Bathythermograph

Device to detect water temperature and pressure

The bathythermograph, or BT, also known as the Mechanical Bathythermograph, or MBT; While the instrument is being dropped, the wire is paid out until it reaches a predetermined depth, then a brake is applied and the BT is drawn back to the surface. as a collaboration between MIT, Woods Hole Oceanographic Institution (WHOI), and the U. S. Navy. The device was modified during World War II to gather information on the varying temperature of the ocean for the U. S. Navy. Originally the slides were prepared "by rubbing a bit of skunk oil on with a finger and then wiping off with the soft side of one's hand," followed by smoking the slide over the flame of a Bunsen burner. Later on the skunk oil was replaced with an evaporated metal film. By monitoring variances, or lack of variances, in underwater temperature or pressure layers, while submerged, the submarine commander could adjust and compensate for temperature layers that could affect sonar accuracy. This was especially important when firing torpedoes at a target based strictly on a sonar fix.</blockquote>

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. Model Applications Maximum Depth Rated Ship Speed Vertical Resolution T 4 Standard probe used by the U. S. Navy for ASW operations 460 m 1500 ft 30 knots 65 cm T 5 Deep ocean scientific and military applications 1830 m 6000 ft 6 knots 65 cm Fast Deep Provides maximum depth capabilities at the highest possible ship speed of any XBT 1000 m3280 ft 20 knots 65 cm T 6 Oceanographic applications 460 m1500 ft 15 knots 65 cm T 7 Increased depth for improved sonar prediction in ASW and other military applications 760 m2500 ft 15 knots 65 cm Deep Blue Increased launch speed for oceanographic and naval applications 760 m2500 ft 20 knots 65 cm T 10 Commercial fisheries applications 200 m600 ft 10 knots 65 cm T 11 High resolution for U. S. Navy mine counter measures and physical oceanographic applications. 460 m1500 ft 6 knots 18 cm

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:

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.

Oceanography and hydrography: to obtain information on the temperature structure of the ocean. A study in 2019 (published 2023) at the outfall of the Totten Glacier in East Antarctica showed that water at depth above freezing temperature was melting the under side of the glacier. Submarine and Anti submarine warfare: to determine the layer depth (thermocline) used by submarines to avoid active sonar search.