considerable phase lag and much scatter depending on flow direction for 

 10- to 20-second swell. It would seem that this type is also readily 

 fouled by kelp or seaweed. 



Electromagnetic (EM) current meters are based on inductance prin- 

 ciples. They are currently the most popular type since first being 

 introduced for surf zone measurements by Thornton (1969). A magnetic 

 field (a.c.) is created by an electromagnet within the probe. Electrodes 

 on the probe measure the voltage induced by the conductor (moving 

 seawater) to generate the output signal. A popular brand is the two- 

 axis, Marsh-McBirney EM current meter probe and electronics package. 

 The Model 512 OEM has a 4-centimeter-diameter spherical probe with four 

 middiameter electrodes, 90° apart and protruding 0.5 centimeter. The 

 EM meter is rugged, has good cosine response, a wide frequency band 

 width, and offers less chance for fouling than the propeller-type meter. 

 During recent extensive use in the NSTS experiments at Torrey Pines, 

 California (Gable, 1979), 22 EM meters withstood forces large enough to 

 bend one support rod but not damage the probe. Entrapment of kelp and 

 seaweed will alter the calibration, so EM probes must be frequently 

 inspected and cleared of debris and marine fouling attachments. 



Calibration problems have recently been reported for the EM probe 

 under both steady-state and oscillatory-flow regimes. LaVelle, et al. 

 (1978) ■'•^ showed the meter to have a transition (change in slope) in 

 steady flow (d.c. gain) at about 80 centimeters per second*. This could 

 possibly be due to nonlinear boundary layer effects including flow sepa- 

 ration as suggested by Cunningham, Guza, and Lowe (1979) and also by 

 Guza and Thornton (1980). Aubrey (NSTS Workshop, Scrlpps Institute of 

 Oceanography, La Jolla, California, personal communication, February, 1981) 

 found. a break in the steady-flow calibration at 60 centimeters per second 

 and attributed it to the electrodes jutting out from the sphere to disturb 

 the boundary layer. This meter has a nominal output d.c, gain of 1 volt 

 at 1 meter per second and calibration is usually made by towing in still 

 water at this speed. Calibration of the 22 meters used in the NSTS 

 experiment at Torrey Pines, California, made both before and after the 

 experiment, showed less than 5 percent variation in d.c. gain for most 

 meters (Gable, 1979). 



But such meters deployed in the surf zone are subject to oscillatory 

 motions over a wide range of frequencies and should be dynamically cali- 

 brated. For this purpose, the Shore Processes Laboratory of the Scripps 

 Institute of Oceanography at La Jolla, California, has recently 

 developed a hydraulically driven mechanical calibration device. A 

 detailed description of the equipment can be found in Cunningham, Guza, 

 and Lowe (1979) . A horizontal arm with rack gear is driven back and 

 forth by a hydraulic servo-controlled motor. The probe is rigidly posi- 

 tioned beneath the arm near its midpoint and kept about 20 centimeters 

 below the water surface. Various types of arm motions were used to study 



LAVELLE, J.W. , et al. , "Near Bottom Sediment Concentration and Fluid 

 Velocity Measurements on the Inner Continental Shelf, New York," Journal 

 of Geophysical Research^ Vol. 82, No. C12, 1979 (not in bibliography). 



56 



