Three sensor types have been used with the Sea-Bird CTD during the HOT program to measure dissolved oxygen in the water column. Sea-Bird’s SBE-13 sensors with Beckman sensor element (referred to as Beckman) were used between 1988 and 1991; SBE-13 with YSI oxygen probe (referred to as YSI) were used between 1991 and 2001; and Sea-Bird SBE-43 sensors have been used since 1999. The Beckman sensors had several problems including unreliable near-surface data, sensor drift, and they were difficult to calibrate. For these reasons their measurements were viewed as being useful primarily to assist in quality controlling water sample oxygens, and for interpolating between the discrete samples. The switch to YSI sensors improved the quality of the CTD oxygen data, however these sensors were not very reliable, had continuing failures, and were often not very stable. Even though these sensors had some undesirable characteristics, they still provided useful information on the variability of O2 on small scales. The use of the Sea-Bird SBE-43 sensors greatly improved the quality of the data, mainly due to their stability and ease of calibration, and although some of the early versions of the sensor had numerous failures, the reliability of more recent units continues to improve.

     A survey of the sensors used during the HOT program, and an evaluation of the data quality is included here. Particular problems with sensors and the affected data are also addressed. The overall quality of the data for each cruise is evaluated by comparing the calibrated CTD data with the oxygen samples.

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