Frequently Asked Questions (FAQs) --
General Oceanographic Questions

FAQs have been organized into the following categories:

General oceanographic questions (this page)

• What is the difference between psu and ppt?
• Is the salinity calculated by Sea-Bird valid above 42 psu?
• Does Sea-Bird software output Practical Salinity (psu) or Absolute Salinity?
• 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?
• Which algorithm for calculating sound velocity (SV) from CTD data should I use?
• 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?

	General instrument questions How do instruments that can be internally or externally powered handle external power if internal batteries are installed? For an RS-232 Sea-Bird instrument, what is the maximum cable length for real-time data? Why do some instruments have zinc anodes, while others do not? Do Anti-Foulant Devices affect the conductivity cell calibration? What is Triton? Does it harm sensors? Do I need to purchase it from Sea-Bird? Where can I get information about safe handling/hazards associated with chemicals used with Sea-Bird equipment, such as Anti-Foulant Devices and Triton X-100 detergent? Why is Teflon tape used? My CTD has a Digiquartz pressure sensor. Can I use it above its rated pressure? My CTD has a Druck pressure sensor. Can I use it above its rated pressure? What do you recommend for cleaning barnacles off the exterior of the instrument? Why is the color on my instrument housing changing? Can I use hydrochloric acid to clean the outside of the housing? Why am I getting negative density values when testing the instrument? What is a wet-pluggable or wet-mateable or MCBH connector? Can I mate and unmate a cable with this connector underwater?
	Recommended practices What are the major steps involved in taking a cast with a Profiling CTD? Should I collect water samples (close bottles) on the downcast or the upcast? Can I deploy my profiling CTD for monitoring an oil spill? What are the major steps involved in deploying a moored instrument? What are the recommended practices for mating and unmating connectors? How can I tell if my connectors have leaked, and what do I do about corrosion on connector pins? What are the recommended practices for replacing a bulkhead connector? What are the recommended practices for inspecting and cleaning o-rings and mating surfaces? How often should I replace o-rings? How should I handle my CTD to avoid cracking the conductivity cell? What is an Anti-Foulant Device? How often should I replace it? Does it require special handling? What are the recommended practices for deploying in frazil or pancake ice, or deploying at low temperatures? Does it matter if I deploy my moored instrument, which includes a conductivity sensor, in a horizontal or vertical position? How many / what kind of spares should I have on ship for my instrument? How many/what kind of spares should I have on ship for my SBE 9plus? How will my CTD be affected by adjacent objects? What are the safety concerns/procedures if the instrument floods? Can the instrument explode? Is it necessary to put my instrument in water to test it? Will I destroy the conductivity cell if I test it in air? How should I store my conductivity sensor if there is danger of freezing? What are the recommended practices for cleaning and lubricating winch cables? What are the recommended practices for splicing cables?
	Service How often do I need to have my instrument and/or auxiliary sensors recalibrated? Can I recalibrate them myself? I am planning to ship my instrument to Sea-Bird for calibration and minor repairs. What is the typical turn-around time? I am planning to ship my instrument to Sea-Bird for calibration and minor repairs. Should I also send the auxiliary sensors from other manufacturers? My auxiliary sensor (not manufactured by Sea-Bird) needs to be repaired / recalibrated. Where should I send it for servicing? I want to add an auxiliary sensor to my CTD. Assuming the auxiliary sensor is compatible with the instrument, what is the procedure? Do I need to remove batteries before shipping my instrument for a deployment or to Sea-Bird? Do I need to clean the exterior of my instrument before shipping it to Sea-Bird for calibration? I want to change the pressure sensor on my CTD, swapping it as needed to get the best data for a given deployment depth. Can I do this myself, or do I need to send the instrument to Sea-Bird? Can I brush clean and replatinize the conductivity cell myself? How often should this be done? I sent my conductivity sensor to Sea-Bird for calibration, and you also performed a Cleaning and Replatinizing (C&P). You sent the instrument back with 2 sets of calibration data. What does this mean? How can I tell if the conductivity cell on my CTD is broken? What are Configuration Sheets, and where can I find them for my instrument? What is Sea-Bird's policy on upgrading instruments? Should I purchase spare pressure sensors for my SBE 9plus? What do I need to send to Sea-Bird for calibration of my SBE 9plus? What do I need to send to Sea-Bird for calibration of my SBE 25?
	Ordering How do I find information about the options available with each instrument? Where do I find pricing information for new instruments, calibration services, and/or repair services? Which Sea-Bird profiling CTD is best for my application? I want to integrate a moored CTD with some auxiliary sensors (transmissometer, fluorometer, etc.). Which CTD should I use? How should I pick the pressure sensor range for my CTD? Would the highest range give me the most flexibility in using the CTD? I am ordering a CTD and want to use auxiliary sensors. Should I order them from Sea-Bird also, or deal directly with the sensors' manufacturers? What are the pros and cons of ordering wet-pluggable connectors? Can Sea-Bird provide some guidance for ordering cable, winch, and deck gear?
	Software I am confused by all these software names. Which software does what? Can I install my Sea-Bird CD-ROM on multiple computers or give it to another interested scientist? How can I copy the setup of my Sea-Bird software onto another computer? How can I view CTD data? What is a configuration (.con or .xmlcon) file and how is it used? What is a .psa file and how is it used? What is a .cfg file and how is it used? Does SEASOFT have a provision for converting to MatLab data files? Can I use SEASOFT on a Macintosh, Unix, or Linux system? Is the Windows version of SEASOFT compatible with Microsoft's Vista operating system? Is the Windows version of SEASOFT compatible with Microsoft's Windows 7 operating system? Can I use SEASOFT-DOS on a computer with Windows 95/98/NT/2000/XP operating system? Your website Download Software from FTP Site page defines the latest software revision, but I cannot find the latest revision on the FTP site. Is it not posted yet? Why am I having trouble downloading software from your FTP site? How does Sea-Bird software calculate conductivity, temperature, and pressure in engineering units? How does Sea-Bird software calculate derived variables such as salinity, sound velocity, density, depth, thermosteric anomaly, specific volume, potential temperature, etc.? What is the flag variable column that is added to the data file by SBE Data Processing's Data Conversion or ASCII In module? What formula does Sea-Bird software use to convert pressure data to depth? In Sea-Bird software, is noon on January 1 Julian Day 0.5 or Julian Day 1.5? Can I edit my .dat data file to add some explanatory notes to header? Can I edit my .hex data file to add some explanatory notes to header? Why am I getting a class not registered error when running SBE Data Processing?
	Data analysis and processing Why and how should I align data from a 911plus CTD? What are the typical data processing steps recommended for each instrument? Where can I find formulas for calculating conductivity, temperature, pressure, and derived parameters such as salinity, sound velocity, density, depth, thermosteric anomaly, specific volume, potential temperature, etc.?
	Manufacturing Does Sea-Bird pressure test each instrument before shipping to verify integrity of housing, o-rings, assembly, etc.? Does Sea-Bird check and calibrate all sensors on a CTD system prior to shipping? Do Sea-Bird instruments have CE certification? Do Sea-Bird instruments have ISO certification?

