Impellor 



O 



Q 

 Cd 

 td 

 CU 



CO 



3: 



O 



Impellor meters, used widely by industry, are employed by oceanographers 

 in a configuration consisting of an axial rotor mounted on bearings inside a length 

 of tubing with circular cross section. On the rotor are equally spaced blades 

 placed at some angle to the pipe axis. The pipe gives the meter a degree of 

 directionality. The angular velocity of the rotor is a function of the current 

 velocity. This angular velocity of the rotor is measured indirectly by photoelec- 

 tric or magnetic pickups mounted on the pipe to sense the passage of the blades. 

 The number of pulses per unit time obtained from such pickups are converted to 

 rotor velocity. 



The dynamic response for commercially available impellor meters is not 

 given, but with low-mass rotors and low-entrained water mass, the response is 

 sufficient for the required oceanographic conditions.^ 



A typical steady-state calibration curve furnished by Marine Advisers 

 (fig. 4) is shown for their Model B-7C ducted current meter. Its specifications 

 will be used for some calculations in order to determine if this type of meter is 

 acceptable for the intended use. In order to depict the nature of the nonlinear 

 output, figure 4 was replotted in figure 5, using linear abscissa and ordinate 

 scales. In the region of interest (below 1 knot), the nonlinearity is due mainly 

 to a threshold. Between the threshold velocity and 1 knot the output is nearly 

 linear. 



0.1 0.2 0.5 1 2 5 10 20 50 100 200 



METER OUTPUT (PULSES/SECOND) 



Figure 4. Calibration curve, Marine Advisers Inc. ducted meter model B-7C (five-bladed 

 impeller); calibration date: 27 February 1963. 



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