Kestrel Instruments Support & FAQs

Why is the impeller of my Kestrel Meter NOT balanced? Why does one blade remain down no matter how I rotate the rest of the instrument?

The Kestrel impeller IS balanced, and to a very high degree. It APPEARS to be imbalanced because of the tiny magnet incased within the impeller. The force acting on your impeller blade is magnetic, not gravitational. If you want to test this out, simply hold your Kestrel horizontally, and then rotate the entire instrument in the horizontal plane. PIC You will see that the impeller does not rotate. It acts like a compass and remains stationary. This is how it SHOULD act when properly operating.

Looking at the issue more technically, there are no known effects from either the weak or strong forces. The earth’s magnetic field has a vertical component everywhere but the equator, and this component creates a torque on the magnet mounted on the impeller shaft. Although the earth’s field does provide a weak aligning force, which causes the impeller to oscillate as it comes to a stop, there is NO effect on the accuracy of the windspeed reading. This is because the force acting on the impeller is “conservative” – it has no effect on the rotation speed. The conservative force causes acceleration of the impeller in equal and opposite measure to the deceleration every time the impeller completes an entire rotation.

Rest Assure that any new impeller from NK has been calibrated and tested to be well within the specified accuracy of plus/minus 3%. The impeller needs NO ADJUSTMENT. If your Kestrel® application involves hours of intense high-speed use, you may want to consider purchasing a replacement impeller ($15.00) once a year to ensure that your Kestrel maintains factory specified accuracy.