Expendable Bathythermograph - XBT

XBT - Sippican Ocean Systems

The Expendable Bathythermograph (XBT) was introduced in 1966 to replace the Mechanical Bathythermograph (MBT), the first vertical profiler in wide use between 1951 and 1975. The XBT is largely used until these days, including in volunteer observing ships for profiling the water column (RUDNICK & KLINKE, 2007; TALLEY et al., 2011). The device has a thermistor, which measures the temperature, and an electronic data acquisition system. It makes 400m, 800m or 1500m profiles and can be launched from moving ships at up to about 30kt of velocity (TALLEY et al., 2011). The XBT has a torpedo-like shape which is connected to two element wire insulated by resin, one inside the probe and one inside the canister (RUDNICK & KLINKE, 2007; TALLEY et al., 2011). As the probe falls into and deeper in the water, the wire pays out from the both spools transferring the temperature signal back to the ship (TALLEY et al., 2011; ULLMAN; HEBERT, 2014). When all the wire is out, it breaks and the XBT is lost (RUDNICK & KLINKE, 2007; TALLEY et al., 2011).

The device is assumed to fall at a known and constant rate, once the cable drag is negligible and thus the velocity of the cable through the water is zero (RUDNICK & KLINKE, 2007), making it possible to infer the depth from which the temperature is being measured (TALLEY et al., 2011). The fall rate is determined practically and it’s known to introduce some errors in depth. Besides, changes in the probe’s density as the wire pays out also introduce depth errors. Currently, the error presented in XBT depth is about 20%. The calibration does not occur individually. Instead, thousands of XBT’s thermistors are purchased and a few hundred are calibrated for accuracy. Then, the ones calibrated are assigned to the rest of them. The thermistors that do not meet the accuracy needed are discarded. The XBTs can be launched from portable or fixed launchers (TALLEY et al., 2011).

The XBT from the Sippican Ocean Systems are packaged in cardboard cartons and must be kept from rain, snow, salt spray etc. Caution must be taken when storing them as well as with the local temperature of the storage room. The best local to launch is the leeward side of the ship and extra care must be taken to not let the copper wire get in touch with the ship’s hull to avoid spikes in the data and/or wire break (SIPPICAN OCEAN SYSTEMS, 2016).


SIPPICAN OCEAN SYSTEMS XBT User's Guide. Acquired from http://www.sippican.com/seaair/xbt.php on February, 20th 2016.

TALLEY, L.D.; PICKARD, G.L.; EMERY, W.J.; SWIFT, J.H. Descriptive Physical Oceanography: An Introduction. 6ª ed. London. Elsevier. 2011. 587p.