after storage. However, the number of these measurements is not large enough to provide statistical evidence.
Please also note that Thies anemometers come with a 5-year warranty. This is a clear sign that the anemometers are quite stable over time, especially in a storage environment.
In-situ comparison 2) for the top 2 anemometers is very important. The campaign manager should monitor the anemometer very regularly and systematically with an In-situ comparison to prevent the anemometers from drifting apart. If the pair of anemometers drift apart, they should be replaced as soon as possible.
As a company carrying out the measurement campaign: If you have stored the anemometer for 12 months without recalibration, but you have proof that your anemometer has remained stable through 3) Post-calibration or 2) In-situ comparison, then you have a good solution from the perspective of IEC 61400-12-1.
However, you have a small commercial risk: If the 3) Post-calibration or 2) In-situ comparison differs from the 1 a) Pre-calibration at the end of the measurement campaign, your customer might argue that this difference is due to the anemometer being stored for too long, although the difference is most likely due to the measurement campaign in the field and not the storage time.
Proof of stability through 3) Post-calibration provides clearer results than 2) In-situ comparison. So if your logistics allow you to perform a 3) Post-calibration, you have the best documented solution there.
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