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.

     Foundation of electronics in Oceanography

Reading Thermometers onboard HMS”Challenger”

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 an electrical thermometer aboard the HMS “Challenger”.

Sir William Siemens (1823-1883)

It was a gift from Sir William Siemens, a complete deep-sea electrical thermometric system, which he designed on the principle of the variation of the electrical resistance of a conductor with its temperature. In the Bakerian Lecture for 1871, 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? This is a mystery. This system was not lacking a vigorous protagonist, Dr. Seimens, a scientist and engineer of great wealth and ingenuity. Since 1850 he established the London sales office of Siemens & Halske, the famous engineering company producing telegraphs, which his brother Werner had founded in 1847 at Berlin.

Sir Siemens’ Electrical Thermometer (form the Challenger Report)

Further, 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 there was something missing in this development. Perhaps something was found to be incompatible with some feature of the sea or maybe it was just poor cable insulation. I would rater blame the conservativeness of Challenger’s naturalists, who were not familiar with electronics 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).

During the autumn of 1881 Siemen’s electrical thermometer was successfully tested onboard U.S.S “Blake” by Commander Bartlett, to show its entire practicability.

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