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Slopes, Offsets and Calibration Data for Anemometers/Rotors    

Slope/Offset figures are not provided as standard on the "Green" Rotor Calibration Data Sheet (RCD-10) which ships with each rotor we manufacture, however Slope/Offset figures for entry into a datalogger can be obtained in several ways as explained below:

ANY figures mentioned below refer ONLY to the pulse/frequency output of our anemometers - we do not advise using the analog/voltage output signal for applications where high accuracy is required, and we are not able to provide slope/offset information for the analog/voltage outputs (refer to datasheets).

The best (preferred) way to get an accurate slope/offset figure for an anemometer/rotor pair is to have a multiple-point/windspeed individual wind tunnel calibration (such as a MEASNET calibration) performed on the anemometer/rotor pair, in which case the calibration certificate will provide a slope/offset for each specific instrument tested. A MEASNET calibration is NOT included as standard with our anemometers/rotors.

As an alternative (less accurate): Every rotor we manufacture IS however tested on a standard anemometer in our own wind tunnel at a single wind speed of around 9m/s by comparison to one of a set of "reference" rotors. This enables us to produce a "rotor calibration figure" (in "RPM per m/s") for each individual rotor. This is the figure that is provided on the "green" Rotor Calibration Data Sheet (RCD-10) which we ship with each individual rotor. That same sheet also contains a table showing how the "RPM per m/s" figure will vary at other wind speeds compared to the quoted figure for around 9m/s - this allows a correction to be applied for the small non-linearity of the rotor (the rotor is quite linear at lower wind speeds, most non-linearity is at very high wind speeds). The calibration/correction information on the RCD-10 sheet can be difficult to interpret/use properly however, so some further guidance follows below:

In the case of loggers which only accept a simple "slope+offset" calibration, it is necessary to calculate the output frequency versus wind speed "characteristic" for the individual rotor/anemometer pair using the figure/curve from the RCD-10 sheet mentioned above. The slope+offset can then be determined by "fitting" a straight line to that "characteristic" curve over the part of the curve covering the wind speed of interest (usually 4 to 16m/s for wind assessment applications). Obviously, this can be quite an involved process which is one reason why many people prefer to have an individual MEASNET wind tunnel test.

We have, however, calculated the "best-fit" data from this theoretical/nominal information for a range of our rotor and anemometer combinations (for best- fit over a wind speed range of 4 to 16m/s) so that if such a MEASNET test has not been done, we can offer "nominal" slope/offset figures. These figures can only be provided if the anemometer and rotor types and serial numbers can be specified, and the rotor has had a recent test in our wind tunnel. These theoretical/nominal slope/offset figures provide end results which correlate reasonably well with actual figures from wind tunnel calibrations.

Contact our Sales Office with the serial numbers of any anemometers/rotors for which you would like these calculated/theoretical slope/offset figures, and we will be happy to provide them (if we have "rotor calibration figure" data available for the rotors in question, and that data/calibration is not outdated).

*** Any calculated theoretical/nominal slope/offset figures which may be quoted by us (here or elsewhere) are provided "in good faith" and you use them at your own risk and we make no guarantees as to their correctness or suitability for your application, and you indemnify us of any liability. Your use of any such figures implies your acceptance of this condition.***

Calculations for a "nominal" A100LK or A100L2(with a rotor having a figure of 46.6 RPM per m/s) suggest that the "nominal" figures for the logger would be: slope=0.0499 offset=0.258 (for results in m/s, best fit over 4-16m/s)

Calculations for a "nominal" A100LM (with a rotor having a figure of 46.3 RPM per m/s) suggest that the "nominal" figures for the logger would be: slope=0.0968 offset=0.249 (for results in m/s, best fit over 4-16m/s)

Calculations for a "nominal" A100R/K with a "K" band rotor (with a figure of 46.6 RPM per m/s), the "nominal" figures for the logger would be: slope=1.250, offset=0.253 (for results in m/s, best fit over 4-16m/s). Note that an A100R/K has a rotor which is the same as that on an A100L2 or A100LK and has "optimum" cosine response, however it spins roughly 3% slower than the rotor on a "standard A100R".

Calculations for a "nominal" (Standard) A100R with a "nominal/standard" A100R rotor ("S" band, with a figure of 48.0 RPM per m/s), the "nominal" figures for the logger would be: slope=1.212, offset=0.252 (for results in m/s, best fit over 4-16m/s)

As a further alternative: "consensus" slope/offset figures could be used: Garrad Hassan recently published a report which quotes "consensus" slope/offset figures for A100L2/A100LK and "Standard A100R" instruments (derived from an analysis of a large number of MEASNET wind tunnel calibrations) which correlate quite closely with the calculated/theoretical values quoted above, particularly for the A100L2/A100LK:

GH "Consensus" figures for Standard A100L2 (or A100LK): slope = 0.0499, offset=0.2400

GH "Consensus" figures for "Standard A100R" (NOT A100R/K): slope = 1.2146, offset=0.2170

These figure are of course "nominal", and you should be aware that small variations exist between different anemometers/rotors due to the manufacturing process which can result in small differences in calibration between individual anemometers/rotors and the "nominal" values.

Some datalogger manufacturers may already include "preset" slope/offset values in their logger for our anemometers (based on calculated/nominal or "consensus" figures) - refer to your data logger manual or manufacturer for more information on how to use any such settings.

Once again, we remind you that for the best accuracy you should always obtain an individual multi-point wind speed calibration in a reputable wind tunnel (such as MEASNET) wherever the highest accuracy results are required.

More Information: FAQ: What is the difference between the "factory calibration" (RCD-10 data) and a "MEASNET calibration"

Windspeed Ltd (Vector Instruments)
Windspeed Limited (trading as Vector Instruments),
113 Marsh Road, RHYL, North Wales, LL18 2AB, United Kingdom
Tel: +44 (0) 1745 350700
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