Salinometer Thermosalinograph (TSG) CTD

1. Salinometer Thermosalinograph (TSG) CTD Clifford Hoang SIO 218A

2. 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).

3. 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 matlabsalinity.m file), k=+0.0162 • Valid over ranges of temperature (-2 to 35C) • Source: Unesco(includes Fortran algorithm) • Matlab

4. 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.

5. Salinometer Guildline8400B Autosal Cost: N/A “GuildlineAutosal Salinometer is the only instrument presently capable of providing the required accuracy in salinity measurements made during WOCE (World Ocean Circulation Experiment—NOAA) cruises.” Figure: Autosal Conductivity Cell. GuildlineAutosal uses an adaptation of the Guildline CTD cell. OSIL ( Ocean Scientific International Ltd.)

6. 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

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

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

9. 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). 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)

10. CTD • Advantages • 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)

11. 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