Posted on Feb 4, 2014

The Mark III conductivity-temperature-depth (CTD) profiler has been the mainstay of modern physical oceanographic research1. The MKIIIB CTD provides high quality oceanographic data when used by skilled personnel and are frequently re-calibrated. The design objective of the Integrated CTD system was to attain the same high level of performance whil

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e reducing the necessity of frequent re-calibration through the enhancement of long term stability. This required re-consideration of both the electronic approach and the re-design of the physical sensors. The result of this work is a CTD system with improved measurement precision. All three primary sensors are newly designed to achieve long term measurement stability and to optimize system sampling performance without the limitations of existing technologies. Advances in the state of the art in electronic and microcontroller technologies has enabled the development of improved methods of analog sensor signal processing. During the MKIII development, micro-processors and their software development tools were primitive and difficult to use. The revolution in high speed micro-controllers has allowed their use in the Integrated CTD allowing for real time numerical correction for drift in the analog signal processing circuitry. The key component of this design is a phase shift oscillator as described by Brown 19682. Through the addition of a precision reference network and real-time numerical correction the performance of the oscillator circuit has been enhanced. This requires that the oscillator output frequency be stable for short periods of time over which it is recalibrated against the precision resistance reference network. The network accurately simulates the output of the sensors for known values of the measured...

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