Frequently Asked Questions (FAQs) --
Data Analysis and Processing

FAQs have been organized into the following categories:

Data Analysis and Processing questions (this page)

• 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 variables such as salinity, sound velocity, density, depth, thermosteric anomaly, specific volume, potential temperature, etc.?

	General instrument questions How do instruments that can be internally or externally powered handle external power if internal batteries are installed? 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? 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?
	General oceanographic questions 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?
	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? 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?
	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 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 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? My auxiliary sensor (not manufactured by Sea-Bird) needs to be repaired / recalibrated. Where should I send it for servicing? 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 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) 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? 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? What is the flag variable column that is added to the data file by SBE Data Processing's Data Conversion or ASCII In module? 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 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?
	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 T-C Duct on a 911plus imposes a fixed delay (lag time) between the temperature measurement and the conductivity measurement reported in a given data scan. The delay is due to the time it takes for water to transit from the thermistor to the conductivity cell, and is determined by flow rate (pump rate). The average flow rate for a 9plus is about 30 ml/sec. The Deck Unit (11plus) automatically advances conductivity (moves it forward in time relative to temperature) on the fly by a user-programmable amount (default value of 0.073 seconds), before the data is logged on your computer. This default value is about right for a typical 9plus flow rate. Any fine-tuning adjustments to this advance are determined by looking for salinity spikes corresponding to sharp temperature steps in the profile and, via the SBE Data Processing module Align CTD, trying different additions (+ or -) to the 0.073 seconds applied by the Deck Unit, until the spikes are minimized. Having found this optimum advance for your CTD (corresponding to its particular flow rate), you can use that value for all future casts (change the value in the Deck Unit) unless the CTD plumbing (hence flow rate) is changed.

Oxygen and other parameters from pumped sensors in the same flow as the CT sensors can also be re-aligned in time relative to temperature, to account for the transit time of water through the plumbing. A typical plumbing delay for the SBE 43 DO Sensor is 2 seconds. However, the DO sensor time constant varies from approximately 2 seconds at 25 °C to up to 10 seconds at 0 °C. So, you should add some advance time for this as well (total delay = plumbing delay + response time ~ 4 - 12 seconds). As for the conductivity alignment, the Deck Unit can automatically advance oxygen on the fly by a user-programmable amount (default value of 0 seconds) before the data is logged on your computer. However, because there is more variability in the advance, most users choose to do the advance in post-processing, via the SBE Data Processing module Align CTD. For additional information and discussion, refer to Module 9 of our training class and the SBE Data Processing manual.

Note: Alignment values are actually entered in the 11plus Deck Unit and in SBE Data Processing relative to the pressure measurement. For the 9plus, it is sufficiently correct to assume that the temperature measurement is made at the same instant in time and space as the pressure measurement.

Section 3: Typical Data Processing Sequences in the SBE Data Processing manual provides typical data processing sequences for our profiling CTDs, moored CTDs, and thermosalinographs. Typical values for aligning, filtering, etc. are provided in the sections detailing each module of the software. This information is also documented in the software's Help file. To download the software and/or manual, go to Software Descriptions and Revisions.

For formulas for the calculation of conductivity, temperature, and pressure from the raw data, see the calibration sheets for your instrument (if you cannot find the calibration sheets, contact us with your instrument serial number at seabird@seabird.com or 425-643-9866).

For derived parameter formulas (salinity, sound velocity, density, etc.), see the SEASAVE and SBE Data Processing manuals, which document these formulas in an Appendix. Additionally, the formulas are documented in the Help files for these programs. To download the software and/or manuals, go to Software Descriptions and Revisions.

Last modified: 06 Apr 2007

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