Bathythermograph

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A Bathythermograph.

The bathythermograph, or BT, is a small torpedo-shaped device that holds a temperature sensor and a transducer to detect changes in hydrostatic pressure. Lowered into the water from an under way ship, the BT records pressure and temperature changes as it is dropped through the water. Because the pressure is a function of depth (see Pascal's law), it can be correlated with temperature.^{[citation needed]} The bathythermograph was first developed by Athelstan Spilhaus.

[edit] World War II use on U.S. submarines

Since water temperature may vary by layer and may affect sonar by producing inaccurate location results, bathothermographs (U.S. World War II spelling) were installed on the outer hulls of U.S. submarines during World War II.

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.

More importantly, when the submarine was under attack by a surface vessel using sonar, the information from the bathothermograph allowed the submarine commander to seek thermoclines, which are colder layers of water, that would distort the pinging from the surface vessel's sonar, allowing the submarine under attack to "disguise" its actual position and to escape depth charge damage and eventually to escape from the surface vessel.

Source:

Blair, Jr., Clay (2001). "(page 458)", Silent Victory, the U.S. Submarine war against Japan. Annapolis, Maryland: Naval Institute Press. ISBN 1-55750-217-X.