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).

The modern oceanographic definition of salinity is the Practical Salinity Scale of 1978 (PSS-78). By definition, PSS-78 is valid only in the range of 2 to 42 psu. Sea-Bird uses the PSS-78 algorithm in our software, without regard to those limitations on the valid range.

Unesco technical papers in marine science 62 "Salinity and density of seawater: Tables for high salinities (42 to 50)" provides a method for calculating salinity in the higher range (Click here to access this paper via Unesco's website).

In June 2009, a new Thermodynamic Equation of State of Seawater, referred to as TEOS-10, was adopted by the Scientific Committee on Oceanic Research and the International Association of Physical Sciences of the Ocean Working Group 127. The new equation incorporates a more accurate representation of salinity known as Absolute Salinity. Application Note 90 discusses this new equation, and Sea-Bird's plans for implementation.

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 outputting or plotting 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.

Sea-Bird real-time data acquisition (Seasave V7) and data processing (SBE Data Processing) software supports calculation of Chen-Millero, Del Grosso, and Wilson sound velocities. The algorithms, as implemented in our software, are provided in the software documentation, which is available via the software Help files or in an Appendix in the software manuals. To download the software and/or manuals, go to Software Descriptions and Revisions.

The Hydrographic Society published Special Publication No. 34 in 1993, "A Comparison Between Algorithms for the Speed of Sound in Seawater", comparing a number of sound velocity algorithms. The report recommends using the Chen-Millero algorithm for water depths less than 1000 meters and the Del Grosso algorithm for water depths greater than 1000 meters, and recommends that the Wilson algorithm should not be used. Click here to access the report via the Hydrographic Society's website.

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.

See Application Note 2D: Instructions for Care and Cleaning of Conductivity Cells for rinsing, cleaning, and storage procedures.

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: 05 Nov 2010

Sea-Bird Home     Phone: (+1) 425-643-9866     Fax: (+1) 425-643-9954    E-mail: seabird@seabird.com