HOT CTD data are collected using a SeaBird CTD 9-11 (9-11 Plus with dual sensors as of HOT-54) at the maximum sampling rate of 24 samples per second (24 Hz). They are screened for errors and processed to 2-dbar averages. Details of the CTD processing for each year can be found in HOT Data Reports #1-8 (see the file Readme.first for information on how to obtain these Data Reports).

CTD data are written to files using formats specified by the WOCE Hydrographic Programme Office. These formats are based on NODC formats, and are detailed in the WHP Office Report WHPO 90-1, available from Steve Diggs, WHPO Data Manager, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093-0214.

The files contain pressure, temperature and salinity profiles for all casts. Some casts also have dissolved oxygen, light transmission, and fluorescence. The data format consists of a self-documenting header followed by the data. The data records are written so that they can be read with a simple FORTRAN read statement. Details of this format are given in the file Readme.format.

The first year's temperatures are reported in IPTS-68. Subsequent temperatures are reported in ITS-90 units. Since temperature sensor calibrations were done in IPTS-68 units, and the UNESCO routines require IPTS-68 temperature, all intermediate processing was done in IPTS-68. As a final step, temperature and potential temperature were converted to ITS-90 using t_90 = 0.99976 t_68.

Since November 2001 CTD fluorescence data have been regularly calibrated against Chlorophyll bottle data and reported in the CTD files as Chloropigments (CHLPIG) in microgram/liter (uG/L). Bottle Fluorometric Chlorophyll-a plus Pheopigments above 175 dbar are matched against the upcast CTD Fluorometry to calibrate the downcast Fluorescence reported in the CTD files. All CTD files for prior cruises were calibrated and updated to reflect this change.

Starting HOT-177 (2006), the Transmissometer data that used to be included in the CTD files have been replaced by continuous Nitrate measured using Satlantic's InSitu Underwater Spectrometer (ISUS V2). Satlantic's ISUS V2 is a chemical free sensor that uses UV absorption technology to provide accurate nitrate concentration measurements in real-time These data are not included since they were uncalibrated and of questionable quality.