Upgraded vector measuring current meter

Vector Measuring Current Meters (VMCM) have supported the work of many investigators from many institutions. The existing VMCM pool forms the heart of the moored measurement capability for investigators working near the surface in both the open ocean and coastal ocean. However, the existing units have data recorders and electronics that are no longer available or maintainable, and no acceptable alternative to the VMCM has been found. In 1974 the VMCM was compared with Aanderaa, Neil Brown ACM and a Savonius rotor instrument. The VMCM was shown to have the least error in relatively small mean flows when high frequency oscillatory flow was also present. Current testing of new acoustic technologies has not shown that these new technologies are ready to meet present scientific needs. The VMCM data acquisition and data storage capabilities have been upgraded while utilizing the existing orthogonal propeller sensor, pressure case and in-line cage assembly. A low-power design provides measurements of existing variables, including vector velocity and thermistor temperature, along with instrument tilt and real-time clock output. Tilt is useful for correction of post-processed data. In addition to this basic capability, the electronic interface and recording capability exists for optional variables, including pressure, PRT temperature, and conductivity (either frequency or RS485). All of these variables are recorded to a PCMCIA flash memory card of 4 to 40 megabytes size. This upgraded electronics (designated the "VMCM2"), together with previous improvements to VMCM propeller bearings and materials and improved anti-fouling coatings, enables the VMCMs to be deployed for up to one year. This paper reports on the design and testing program funded by the National Science Foundation (NSF). Fabrication of upgrade "kits" for all of the VMCMs in the research community is planned for 1999 with funding from NSF, the Office of Naval Research (ONR), and the United States Geological Survey (USGS).