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Auxiliary Hydraulic Variables
Published in Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste, Experimental Hydraulics: Methods, Instrumentation, Data Processing and Management, 2017
Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste
Conductivity is measured in units of siemens/m (S/m) as a surrogate for chemical composition, because the electrical conductivity of water increases with salinity. The measured conductivity is calibrated using a standard solution of known salinity. Conductance is the inverse of electrical resistance, and thus can be obtained by dividing a measured electrical current by the voltage of the electric field. The conductance depends on both the conductivity of the fluid and the length and cross-sectional area of the electrical current flow. A conductivity probe basically consists of a flow cell with either electrodes or transformers that apply voltage to induce current. The specific design of the sensor determines accuracy, which can be degraded by uncertainty in the electrical current cross-sectional area and resistivity of the sensor components. It is worth noting that both of these sensor properties can be altered by biofouling and application of anti-biofouling agents, which is particularly problematic if a portion of the electrical current is external to the flow cell. The SBE 4 conductivity sensor used in Seabird profiling CTDs minimizes these uncertainties by employing three low-resistance platinum electrodes placed within the flow cell such that the electrical current is entirely contained within the cell (http://www.seabird.com/document/conductivity-sensors-moored-and-autonomous-operation; accessed 19th October 2016). The rated precision and accuracy of the SBE 4 are 0.00004 S/m and ±0.0003 S/m, which corresponds to a practical salinity accuracy of ±0.01 [dimensionless, but approximately units of g/kg] using the Practical Salinity Scale 1978 (PSS-78; IEEE Journal of Oceanic Engineering, Vol. OE-5, No. 1, January 1980 ) at standard temperature and pressure. The absolute salinity of seawater of a certain composition from a given location can be calculated from measurements of practical salinity using the TEOS-10 standard.
Thermohaline equation of state for pure water, seawater and brine
Published in Journal of Hydraulic Research, 2023
It is essential to develop tools to determine the density of water when studying and modelling flows in natural and constructed water bodies. The international equation of state, EOS-80 (UNESCO, 1981), for seawater by Millero and Poisson (1981) has provided such a tool for the oceanographic range of temperature T = 0–40°C and practical salinity = 0–40 ppt. A more recent method, TEOS-10, has also been developed to determine the density as well as all the thermodynamic properties of seawater (IOC, SCOR and IAPSO, 2010). Unlike EOS-80, which is based on experimental measurements, TEOS-10 and similar methods (e.g. Jackett et al., 2006) rely on theoretical geometrical representations of the thermodynamic properties, as initially proposed by Gibbs (1873). A notable difference between EOS-80 and TEOS-10 is the adoption of absolute salinity S instead of practical salinity Sp, which has been used for many years by oceanographers. Absolute salinity S (g kg−1) is the amount of salts in grams dissolved in one kilogram of seawater. The practical salinity is determined indirectly from electrical conductivity measurements and normally expressed in parts per thousand (ppt) or practical salinity units (psu). Millero and Huang (2009) found the relationship between absolute and practical salinity useful for comparing data from different studies.