mechanical bathythermograph

This was a widely used device for continuous temperature profiling. It can be used from a vessel steaming at up to 12 knots. The BT is lowered on a cable, and as it falls through the water, the xylene in the thin copper tube wound around the tail expands or contracts with the change in temperature causing the Bourdon tube to change shape, so moving a stylus across a plated glass slide inserted in the holder. A bellows attached to the holder contracts under increasing pressure drawing the slide at right angles to the stylus motion. So a profile of temperature versus pressure is scratched on the glass slide.

expendable bathythermograph

The first step towards modern electronic profiling instruments was a development of the BT, the so-called Expendable BathyThermograph or XBT. This instrument uses a thermistor to sense temperature while its depth is known from its well behaved fall characteristics through the water. The thermistor in the nose of the probe is connected electrically to the ship by means of two thin wires coated with four epoxy and nylon layers of insulation fused together into a single filament. The filament link is .00016 cm in diameter and is wound onto two spools, one in the XBT launcher and the other on the probe. As the XBT falls through the ocean, wire is unwound from both reels simultaneously. This means that the XBT can be used from high speed boats and even aeroplanes since the fall speed of the probe is
completely independent of the motion of the launch platform. XBT probes are available for depths of 200 m to 1830 m. Accuracy of the probes is 0.1 C and 2% of depth. The XBT is invaluable for ocean monitoring because any vessel, merchant or whatever, can make frequent continuous profiles without needing to slow down at all for the cost of only the relatively cheap probes. Extensive data data sets of upper ocean variability have been assembled in this way along the major shipping routes. The recent availability of cheap microcomputers has meant that the data can be stored digitally for subsequent transfer direct to main frame machines, instead of having to go through the labour intensive and inaccurate reading of analogue records.

platinum resistance thermometer

The most widely used, accurate and reliable method for determining sea water temperature at all depths is used in the sophisticated package known as a CTD, which also provides precise pressure and salinity values in situ. As metals heat and cool their electrical resistance changes. Platinum can be purified to a very high degree so providing an extremely stable and sensitive thermometer. A practical PRT consists of a coil of platinum wire wound around an inert former or support inside a protective sheath of stainless steel. Its size is only a few centimetres so it responds quickly to small temperature changes.

source: www.sos.bangor.ac.uk (University of Wales, Bangor, Ocean Sciences)