|
A vector measuring current meter (VMCM) is an instrument used for obtaining measurements of horizontal velocity in the upper ocean, which exploits two orthogonal cosine response propeller sensors that directly measure the components of horizontal velocity.〔Weller, R. A., and R. E. Davis, 1980: (A vector measuring current meter ). Deep-Sea Res., 27A, 565–582.〕 VMCM was developed in the late 1970s by Drs. Robert Weller and Russ Davis and commercially produced by EG&G Sealink System (currently EdgeTech). The instrument has the capability of one year long deployment at depths of up to 5000 m. Both laboratory and field test results show that the VMCM is capable of making accurate measurements of horizontal velocity in the upper ocean. The VMCM is the current standard for making high quality velocity measurements in near-surface regions 〔Way, B. S., D. S. Hosom, J. D. Ware, R. P. Trask, and G. P. Allsup, 1996: (Vector Measuring Current Meter (VMCM) Upgrade ), Woods Hole Oceanographic Institution, Department of Physical Oceanography〕 and it has been used for benchmarking other current meters.〔Dickey, T. D., A. J. Plueddemann, and R. A. Weller, 1998: (Current and water property measurements in the coastal ocean ). ''The Sea'', KH Brink and AR Robinson, Eds., Vol. 10, John Wiley and Sons, 367–398〕〔Gilboy, T. P., T. D. Dickey, D. E. Sigurdson, X. Yu, and D. Manov, 2000: (An Intercomparison of Current Measurements Using a Vector Measuring Current Meter, an Acoustic Doppler Current Profiler, and a Recently Developed Acoustic Current Meter )〕 ==Equipment== The main components of a VMCM are its two orthogonal cosine response propeller sensors, that directly measure the components of horizontal velocity parallel to their axes. The orientation of the instrument with respect to magnetic north is sensed with a flux-gate compass, which permits to evaluate the direction of flux, providing the angle of the Y axis with respect to the magnetic North. A microprocessor rotates the X-Y coordinates in the conventional East-West and North-South components of velocity. This is done once each sample interval and, at the end of the record interval, the conventional components of velocity are averaged and the averages are stored on a cassette magnetic tape. Other components of the system are a bearing retainer, an end cap, an outer bearing race, a ball retainer and bearing balls, an encoder and an epoxy or Noryl plastic disk with four magnets, pressure window, an aluminum disk, two magnetodiodes mounted asymmetrically on a printed circuit ring, a hub, and a shaft with inner races machined in it. The function of the magnetodiodes is detecting the rotation of the propeller sensors. Incorporated in the system there is the vector averaging electronics, that uses the pulses from the magnetodiodes and the instrument heading from the flux-gate compass to calculate and record the velocity components. In the 1990s, Way et al.〔 upgraded the electronics by redesigning the vector measuring circuitry, data acquisition, and storage components and retaining instead the propeller sensors assembly, which proved to be reliable in the several tests accomplished.〔〔Halpern, D., R. A. Weller, M. G. Briscoe, R. E. Davis, and J. R. McCullough, 1981: Intercomparison tests of moored current measurements in the upper ocean, ''Journal of Geophysical Research'' 86, No. C1, pages 419–428〕 A pressure case houses the electronics and the appendage on which the propellers are mounted on. In its first design of the late 1970s, a VMCM was approximately 2.56 m high and had a mass of 34.5 kg in air. The original VMCM is no longer commercially available from EG&G (currently EdgeTech). The 1970s electronics components are outdated and difficult, if not impossible, to find. Like many of the electronic components the original flux gate compass is no longer available.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Vector measuring current meter」の詳細全文を読む スポンサード リンク
|