Frequently Asked Questions (FAQs) --
General Oceanographic Questions
FAQs have been organized into the following categories:
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General oceanographic questions (this page) |
- What is the difference between psu and ppt?
- What is the difference between IPTS-68 and ITS-90?
- What is the difference between initial accuracy and resolution?
- Why is sound velocity (SV) computed from a CTD better than sound velocity from direct measuring instruments?
- What is hydrostatic head effect?
- How can I find the density of seawater at different temperatures and/or salinities?
- What is the cause of conductivity drift?
- Does it matter whether you use natural or artificial seawater for calibrations? Which does Sea-Bird use?
- Where can I purchase standard seawater?
- Why is my CTD data showing hysteresis?
Our Glossary page is another good source of information.
The numeric difference between psu and ppt is small; both indicate ocean salinity.
The modern oceanographic definition of salinity is the Practical Salinity Scale of 1978 (PSS-78). It yields a practical salinity from new equations, smooth expansions of conductivity ratio, which were carefully fit to the real salinity of diluted North Atlantic seawater. The numeric unit from PSS-78 is psu (practical salinity unit) and is distinct from the previous physical quantity ppt (kg salt per kg water in parts per thousand). The primary motivation for psu was consistency; it focused on a trace to a primary conductivity standard (K15) and recognition that ocean ion ratios were not identical. Salinometer work was plagued by an inconsistent standard and the ppt equations included ion ratios from different oceans. So, the trade was a consistent standard and equation that works for a single ion mix instead of exact salinity in other ocean basins. G. Siedler and H. Peters highlighted where PSS-78 and EOS-80 formulas deviate from real salinity and density (e.g., Baltic Sea is difficult, but the deep Pacific has EOS-80 deviations of up to 0.02 kg/m3, implying salinity errors of order 0.02 psu).
ITS-90 is the new (as of 1990) temperature scale; IPTS-68 was the previous standard. The differences are related to redefining certain triple points and other melt or freeze cells that are used as the fundamental standards for temperature. Over the oceanographic ranges of temperature, a linear approximation is used to convert:
IPTS-68 = 1.00024 * ITS-90
The difference is small, but at WOCE levels it is significant.
Note: Salinity, density, and sound velocity are still defined in terms of IPTS-68 temperature. Sea-Bird’s software uses IPTS-68 temperature to calculate these derived parameters, regardless of which temperature scale you select for temperature.
Application Note 42: ITS-90 Temperature Scale provides a more detailed description.
Upon receipt of an instrument, the initial accuracy is the accuracy when comparing to a known standard. Resolution is the smallest amount of change that a sensor can see.
Direct SV probes measure the time (flight time) required for a sound pulse to travel over a fixed length, using a high-speed clock to measure time. The clock starts when the pulse is emitted, and stops when the pulse is received. Theoretically, you only need to know the length of the path (and the frequency of the clock ‑ an easy matter) to compute SV. SV is calculated as:
SV = length of acoustic path / flight time.
For a typical acoustic path of 0.1 m, a flight time of 67 microseconds is expected for SV = 1492 m/s.
Two problems associated with direct SV probes are:
The length is not readily determined by a ruler measurement. The true length includes some depth into the acoustic transducer at which the pulse actually arises and again some depth where it is actually detected. Consider for example the SVplus instrument made by Applied Microsystems in Canada. The claimed accuracy for this probe is 0.06 m/s. For a typical water SV of 1500 m/s and a probe acoustic path of 100 mm, achieving this accuracy requires that length be determined to within (0.06/1500) x 100 mm = 0.004 mm (approximately 1/25 the thickness of a sheet of paper). The acoustic transducer would be of order 1 mm thick, so its dimension is much larger (250 times) than the length associated with the specified accuracy.
Determining the actual flight time is not as simple as counting clock pulses. There are other time delays in determining both the start of the acoustic pulse and the time of its reception. Recalling that the time sound requires to travel 100 mm is approximately 67 microseconds, to measure SV to within 0.06 m, the flight time must be determined to within (0.06/1492) x 67 microseconds = 2.7 nanoseconds. It is exceedingly difficult to measure time to such precision, especially as the time lag associated with the acoustic transducer is much larger than this ‑ typically of order 1 microsecond (hundreds of times larger than the permitted error).
