12 



Analysis of the 



ANEMOMETER WITH CONSTANT ERROR 



For a bearing of known frictional torque the curves h and k which determine the calibration 

 which would result by the use of various diameter cups and arm lengths have been obtained. 



In any practical anemometer the indicating scale must be connected to the cup wheel by 

 some mechanical means and the indicated velocity, Vi is given by Vi = Fv where F is some gear 

 train factor. It is now well known that, with a constant gear train factor F, it is impossible to 

 have Vi=V except at the one point where the line V/v = F cuts the hyperbola (V/v — h) 

 (V/v -l) = k. 



The procedure up to the present has been to choose F in a rather arbitrary fashion so that 

 it fitted fairly well over the range of speeds to be measured. Thus at low speeds V, would be too 

 low, the error decreasing up to the point V/v — F and after that Vi would become too high and a 

 variable correction would be applied depending on Vi. This is extremely cumbersome and it will 

 be shown that, by suitably designing the anemometer, the error can be reduced to an additive 

 constant over the entire range of velocities. 



Consider the equation 



(V/v - h)(V/V - i) = k. 



If we arrange h = F (the gear train factor), then 



Vi = hv 

 and 



V 



Vi + V ± ^{Vi + V y- - 4 (A - k)vV 



