In a search of stability
The NBIS CTD-systems provided high-quality oceanographic data when used by skilled personnel and are frequently re-calibrated at sea with using the bottle samples and a laboratory salinometer (“the golden age of salinometry!”). For Neil Brown’s future development at the WHOI, the design objective of the Integrated CTD system (ICTD) was to attain the same high level of performance while reducing the necessity of frequent re-calibration through the enhancement of long term stability. That required re-consideration of both the electronic approach and the re-design of the physical sensors. The result of his work in collaboration with Falmouth Scientific, Inc (FSI) was the ICTD 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.
A serious limitation to the long term stability of NBIS CTD’s was the use of electrode type conductivity sensors. Since the electrodes must be exposed directly to seawater they cannot be reliably protected from marine fouling. The small dimensions of these cells make them particularly vulnerable to even minute amounts of fouling. Earlier applications of inductively-coupled conductivity sensors in STD systems were not particularly successful for a number of reasons as follows:
1. The toroidal transformers in these sensors had to be pressure-protected (Brown 1968) to eliminate the effects of pressure on the electrical parameters of the transformers. The required electrically insulated pressure housing dramatically increased the thermal mass of the sensor, which in turn resulted in substantial thermal contamination of the seawater being measured. The pressure housing also resulted in a small hole through the center which restricted the seawater path, thus reducing the sensitivity and the sensors signal to noise ratio.
2. The major problem with these sensors was the instability of the voltage ratios caused by the combination of the finite electrical resistance of the transformers’ single windings and the variability of the inductance of these windings. These variations are unpredictable and are influenced by pressure and temperature effects as well as previous magnetic history (magnetic hysteresis) of the magnetic core. The current induced in the seawater circuit is directly proportional to the product of conductivity and the voltage induced in the seawater. Hence changes in the voltage ratio were indistinguishable from changes in seawater conductivity.
The key component of this design is a phase shift oscillator as described by Brown in 1968. 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. The above graph shows the detailed implementation of the conductivity circuit in the ICTD. The circuit consists of two basic sections. The first section is the phase shift oscillator which acts as the signal generator for the drive winding of the input toroid of the inductively coupled sensor. The second section is the current balancing circuit which provides a current through the balance winding of the output toroid that exactly balances the current induced in the seawater circuit. This balance current is passed through a precision resistor (Rs) thus generating a voltage (Es) that is exactly proportional to conductivity. This sensor voltage (Es) is added to the quadrature voltage (Eq) of the phase shift oscillator in the same way as was done for the temperature circuit.
Extreme accuracy was achieved by the use of a simple network of ultra-stable precision resistors that simulate the output of the transducer at values of the sensed parameter which are determined at the time of the lab calibration. This simulation network is periodically used in conjunction with the microprocessor to calibrate the oscillator thus eliminating the effect of calibration drift due to temperature, ageing or supply voltage variations on the electronics.
Features of ICTD:
- 32Hz sampling rate
- High-Accuracy measurement
- 7000m titanium housing
- Optional 4MB internal memory
- 8 A/D and RS-232 serial inputs for a variety of external sensors
- Water Sampler Control