TR No. 22 
Additional tests were performed to determine the variation of the 
calibration coefficient with flow direction. For these tests the axis 
of the current meter was set at various known angles relative to the 
towing direction, and the frequency output was measured at known, con- 
stant velocities. The variation of k as a function of GC, the angle 
between the axis of the current meter and the towing direction, is 
shown in figure 6, which indicates that k is given very closely by 
Ix (&) =K(6) Gs 2 B12 2 Z2ec (5) 
The largest deviation occurred at values of @ near "7/2 and was probably 
caused by asymmetry in the mounting arrangement. Since the component of 
velocity 
= “ A A 
q = iu + jv + kw 
in the x direction (taken along the axis of the current meter) is 
u = [7 | cos B 5 
the current meter is sensitive to the component of velocity along the axis 
and insensitive to the components perpendicular to the axis. A second 
calibration of the current meter was obtained using a low speed wind tunnel 
(appendix A), The calibration curve is shown in figure 7. The slope of 
the straight line is the same as that obtained from the in-water calibration, 
but the straight line intercepts the U axis at 10 cm-sec™/ instead of 
passing through the origin. Since the measurements were performed at relatively 
low wind tunnel velocities, the difference is attributed to error in measuring 
the low velocities with a pitot static probe. The correct value of the cali- 
bration coefficient is assumed to be the in-water value. 
Response to Accelerated Flow 
The current meter has been used (Shonting, 8, 9, 15, 16) previously to 
make measurements of the particle motions in ocean waves. For those measure= 
ments the mean water velocity was zero or near zero. Under such conditions 
it was determined through wind tunnel and in-water tests (8, 22) that the 
response time of the current meter for a step function change in water velocity 