The fact is that in designing a direct path SV probe, the determination of length by ruler is only good to 5 or 10% (approximately 100 m/s equivalent uncertainty in SV). The actual determination of SV response therefore must be made in a calibration bath (using a CTD as a reference!), which is how all SV probes are calibrated.
Direct SV probes are often marketed on the principle that the measurement is based only on fundamental physical values of length and time. That is true in theory, but the practice is a different story! Direct SV probe manufacturers do not know the length (or the time) ‑ they just fit the probe response to CTD-computed SV.
There is a place for direct SV probes. Having been calibrated in water against a CTD, they do a competent job of measuring SV in other liquids. They will go on working in oil, petrol, milk, beer, etc. ‑ liquids in which CTD measurements have no meaning.
This effect is not the oceanography adiabatic temperature gradient, but rather the effect of pressure on the triple-point-of-water (TPW) (pressure effect on the phase point). It is unfortunate that oceanographers and metrologists have a common terminology for 2 different physical effects.
The adiabatic lapse rate (oceanography hydrostatic head effect) for pure water (+0.010 C, +6.11 mbar) is -0.3059 microK/cm. The pressure effect on the TPW is -7.3 microK/cm and does not require the presence of all 3 phases (i.e., applies to ice/water interface only). The reason the adiabatic lapse rate does not affect measurements in the TPW cell is because the water at the inner edge of the ice mantle and the water surrounding the thermometer sheath does not convect. This motionless water therefore comes into thermal equilibrium with the vertical temperature gradient at the ice interface (the metrology hydrostatic head effect).
SBE Data Processing includes a module called SeacalcW. SeacalcW can calculate density, sound velocity, and a number of other parameters for a given user input of pressure, temperature, and conductivity (or salinity). You can download SBE Data Processing from our FTP page.
Conductivity cells drift primarily as a function of cell fouling. There are several sources of the fouling:
Biological growth is the primary source of cell fouling. Rinsing the conductivity cell with clean de-ionized water after each cast helps prevent most growth in the cell. If the cell is not rinsed, or standard tap water is used, growth rates can be severe. As the cell fouls, it will drift towards lower salinity values.
Surface oil slicks also cause cell fouling. Avoid deploying the CTD through obvious slicks. When working in coastal areas, with higher chances of oil fouling, rinse and soak the cell with a 1% Triton X-100 solution (diluted in clean DI water) to help prevent oil fouling.
Because of the nature of fouling, the total cell drift may not be linear. It exhibits rapid small shifts (especially if related to oil fouling) on top of a base line drift. It is important to take water samples to document the behavior. Application Note 31: Computing Temperature and Conductivity Slope and Offset Correction Coefficients from Laboratory Calibrations and Salinity Bottle Samples discusses how to correct the data.
For SBE 4 conductivity calibrations, Sea-Bird uses natural seawater that has been carefully collected, stored, UV irradiated, and filtered. Artificial seawater is not adequate if calibration errors are to be kept below 0.010 psu.
The primary difference between natural and artificial seawater is the behavior of conductivity versus temperature. The practical salinity scale 1978 equations include a term rt. This term is expanded into a fourth order equation that describes the variation of conductivity versus temperature for a sample of constant salinity. The equation’s coefficients are derived by fitting to natural seawater samples. Artificial seawater does not have the same conductivity versus temperature characteristic, providing incorrect coefficients and causing a slope error in the calibration.
For calibrations of conductivity sensors other than the SBE 4, Sea-Bird uses artificial seawater (NaCl solution). However, we place an SBE 4 conductivity sensor in each bath, providing a standard for reference to the natural seawater calibration. This allows us to correct errors in the coefficients and slope introduced with the artificial seawater calibration.
For calibration of temperature sensors, Sea-Bird uses artificial seawater (NaCl solution).
IAPSO standard seawater is available in 250 ml vials. For more information and purchase inquiry, e-mail osil@oceanscientific.co.uk.
The difference between downcast and upcast is most likely related to package wake. When the CTD is mounted under a large water sampler, the variation can be on the order of 5 to 8 meters. This is due to the shadowing of the CTD sensors by the water sampler.
Last modified: 06 Apr 2007
Sea-Bird Home Phone: 425-643-9866 Fax: 425-643-9954 E-mail: seabird@seabird.com
