Development of CTD-systems


             The primary tool for a physical oceanographer is an instrument for measuring electrical conductivity, temperature, and pressure, which historically called a CTD-system (as the abbreviation of Conductivity, Temperature, Depth), also being named at beginning a “submersible salinometer” or an STD. The output parameters of a CTD-system are used to calculate the practical salinity and density of seawater using the Practical Salinity Scale 1978 and the Thermodynamic Equation of State 2010 of seawater, based on these three measured physical parameters.

     Foundation of electronics in Oceanography

Reading mercury thermometers onboard HMS”Challenger”

How did it start? When electricians come with a bright idea to  electronize the oceanographic measurements? The answer lies deep in the ocean with the first attempts to lie down the telegraph cable between Europe and America in the middle of the 19th century.  Electricians asked oceanographers why their cables break, why the resistance of cables changes at depth, and oceanographers could not give a complete picture of what affects the cables, what generally happens deep on the ocean floor. This fact is not very well known, and it is a particularly interesting story for finding roots in the development of electronic devices in Oceanography.   During the famous Challenger Expedition 1872-1876, which is noted as the beginning of Oceanography as a Science, there was a request from the Telegraph companies and investors to light up the dark secrets of the ocean and provide them with a detailed map of depths and temperature of the oceans. To fulfill their request for more detailed temperature profiles, they even provided a special thermometer aboard the HMS “Challenger”.

Sir William Siemens (1823-1883)

It was a complete deep-sea electrical thermometric system, a gift from Sir William Siemens, which he designed on the principle of the variation of the electrical resistance of a conductor with its temperature. Sir Charles William (Carl Wilhelm) Siemens was a great German-British electrical engineer and businessman. In 1850 he established the London sales office of Siemens & Halske, the engineering company producing telegraphs, which his brother Werner had founded in 1847 at Berlin. He started selling such devices to the wire rope producer Newall&Co, which soon  outsourced test jobs for cables to Siemens and such enabled the new company to enter the ocean cable-laying business.  Siemens pursued two major themes in his inventive efforts, one based upon the science of heat, the other based upon the science of electricity; and the electric thermometer was, as it were, a delicate cross-coupling which connected both. In the Bakerian Lecture for 1871 “On the Increase of Electrical Resistance in Conductors with Rise of Temperature”, he showed that this principle might be applied to the construction of an electrical thermometer, which would be of use in cases where a mercurial thermometer was not available. He devised an instrument for measurement temperatures where the greatest degree of accuracy is required, as in the case of deep-sea observations.

      It employed a simple bridge circuit with null indicated by a marine galvanometer of the type invented for cable laying by Sir William Thompson some twenty years earlier. The measuring transducer, a 15mm diameter coil of silk-covered metal wire with 432 Ohm resistance provided with three leads to compensate for cable resistance. The electrical balance was attained by a method truly beautiful for its certainty and simplicity. The reference coil (S) was merely put into a water bath along with hot and cold water in the proportions required to balance the galvanometer (G).

Siemens Thermometer’s Sinker and Resistance Coil

A good mercury-in-glass thermometer then indicated the reference temperature for calibration of the measuring system. It was an elegant experimental method limited hardly at all as to precision, certainly not by “contact resistance”. As it was stated in the Challenger Report:  “Only the briefest of tests of Sir Siemens ‘ gift was carried out during the expedition even though it apparently was capable of very fine measurements. Its design was sensible and conservative”

Why did this prototype not start a tradition long ago, leading to routine electrical wire hydrographic casting? 

Sir Siemens’ Electrical Thermometer (form the Challenger Report)

The “electricians” were at that time quite prominent in natural philosophy and were the ones who first floundered out to sea with machinery to lay cables. No doubt something missing in this story. Perhaps something was found to be incompatible with some feature of the sea or maybe it was just poor cable insulation. I would also suspect the lack of practical expiernce  with electrical devices of Challenger’s Lieutenants.  They were in charge for temperature measurement in the expedition, but were not familiar with handling electric circuitry and had no personal scientific interest in testing Seimens’ thermometer, rather put effort into extensive testing glass-mercury reversible thermometers, widely used in the expedition.    “No permanent place was fitted for galvanometer or apparatus, and in consequence continuous and careful observations were not made”(Challenger Report).  It is hard to tell why it happened because the records of mild successes soon become dim for many years ahead. ” When accurate temperature observations are required from the intermediate depths, this instrument is especially valuable, and it will in all probability be intensively used in future deep-sea investigations.” (Challenger Report). 

Later, the Siemens brothers developed a generator to replace the original batteries

    During the autumn of 1881 Siemen’s electrical thermometer was successfully tested in the Gulf Stream onboard U.S.S “Blake” by Commander Bartlett, to show its entire practicability. A series of observations were taken using Miller-Casella thermometers immidiately proior to the Siemens’ measurements. Five minutes were allowed  for the coil to assume the ambient temperature, and after ballancing the bridge, another 5 minutes were allowed to elapse to insure the stability of both coils. In majority of cases the results were identical to those made by the Miller-Casella thermometers:


Commander Bartlett’s Report on comparison of Siemen’s electrical thermometer with Miller-Casella mercury thermometer