d. Current Meters . Two Marsh-McBirney (MMI) 551 electromagnetic current 

 meters are deployed at the FRF, one at station 19+40 under the seaward end of 

 the pier and the other at approximately the same distance from shore but 500 

 feet north of the pier (Fig. 2). The meter under the pier is attached to the 

 T-bracket shown in Figure 8; the other meter installation is shown in Figure 9. 

 The MMI current meters are solid-state instruments with no moving parts. The 

 meter's operation is based on Faraday's principle of electromagnetic induction, 

 i.e., a conductor moving through a magnetic field will produce a voltage pro- 

 portional to the speed of the conductor. The probe for the meter consists of 



a 4-inch-diameter sphere which contains an electromagnet and two pairs of elec- 

 trodes . The two orthogonal electrode pairs sense the vector component voltages 

 induced when the water moves through the magnetic field produced by the elec- 

 tromagnet. The instrument range is ±10 feet per second with an accuracy of ±2 

 percent or ±0.07 foot per second and a 0.2-second time constant; zero drift is 

 less than 0.07 foot per second. Additional details on the MMI 551 meters are 

 in Marsh-McBirney, Inc. (1978). 



e. Conductivity and Temperature Meters . Two Hydrolab Model TC-2 conduc- 

 tivity probes (Fig. 10) and two Hydrolab Model T-4 temperature probes (Fig. 11) 

 are located under the seaward end of the pier at station I960. One conduc- 

 tivity and one temperature probe is 10 feet below MSL; the other pair is 22 

 feet below MSL. The TC-2 determines the total concentration of ions in solu- 

 tion using the four-electrode method of conductivity measurement. The method 

 consists of separating the electrodes that supply the current in the conduc- 

 tivity cell from the electrodes that measure the voltage produced in the 

 solution by passage of the current through the cell. This technique eliminates 

 errors due to cable resistance and electrode polarization, and diminishes the 

 effects of electrode fouling (Garner, 1972). Measurements of conductivity also 

 vary with temperature; the TC-2 instruments are electronically corrected to 25° 

 Celsius for temperature variations. Water temperature is measured by the TC-2 

 using a precision thermistor temperature sensor contained in the conductivity 

 probe. The conductivity output data signal is stated to be accurate to within 

 2.5 percent of the reading and the temperature readings to within ±0.5° Celsius. 

 Mounted a few inches above the TC-2 probes are T-4 Marine Hydrographic ther- 

 mometers which have a range from -2° to 49° Celsius. The conductivity and tem- 

 perature values are obtained by reading Hydrolab meters located in a stainless- 

 steel box on the pier deck (Fig. 12). (Additional details on the conductivity 

 and temperature probes may be obtained from the Hydrolab Corporation, Austin, 

 Texas.) 



f. Tide Gages . Three NOS tide stations consisting of four gages are in- 

 stalled at the FRF. The NOS control station (Fig. 13) at the seaward end of 

 the research pier (Fig. 2) consists of a Leupold-Stevens gage (Fig. 14; manu- 

 factured by Leupold and Stevens, Inc., Beaverton, Oregon) and a Bristol pressure 

 gage (Fig 15; manufactured by the Bristol Company, Waterbury, Connecticut). 



The second tide station (located at station 7+20) has a Fischer-Porter tide gage 

 (Fig. 16; manufactured by Fischer and Porter Company, Warminster, Pennsylvania). 

 The third station (located about 1,000 feet west of the shore in the Currituck 

 Sound) has a Metercraft pressure tide gage (Fig. 17; manufactured by Metercraft 

 Corporation, Perry Hall, Maryland). 



Both the Leupold-Stevens and Fischer-Porter analog-to-digital recorders 

 (ADR) are float-activated, negator-spring, counterpoised instruments that me- 

 chanically convert the vertical motion of a float into a coded, punched 



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