6.7 Measurement of Conductivity
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 elecrodes are in a tube of non-conducting glass, the volume of water is accurately known, and the current is independent of other objects near the conductivity cell (Figure 6.13). The best measurements of salinity from conductivity give salinity with an accuracy of ±0.005.
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 ±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 out flow. 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°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 1500m 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 6.14). Recent measurements of salinity are the most accurate. The standard deviation of modern 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. More recent instruments such as the Sea-Bird Electronics Model 911 Plus have an accuracy of better than 0.005psu without adjustments. A careful comparison of salinity measured at 43° 10'N, 14° 4.5'W by the 911 Plus with historic data collected by Saunders (1986) gives an accuracy of 0.002 (Figure 6.15).
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
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Updated on September 15, 2006