A low-noise conductivity microstructure instrument

In rapid sampling of oceanic scalar microstructure, conductivity measurement offers advantages over temperature measurement in that conductivity probes can be designed to have fixed, free-flushing sample volumes, and their response is not limited by thermal diffusion times (an inherent problem with temperature sensors). Development of a "dual-needle" microstructure conductivity instrument is described, from its first use as a reference cell on a dynamic sensor calibration facility (salt-stratified tank) to more recent modifications and use in the open ocean. The cell used in the ocean had a centimeter-scale spatial resolution, compared with the millimeter-scale resolution achieved with the smaller version used in the stratified tank. An rms noise level of s/m was achieved over a 1 to 100 Hz bandwidth (which at constant salinity corresponds to an equivalent rms temperature noise of 10 microdegrees C).