Frequency Response Characterization of Current Meters

The dynamic characteristics of current meters are required to interpret data which includes waves and mooring line motion superimposed on slowly varying currents. Current meter outputs are analyzed as 2nd order differential equations. The coefficients of each ordered derivative characterize the instrument response. The zeroeth order coefficient is the static sensitivity and is prescribed by constant speed tow tests. The first order coefficient is related to the time constant which is prescribed by a rapid deceleration ramp test. The second order coefficients are related to the undamped natural frequency and the damping ratio and are determined from an analysis of pendulum oscillator tests. The test results yield and equation for the instrument behavior which may be used to determine the suitability of an instrument for a particular application and to correct for velocity amplitude attenuation and phase shift in the meter output. An electromagnetic and an acoustic travel time meter are examined.