TR No. 22 
Sensitivity 
The lowest water velocity sufficient to maintain a constant angular 
velocity of the impeller is of the order of 5 to 7 em-sec™+, No measure-~ 
ments were made to determine the sensitvity of the current meter as a 
function of velocity, but typical commercially available turbine flow 
meters have sensitivities equal to +0.25% or less of the mean velocity. 
If the performance of the ducted impeller current meter is assumed equal 
to that of commercial flow meters, it has a sensitivity of f1 cm-sec~+ 
Output 
From the calibration coefficient, the distance required for the 
current meter to advance relative to the water in order for the impeller to 
complete one rotation is 
21K = (6.28)(3.12 cm) = 19.61 cm, 
The output of the current meter is six pulses per rotation or 6 pulses/19.61 cm = 
0.306 pulses per cm advance. In practice the output of the current meter was 
modified using a Schmidt trigger-binomial counter circuit in a divide-by-six 
mode to obtain one pulse instead of six per rotation of the impeller. This was 
found necessary because of the approximately 410% variation in angular spacing 
between adjacent impeller blades, which otherwise would have resulted in a noise 
level (measurable) corresponding to variations in velocity t40 cm=sec"+, The 
practical output of the current meter is 1/19.61 cm = 0.051 pulses per cm 
advance. 
The recorded data consists of successive periods per rotation of the impeller; 
corresponding values of the water velocity can be computed using the calibration 
coefficient: 
hi ae 
Uy = ae Sy ean Ueki eS (10) 
The term u; is the average value of the instantaneous velocity u(x) over the 
interval of time T;. Since a mean velocity is superimposed on the turbulent 
