Development of CTD-systems

       The main tool for a physical oceanographer is an instrument for measuring electrical conductivity, temperature, and pressure/depth (CTD), whose output parameters can be used to calculate the practical salinity and density of seawater using an accepted “equation of state” based on these three physical parameters.


Bruce Hamon (1917-2014)

Restrictions on collecting seawater samples and analyzing them with salinometers led to the development in 1948 of the first in situ system developed by A. W. Jacobsen, which worked for the Bristol Corporation in Waterbury, Connecticut. Its use was limited to 400 meters. This device used a carrier cable and another multi-core cable to get data. Although this system was very simple, it opened the way for new measurement methods in Oceanography.

In 1958, in Australia, at CSIRO Division of Fisheries and Oceanography Bruce Hamon (1917-2014) and Neil Brown (1927-2005), the ”fathers” of the modern CTD-systems, described a “Temperature-Chlorinity-Depth”  recorder, designed for use at depth to 1000 meters. The accuracy for temperature was within ± 0.15°C,  for chlorinity ± 0.03ppt and depth ± 20m. Chlorinity was measured using a conductivity cell with two electrodes and a circuit including thermistors and resistors to compensate for the effect of temperature on conductivity. A simple phase shift oscillator converts the information coming from each sensor into a frequency-modulated audio signal. This signal was transmitted to a ship, where it was converted to direct DC voltage and recorded on a strip-chart recorder.

A.Bradshaw(L) and N.Brown(R) are setting up the WHOI STD-system, c.1960

    In 1959, Neil Brown left Australia to settle in Woods Hole (MA, USA) with WHOI and work with Bradshaw and Schleicher to understand what could be done with a more accurate in situ measuring instrument. It used an inductive conductivity sensor and a sealed compensation cell filled with standard seawater. This cell was intended to accurately compensate for the simultaneous effects of temperature and pressure.

    It was a brilliant solution in the absence of relations between conductivity, temperature and pressure of seawater, implemented in the form of measurements of relative conductivity (Rt). Such a measuring system has a significant time constant due to slow temperature stabilization in the sealed reference cell, and its application requires special methodological solutions when conducting measurements in profiling mode. I know it from my own experience of developing a Microsalinometer MS-310 with a similar ratiometric principle of measurement. Unfortunately, WHOI did not recognize the uniqueness of this measurement system and work to improve it was abandoned. N.Brown returned to Australia in 1961.

STD system rocketing launch.

In 1962 Neil Brown returned to the United States to join a company called Hytech in San Diego, California, to continue the development work started at WHOI. Hytech was formed in 1960 by Don Cretzler and initially, it made bathythermographs, current meters, and a wave and tide monitor. These were amongst the first electronic ocean instruments.  Neil Brown joined the company to develop a laboratory salinometer (see Development of Salinometers).

Hytech STD

Soon after, he designed the electronic components that were
the key to making in-situ measurements of salinity enabling the first STD to be built in 1964. The following year the Navy contracted with Hytech and Santa Monica, CA based Bissett-Berman, who was one of the few companies at the time with computer technology. Together they made the first automated ocean data collection system that was used to collect large
volumes of data. As a result of this collaboration, Bissett-Berman acquired Hytech. In 1964, the Navy awarded them a contract to develop the first oceanographic sensors capable of continuous operation for up to one year in-situ. This was followed by other Navy contracts for specialized sensor systems. In 1969, the San Diego division moved into a 33,000 square foot purpose-built building in Kearny Mesa, CA. In 1970, Bissett-Berman was purchased by the British company Plessey Ltd. and the San Diego Division was renamed Plessey Environmental Systems. 

This instrument measured Conductivity, Temperature, Pressure (CTP), but because the instrument implemented the “salinity and depth equations” in complex adjustments of internal analog electronics, and these values of Salinity, Temperature, and Depth were transmitted to the surface, the instrument became known as ” STD”, or “S/T/D”.

Commercial of Model 9040 S/T/D System, ca.1976

The Plessey S/T/D systems were on the market from 1964 to 1980,  with popular Telemetering S/T/D 9040 and Self-recording S/T/D 9060.

Commercial of Model 9060, ca.1976

      At that time, computers and their peripherals were expensive, not very reliable, and difficult to use at sea. Since it was essential that salinity measurements were available immediately, the complex relationships between salinity, temperature, pressure, and conductivity had to be simulated using a “salinity bridge”. This bridge consisted of two platinum temperature sensors and three thermistors, two pressure sensors and six transformers, as well as an inductive conductivity cell and important electronic circuits. Its calibration required numerous and complex settings and adjustments.    

Schematic of STD showing available sensors

 The STD’s accuracy was not adequate for depths much greater than 1,000 meters, and there was a severe “spiking” in the salinity data, due to the slow response of the temperature sensor relative to the conductivity sensors. Also, its accuracy was limited by systematic errors in the salinity bridge emulation of the salinity, pressure, temperature, and conductivity relationships.

     Two aspects of the inductive conductivity sensor have limited its commercial applications to the measurement of Oceanographic salinity and density. First, the large thermal mass of the sensor relative to the volume of water that passed through it from the inside, which led to a thermal error in the measured conductivity. The electrical conductivity of a liquid depends strongly on the temperature at which it is measured. If, due to the presence of the thermal mass of the sensor, the conductivity sensor heats or cools the volume of water it is trying to measure, this will lead to inaccuracies in the calculated salinity/density.

Plessey 9040 STD with bottle samplers and frames for reversing thermometers.

Since Oceanographic sensors are often used at significant depths where the surrounding water pressure can reach 700 bar, the magnetic cores of the sensors were installed in a pressure-proof metal housing. Then the pressure protection housing had to be covered with a dielectric material to prevent the metal housing from short-circuiting around the sensor.

      Second, the large non-contact external field can be affected by other structures associated with the CTD device, such as auxiliary sensors, protective frames, etc. These external structures change the free current flowing through seawater. Consequently, their effect cannot be distinguished from changes in the conductivity of seawater, and they introduce a mixing variable.