Salinometer Thermosalinograph (TSG) CTD Clifford Hoang SIO 218A

Standard Seawater All conductivity methods require a chemical standard for calibration. Practical Salinity is defined in terms of the ratio K15 of the electrical conductivity of the seawater sample at temperature of 15C and pressure of 1 atmosphere, to that of a potassium chloride (KCl) solution. Common standard for salinity reduces the possibility of systematic errors. Makes it possible to combine data from different expeditions or surveys in the same area or worldwide. Salinity is measured both on seawater samples collected from bottles such as on a rosette sampler, and through paired conductivity and temperature sensors deployed in the water. Figure: Standard Seawater prepared by Ocean Scientific International Ltd. (OSI) is the recognized standard for the calibration of instruments measuring conductivity (salinity).

Calculating Salinity Conductivity Ratio (R): R = C(S,t,p)/C(35,15,0) where C(35,15,0) is 42.914 mS/cm for t=15C Salinity(S): S=a0+a1Rt1/2+a2Rt+a3Rt3/2+a4Rt2+a5Rt5/2+ΔS where ΔS= (t-15)/(1+k(t-15))(b0+b1Rt1/2+b2Rt+b3Rt3/2+b4Rt2+b5Rt5/2) 2<S<42 ai and bi are constants (see matlab salinity.m file), k=+0.0162 Valid over ranges of temperature (-2 to 35C) Source: Unesco (includes Fortran algorithm) Matlab

Salinometer Salinity Measurements Technique used at sea for 20+ years, yields salinity measurements that have a standard deviation of ~0.0010. Salinity Sample Collection (special care) On research cruises, salinity samples collected from a CTD station are stored in flint glass bottles (3x rinsed w/ 30-40 ml sample water). Samples analyzed onboard within a day of collection as soon as they reach laboratory temperature (room temp), to minimize evaporation from the sample. Processed with a laboratory salinometer that measures the conductivity of each sample in comparison with a carefully prepared standard. IAPSO Standard Seawater is used to standardize the salinometer.

Salinometer Guildline 8400B Autosal Cost: N/A “Guildline Autosal Salinometer is the only instrument presently capable of providing the required accuracy in salinity measurements made during WOCE (World Ocean Circulation Experiment—NOAA) cruises.” Uses a four-electrode conductance cell of small dimensions in a thermostat bath. The seawater flows continuously from the sample bottle through a heat exchanger in the thermostat, to bring it to a specified temperature, and then through the cell. The conductance-bridge is balanced semi-automatically and the conductivity ratio of the sample relative to that of Standard Seawater is displayed digitally. Salinity is then obtained from the conductivity ratio and the temperature using the UNESCO/N.I.O International Oceanographic Tables or the Practical Salinity Scale 1978 Formula. The circuits are such that variations of electrode surface conditions do not affect the measurement. Figure: Tubes connected to the cell come out into the air. This prevent the flow of electrical current external to the cell. Autosal’s notable accuracy results from the fact that cell’s hard parts uniquely define the cell constant. Figure: Autosal Conductivity Cell. Guildline Autosal uses an adaptation of the Guildline CTD cell. OSIL ( Ocean Scientific International Ltd.)

Thermosalinograph (TSG) Measures the properties of sea surface water Salinity, Density, Sound Velocity, and other parameters of interest. Typically in flow-through systems operating continuously throughout a cruise Installed inside or near hull of ship. Intake is usually located as close to the ship’s bow as possible to collect a sample with little influence/uncontaminated from the ship. TSGs are often integrated with other underway data collection systems. Figure: Mid-Atlantic Bight region and track for cruise EN534, October 22-28 2013 NOAA Thermosalinograph Data

Thermosalinograph (TSG) Thermosalinograph: (a) SBE 45 unit and (b) schematic of its operation. Source: Sea-Bird Electronics, Inc. (2012).

Thermosalinograph (TSG) SBE 45 MicroTSG Thermosalinograph Cost: N/A Source: SBE 45 MicroTSG Manual

Conductivity, Temperature, and Depth (CTD) In-situ conductivity measurements are made by CTDs (and their predecessors the STDs) and on other subsurface devices where salinity observations are desired (e.g. moorings and floats). Continuously records salinity (by measuring conductivity), temperature, and depth (by measuring pressure). Temp. must be measured simultaneously with conductivity. When processing CTD data, it is important to account for the sensor response time mismatch (erroneous spiking in derived salinity). Since conductivity depends primarily on temperature and only secondarily on salinity. SBE Cell: Has no external field b/c its outer electrodes are connected together. No voltage difference exists to create an external electrical current. Two-terminal cell which electrode resistances are in series with the cell resistance proper. Electrode’s low resistance and cell’s high resistance, errors resulting from changes in the electrode resistances are small. Immune to proximity errors and protected by anti-fouling items places at ends of the cell. Figure: SBE Conductivity Cell Figure: Anti-biology agent placed at both ends of the cell SBE 37-IM Data Sheet Sea-Bird Electronics, Inc. From left, SBE 37-IM, 37-IMP, 37-IMP-IDO, 37-IMP-ODO (7,000m Depth)

CTD Advantages Disadvantages Remote sensing Very accurate Light weight (CTD only) Can be used at depths up to several thousand meters SeaBird sensor’s electrode cells derive from fully internal field (prevent internal fouling and attendant drift) Disadvantages Small, low-powered CTD sensors require calibration of individual sensors (especially those deployed for long periods) Sensors must be stable for period of deployment Assumptions about the water properties must be made and referenced to the sensor data. Fouling SBE sensors poor flushing, cannot be cleaned in the field (not important for moored applications) Figure: Rosette sampler. Large sampler used in the World Ocean Circulation Experiment, with 36 10-liter Niskin bottles, an acoustic pinger (lower left), an LADCP (center, yellow long), a CTD (bottom, horizontal), and transmissometer (center, yellow short). (Photo courtesy of L. Talley)

CTD SBE 37-IM (Inductive Modem) MicroCAT C-T Recorder Cost: ~$5,000 for most basic. For most advanced, could go up to ~$12,000. IM sensors store data internally, so data can be directly extracted from instruments in event of failure of any part of communication link. Battery life-time of CTDs covered in class (see chapter 5). SBE 37-IM Data Sheet