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Windspeed Limited (trading as Vector Instruments)
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What is the difference between your "factory calibration" and a "MEASNET calibration" ? |
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Answer:
Our "factory calibration" is included as standard with every anemometer we manufacture and involves a test of the rotor/cups to be supplied with the anemometer in our own wind tunnel at a single windspeed (approx. 9m/s). The rotor/cups are compared against one of a set of "reference" rotors which themselves have calibrations traceable to national/international standards. The results of this "factory calibration test" are regularly compared to the results of external calibrations (e.g. MEASNET) to ensure consistency. Experience over the years has shown that while we do not test using the "actual" anemometer body with which the rotor will be shipped, this does not have any significant effect on the results/calibration. The "factory calibration" test gives a single result which is expressed as "RPM per m/s" at the 9m/s test windspeed and is provided on a "Rotor Calibration Data Sheet" - this figure can be used in conjunction with information about "nominal" (slight) non-linearities on the same sheet and/or with information provided in the anemometer Operating Instructions to configure the measuring equipment. "Slope and Offset" figures are not provided directly, but can be calculated/derived if necessary (although the procedure can be rather involved, so we are able to provide calculated/estimated figures on request in many cases). Because this calibration test is only done at a single windspeed, it cannot take into account any slight variations in non-linearity due to the individual rotor/bearings or anemometer (unlike a multiple-speed calibration such as MEASNET).
MEASNET calibration tests are an extra-cost option because they involve shipping the anemometer/rotor to a MEASNET approved facility where the actual anemometer body and rotor/cups are tested as a "set" at more than one wind speed. The test is carried out in a larger cross-section, low-turbulence wind tunnel, at multiple windspeeds (usually over the 4 to 16m/s range) while the wind tunnel speed is both increased and decreased. The speed of the wind (air) in the wind tunnel is calculated during the test from pressure measurements taken using pitot tubes (so the calibration is traceable to "basic standards" rather than to a "reference instrument or rotor/cups"). The MEASNET calibration certificate provides a table of anemometer output signal vs. tunnel wind speed together with "slope and offset" figures (convenient for entry into many data loggers) from a linear-regression calculation performed on that data. The whole MEASNET procedure is tightly controlled, and test facilities in the MEASNET group have to show that their calibrations are accurate, repeatable and consistent. As part of the MEANSET groups checking, instruments are periodically calibrated at different MEASNET authorised facilities to ensure that consistent calibrations are obtained whichever MEASNET approved facility is used. MEASNET anemometer calibration facilities also generally have ISO17025 and other QA certifications. MEASNET calibrations are recommended for high-accuracy applications such as wind-assessment. |
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Windspeed Limited (trading as Vector Instruments),
113 Marsh Road, RHYL, North Wales, LL18 2AB, United Kingdom
Tel: +44 (0) 1745 350700 Fax: +44 (0) 1745 344206
Email: sales@windspeed.co.uk
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Website design and content is copyright (c) 2003-2024 Windspeed Limited (unless otherise stated). All trademarks are acknowledged.
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