After witnessing firsthand the dangers of deploying and retrieving BTs, James M. Snodgrass began developing the expendable bathythermograph (XBT). Snodgrass' description of the XBT:Briefly, the unit would break down in two components, as follows: the ship to surface unit, and surface to expendable unit. I have in mind a package which could be jettisoned, either by the "Armstrong" method, or some simple mechanical device, which would at all times be connected to the surface vessel. The wire would be paid out from the surface ship and not from the surface float unit. The surface float would require a minimum of flotation and a small, very simple sea anchor. From this simple platform the expendable BT unit would sink as outlined for the acoustic unit. However, it would unwind as it goes a very fine thread of probably neutrally buoyant conductor terminating at the float unit, thence connected to the wire leading to the ship. In the early 1960s the
U.S. Navy contracted Sippican Corporation of Marion, Massachusetts to develop the XBT, who became the sole supplier.
Participation by Month of Country and Institutions deploying XBTs Below is the list of XBT deployments for 2013:
XBT Fall Rate Bias Since XBTs do not measure depth (e.g. via pressure), fall-rate equations are used to derive depth profiles from what is essentially a time series. The fall rate equation takes the form: :z(t)=at^2+bt where, z(t) is the depth of the XBT in meters; t is time; and a & b are coefficients determined using theoretical and empirical methods. The coefficient b can be thought of as the initial speed as the probe hits the water. The coefficient a can be thought of as the reduction in mass with time as the wire spools off. For a considerable time, these equations were relatively well-established, however in 2007 Gouretski and Koltermann showed a bias between XBT temperature measurements and
CTD temperature measurements. They also showed that this varies over time and could be due to both errors in the calculation of depth and in measurement of the temperature. From that the 2008 NOAA XBT Fall Rate Workshop began to address the problem, with no viable conclusion as to how to proceed with adjusting the measurements. In 2010 the second XBT Fall Rate Workshop was held in Hamburg, Germany to continue discussing the problem and forge a way forward. A major implication of this is that a depth-temperature profile can be integrated to estimate upper ocean heat content; the bias in these equations lead to a warm bias in the heat content estimations. The introduction of
Argo floats has provided a much more reliable source of temperature profiles than XBTs, however the XBT record remains important for estimating decadal trends and variability and hence much effort has been put into resolving these systematic biases. XBT correction needs to include both a drop-rate correction and a temperature correction. ==Uses==