Bathythermograph in a fitted box. A bathythermograph (BT) is a device for obtaining, from a ship under way, a record of temperature against depth and pressure in the upper 300 m of the ocean. For a thermal element it has a xylene-filled copper coil, which actuates a stylus through a Bourdon tube. The pressure element is a copper aneroid capsule that moves a smoked glass slide at right angles to the motion of the stylus. A double analog record is thus obtained as the BT is lowered and recovered. This device has generally been replaced by the expendable bathythermograph (XBT).