6.7: Measurement of Conductivity or Salinity

Conductivity is measured by placing platinum electrodes in seawater and measuring the current that flows when there is a known voltage between the electrodes. The current depends on conductivity, voltage, and volume of sea water in the path between electrodes. If the electrodes are in a tube of nonconducting glass, the volume of water is accurately known, and the current is independent of other objects near the conductivity cell (figure \(\PageIndex{1}\)). The best measurements of salinity from conductivity give salinity with an accuracy of \(\pm 0.005\).

Figure \(\PageIndex{1}\): A conductivity cell. Current flows through the seawater between platinum electrodes in a cylinder of borosilicate glass 191 mm long with an inside diameter between the electrodes of 4 mm. The electric field lines (solid lines) are confined to the interior of the cell in this design making the measured conductivity (and instrument calibration) independent of objects near the cell. This is the cell used to measure conductivity and salinity shown in figure \(\PageIndex{3}\). From Sea-Bird Electronics.

Before conductivity measurements were widely used, salinity was measured using chemical titration of the water sample with silver salts. The best measurements of salinity from titration give salinity with an accuracy of \(\pm 0.02\).

Individual salinity measurements are calibrated using standard seawater. Long-term studies of accuracy use data from measurements of deep water masses of known, stable, salinity. For example, Saunders (1986) noted that temperature is very accurately related to salinity for a large volume of water contained in the deep basin of the northwest Atlantic under the Mediterranean outflow. He used the consistency of measurements of temperature and salinity made at many hydrographic stations in the area to estimate the accuracy of temperature, salinity and oxygen measurements. He concluded that the most careful measurements made since 1970 have an accuracy of \(0.005\) for salinity and \(0.005^{\circ}\text{C}\) for temperature. The largest source of salinity error was the error in determination of the standard water used for calibrating the salinity measurements.

Gouretski and Jancke (1995) estimated accuracy of salinity measurements as a function of time. Using high quality measurements from 16,000 hydrographic stations in the south Atlantic from 1912 to 1991, they estimated accuracy by plotting salinity as a function of temperature using all data collected below 1500 m in twelve regions for each decade from 1920 to 1990. A plot of accuracy as a function of time since 1920 shows consistent improvement in accuracy since 1950 (figure \(\PageIndex{2}\)). Recent measurements of salinity are the most accurate. The standard deviation of salinity data collected from all areas in the south Atlantic from 1970 to 1993 adjusted as described by Gouretski and Jancke (1995) was \(0.0033\). Recent instruments such as the Sea-Bird Electronics Model 911 Plus have an accuracy of better than \(0.005\) without adjustments. A comparison of salinity measured at 43\(^{\circ}\)10 ́N, 14\(^{\circ}\)4.5 ́W by the 911 Plus with historic data collected by Saunders (1986) gives an accuracy of \(0.002\) (figure \(\PageIndex{3}\)).

Figure \(\PageIndex{2}\): Standard deviation of salinity measurements below 1500 m in the south Atlantic. Each point is the average for the decade centered on the point. The value for 1995 is an estimate of the accuracy of recent measurements. From Gouretski and Jancke (1995).

Figure \(\PageIndex{3}\): Results from a test of the Sea-Bird Electronics 911 Plus CTD in the North Atlantic Deep Water in 1992. Data were collected at 43.17\(^{\circ}\)N and 14.08\(^{circ}\)W from the R/V Poseidon. From Sea-Bird Electronics (1992).