Underway CTD


The Underway Conductivity-Temperature-Depth instrument (UCTD) was developed to fill a need for inexpensive profiles of temperature and salinity from underway vessels, including volunteer observing ships (VOS) and research vessels. The UCTD operates under the same principle as an expendable probe. By spooling tether line from both the probe and a winch aboard ship, the velocity of the line through the water is zero, the line drag is negligible, and the probe can get arbitrarily deep. Recovery is accomplished by reeling the line back in. Recovering the UCTD has some advantages: 1) the cost per profile decreases with increasing use, 2) sensors can be calibrated postdeployment, 3) the UCTD carries a pressure sensor so depth is measured directly, and 4) no hazardous materials are left behind. The design goal for the UCTD was to obtain profiles deeper than 100 m at 20 knots (typical of a VOS). This goal has been surpassed, as it is able to profile to over 150 m at 20 knots and to over 400 m at 10 knots.

Figure 1. The UCTD being deployed by dropping off the stern of a moving vessel. The yellow on the tail of the probe is 400 m of Spectra line.

Acknowledgements. The development and use of the UCTD was supported by the National Oceanic and Atmospheric Administration and the Office of Naval Research. The Instrument Development Group designed and built many of the components of the UCTD.