TR No. 22 
INSTRUMENT ATION 
The ducted impeller oceanographic current meter (figures 1 and 2) 
consists of a six-bladed impeller axially mounted in the center of a 
brass cylinder approximately 8.5 cm in diameter and 15 cm long. The 
impeller is manufactured of micarta (laminated phenol formaldehyde). 
The impeller shaft is terminated at either end with carbide pins 
which rest in quartz V-bearings mounted in neoprene; it is supported 
at either end by three struts spaced 120 degrees apart. A miniature 
Magnet (weighing around 5 grams) is imbedded in the tip of each blade, 
and a coil is potted with epoxy resin in a housing mounted externally 
on the cylinder. 
In operation, the instrument is aligned with the water flow which, 
impinging on the blades of the impeller, is defiected with a resultant 
force exerted on the blade surface causing the impeller to rotate. When 
a constant angular velocity has been achieved, the angular velocity is 
directly proportional to the water current over the specified linear 
operating range of the instrument; the constant of proportionality is the 
calibration coefficient, k, for the current meter. The rotation of the 
impeller, and consequently the passage of the magnets in the tip of each 
blade past the coil, induces a series of voltage pulses which are trans-= 
mitted through a two-conductor waterproof cable to appropriate recording 
instrumentation. The frequency of the pulses generated thus becomes a 
measure of the water velocity. The waveform obtained from the current 
meter is shown in figures 3 and 1, 
Calibration 
The current meter was calibrated in a water tank by towing the 
instrument at various known, constant velocities and measuring the fre- 
quency of the pulses generated. For the calibration, the axis of the 
current meter was aligned with the towing direction. The calibration 
curve is shown in figure 5, from which the calibration coefficient, the 
slope of the calibration curve in the linear range, was determined as 
3.12 cm. Thus, 
(LC) (me Pas) 2 SD (red ee BS cm ie (4) 
