Chapter 6 - Temperature, Salinity, and Density

Chapter 6 Contents

6.9 Measurement of Temperature and Salinity with Depth

Temperature, salinity, and pressure are measured as a function of depth using various instruments or techniques, and density is calculated from the measurements.

Bathythermograph (BT)
A mechanical device that measured temperature vs depth on a smoked glass slide. The device was widely used to map the thermal structure of the upper ocean, including the depth of the mixed layer before being replaced by the expendable bathythermograph in the 1970s.

Expendable Bathythermograph (XBT)
An electronic device that measures temperature vs depth using a thermistor on a free-falling streamlined weight. The thermistor is connected to an ohm-meter on the ship by a thin copper wire that is spooled out from the sinking weight and from the moving ship. The XBT is now the most widely used instrument for measuring the thermal structure of the upper ocean. Approximately 65,000 are used each year.

The streamlined weight falls through the water at a constant velocity. So depth can be calculated from fall time with an accuracy of 2%. Temperature accuracy is 0.1°C. And, vertical resolution is typically 65 cm. Probes reach to depths of 200 m to 1830 m depending on model.

Nansen Bottles
(Figure 6.16) Deployed from ships stopped at hydrographic stations. Hydrographic stations are places where oceanographers measure water properties from the surface to some depth, or to the bottom, using instruments lowered from a ship. Usually 20 bottles were attached at intervals of a few tens to hundreds of meters to a wire lowered over the side of the ship. The distribution with depth was selected so that most bottles are in the upper layers of the water column where the rate of change of temperature in the vertical is greatest. A protected reversing thermometer for measuring temperature was attached to each bottle along with an unprotected reversing thermometer for measuring depth. The bottle contains a tube with valves on each end to collect sea water at depth. Salinity was determined by laboratory analysis of water sample collected at depth.

After bottles had been attached to the wire and all had been lowered to their selected depths, a lead weight was dropped down the wire. The weight tripped a mechanism on each bottle, and the bottle flipped over, reversing the thermometers, shutting the valves and trapping water in the tube, and releasing another weight. When all bottles had been tripped, the string of bottles was recovered. The deployment and retrieval typically took several hours.

Figure 6.16 Left: A CTD ready to be lowered over the side of a ship. From Davis (1987). Right: Nansen water bottles before (I), during (II), and after (III) reversing. Both instruments are shown at close to the same scale. From Defant (1961: 33).

Mechanical instruments on Nansen bottles were replaced beginning in the 1960s by an electronic instrument, called a CTD, that measured conductivity, temperature, and depth (Figure 6.16). The measurements are recorded in digital form either within the instrument as it is lowered from a ship or on the ship. Temperature is usually measured by a thermistor; conductivity is measured by induction; pressure is measured by a quartz crystal. Modern instruments have accuracy summarized in Table 6.2.

Table 6.2 Summary of Measurement Accuracy
Best Accuracy 
±0.02 by titration
±0.005 by conductivity
10,000 dbar
± 0.65 dbar
2 kg/m3
± 0.005 kg/m3
Equation of State
± 0.005 kg/m3

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