ictd92

Description: The Mark III conductivity-temperature-depth (CTD) profiler has been an essential tool in modern physical oceanographic research. The MKIIIB CTD delivers high-quality oceanographic data when operated by skilled personnel and is frequently recalibrated. The Integrated CTD system was designed to achieve the same high performance level while reducing the need for frequent recalibration by enhancing long-term stability. This necessitated a reevaluation of both the electronic approach and the redesign of the physical sensors. The outcome is a CTD system with improved measurement precision. All three primary sensors have been newly designed to achieve long-term measurement stability and optimize system sampling performance without the constraints of existing technologies. Advances in electronic and microcontroller technologies have facilitated the development of improved methods for analog sensor signal processing. During the MKIII development, microprocessors and their software tools were rudimentary and challenging to utilize. The advent of high-speed microcontrollers has enabled their implementation in the Integrated CTD, allowing for real-time numerical correction for drift in the analog signal processing circuitry. A key component of this design is a phase shift oscillator, as described by Brown in 1968. The addition of a precision reference network and real-time numerical correction has enhanced the performance of the oscillator circuit. This requires the oscillator output frequency to be stable for short durations during 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 parameters. The internal microprocessor is then employed to mathematically correct for drift in the electronics. Consideration was given to using a Wien Bridge Oscillator. However, this technique is only applicable to two-terminal high resistance devices, such as thermistors or high resistance two-electrode conductivity cells, and cannot be calibrated against reference resistors due to errors introduced by the unknown resistance of the required electronic switches. Additionally, it is incompatible with four-terminal devices such as strain gauge bridges, inductively coupled conductivity sensors, or low output sensors like platinum resistance thermometers. Calibration in situ is not feasible, necessitating the selection of electronic components with extremely low temperature coefficients and long-term drift. Despite these precautions, achieving the necessary long-term stability in electronics is nearly impossible, particularly in frequency-determining capacitors. Consequently, Wien Bridge technology was dismissed. The design incorporates a unique phase shift oscillator capable of converting the output of a four-terminal network, such as a temperature bridge or strain gauge bridge, into frequency with high linearity and sensitivity. It can also be configured to work with a complex network of sensors and active electronics, such as the conductivity circuit described. For instance, it can be configured to produce a 50% change in frequency for a 1% change in one arm of an equal-arm bridge, making it ideal for low output devices like platinum thermometers and strain gauge pressure sensors. Extreme accuracy is achieved through a simple network of ultra-stable precision resistors that simulate the output of the sensor network at three precisely known values of the sensed parameter, determined by calibration. This simulation network is periodically utilized (once per second) in conjunction with the microprocessor to calibrate the oscillator, thereby eliminating the effects of calibration drift due to temperature variations.

The Mark III CTD profiler represents a significant advancement in oceanographic measurement technology. The system's design focuses on enhancing measurement precision and stability over extended periods, addressing the challenges faced by previous models. The integration of a phase shift oscillator is pivotal, as it allows for high linearity and sensitivity in converting sensor outputs to frequency, enabling accurate readings from various sensor types. This versatility is particularly beneficial for applications requiring precise measurements from low-output devices, where traditional methods may falter.

The calibration process, which occurs once per second, ensures that the system maintains accuracy amidst environmental changes. The use of ultra-stable precision resistors in the simulation network is crucial for maintaining the integrity of the measurements, as it allows for real-time adjustments based on known reference points. This innovative approach minimizes the drift typically associated with electronic components, setting a new standard for long-term stability in oceanographic instruments.

In conclusion, the Mark III CTD profiler exemplifies the successful integration of advanced electronic design and sensor technology, resulting in a robust system capable of delivering reliable and precise oceanographic data essential for ongoing research and exploration.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 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 parameters.

The internal microprocessor is then used to mathematically correct for drift in the electronics. Consideration was given to the use of a Wein Bridge Oscillator3. This technique can only be used with two terminal high resistance devices, such as Thermistor or high resistance two electrode conductivity cells (i. e. , cells which have relatively small inside diameter and long length); and cannot be calibrated against reference resistors due to the error introduced by the unknown resistance of the required electronic switches.

Additionally, it cannot be used with four terminal devices such as strain gage bridges, inductively-coupled conductivity sensors or low output sensors, such as platinum resistance thermometers. Since it cannot be calibrated in situ the electronic components must be selected to have extremely low temperature coefficients and extremely low long term drift.

Even with all these steps the requisite long term stability in the electronics is almost impossible to achieve particularly in the frequency determining capacitors. For these reasons Wien Bridge technology was dismissed. The design utilizes a unique phase shift oscillator that has the ability to convert the output of a 4-terminal network such as a temperature bridge or strain gage bridge to frequency with a high degree of linearity and sensitivity.

It can also be used with a complex network of sensors and active electronics such as the conductivity circuit described below. For example it can be readily configured to produce a 50% change in frequency for a 1% change of one arm of an equal arm bridge thus making it ideal for use with low output devices such as platinum thermometers and strain gage pressure sensors.

Extreme accuracy is achieved by the use of a simple network of ultra stable precision resistors which simulate the output of the sensor network at 3 precisely known values of the sensed parameter, which are determined by calibration. This simulation network is periodically used (once per second) in conjunction with the microprocessor to calibrate the oscillator thus eliminating the effect of calibration drift due to tempe