Evaluation of the fall rates of the present and developmental XCTDs

The fall rates of the current types of expendable conductivity-temperature-depth (XCTD) profilers and one under development are evaluated based on a series of co-located measurements with conventional conductivity-temperature-depth (CTD) profilers in the North Pacific. The types of probes investigated are the XCTD-1, the XCTD-2, the XCTD-3, and the XCTD-5 manufactured by Tsurumi Seiki Co., Ltd. It is confirmed that the present manufacturer's fall-rate coefficients for the XCTD-1/2 satisfy the accuracy guarantee of 2% of depth, at depth greater than 20 m. The coefficients of all XCTD types tested are dependent on water temperature, and the probes tend to fall slightly faster in warmer water. New sets of coefficients are given for the individual types, in a form that includes a correction for water temperature. It is also found that the ring hood structure of the XCTD-1/2 is effective in stabilizing the fall rates. The newer XCTD-3/5, which lack the hood, show larger scatter in the fall rate and occasionally violates the guaranteed depth accuracy.