Light and Temperature
23
Reversing Thermometers. Among the most accurate devices for measuring subsurface temperatures is the reversing thermometer. After this instrument is lowered to
depth and has equilibrated to temperature, a release mechanism is tripped by a
messenger (metal weight) sent down the line from the surface. Upon release, the
thermometer pivots vertically 180°. The thermometer is constructed in such a way that
the stem is coiled and constricted. When the position of the thermometer is reversed
(inverted), the mercury column separates at the point of constriction. As a result, the
position of the detached mercury column will not change if subjected to different
temperatures. With corrections for slight thermal expansions of mercury in glass, most
calibrated reversing thermometers are accurate to at least 0.01 °C. These thermometers
are expensive and fragile, requiring extreme care during their use. A full description of
their characteristics and use is given in Welch (1948).
Thermistor Electrical Thermometers. The electrical resistance of many substances
changes as temperature changes. Early electrical thermometers utilized this relationship to measure temperature. In combination with a simple Wheatstone-Bridge, the
change in resistance (or current) of a conductor, usually ceramic, was measured and
calibrated in units of temperature. More recent instruments are all of the thermistor
type in which, by similar principle, resistance decreases with increasing temperature.
Thermistor response is rapid, requiring only a few seconds to come into equilibrium,
and is quite accurate within the range of temperatures encountered in fresh waters. All
such instruments must be calibrated against reference thermometers periodically.
Bathythermograph. The bathythermograph is an instrument for measuring and
recording continuous vertical profiles of temperature with depth. The torpedo-shaped
instrument consists of a heavy nose-piece to which a cable shackle and swivel are
attached, a body tube housing pressure and thermal elements, and a protecting tail cage
housing the capillary tubing of the thermal element.
The thermal element of traditional, older bathythermographs is a temperaturecompensated Bourdon tube, actuated by a xylene-filled capillary about IS-m long, that
is exposed to uniform flow of water over its surface. A pen and stylus are attached to the
Bourdon tube. The pressure element consists of an evacuated bellows, movable at one
end and fixed at the other, which compresses with increasing water pressure. An
accurately wound calibration spring resists this compressing action and is attached to a
slide holder at the end of the bellows.
In operation, a glass slide (smoked or coated with an extremely thin layer of a gold
compound) is placed into the slide holder on the free end of the bellows assembly, and
the stylus is positioned to make contact with the slide. As the bathythermograph is
lowered in the water, the slide moves vertically as increasing water pressure compresses
the bellows assembly. The stylus is fixed in the vertical direction. Horizontal movement
of the stylus with respect to the slide, which is fixed in position in this direction, is
produced by movement of the Bourdon tube caused by temperature change. The
resulting trace is a composite of depth on the vertical ordinate and temperature on the
horizontal ordinate. The accuracy ofthe temperature and depth values is about ± 1°C
and ± 1 % of instrument depth range. New oceanographic bathythermographs are
"disposable" in that data are transmitted electronically to the ship as the instrument
descends through the water column to the sediments.
23
Reversing Thermometers. Among the most accurate devices for measuring subsurface temperatures is the reversing thermometer. After this instrument is lowered to
depth and has equilibrated to temperature, a release mechanism is tripped by a
messenger (metal weight) sent down the line from the surface. Upon release, the
thermometer pivots vertically 180°. The thermometer is constructed in such a way that
the stem is coiled and constricted. When the position of the thermometer is reversed
(inverted), the mercury column separates at the point of constriction. As a result, the
position of the detached mercury column will not change if subjected to different
temperatures. With corrections for slight thermal expansions of mercury in glass, most
calibrated reversing thermometers are accurate to at least 0.01 °C. These thermometers
are expensive and fragile, requiring extreme care during their use. A full description of
their characteristics and use is given in Welch (1948).
Thermistor Electrical Thermometers. The electrical resistance of many substances
changes as temperature changes. Early electrical thermometers utilized this relationship to measure temperature. In combination with a simple Wheatstone-Bridge, the
change in resistance (or current) of a conductor, usually ceramic, was measured and
calibrated in units of temperature. More recent instruments are all of the thermistor
type in which, by similar principle, resistance decreases with increasing temperature.
Thermistor response is rapid, requiring only a few seconds to come into equilibrium,
and is quite accurate within the range of temperatures encountered in fresh waters. All
such instruments must be calibrated against reference thermometers periodically.
Bathythermograph. The bathythermograph is an instrument for measuring and
recording continuous vertical profiles of temperature with depth. The torpedo-shaped
instrument consists of a heavy nose-piece to which a cable shackle and swivel are
attached, a body tube housing pressure and thermal elements, and a protecting tail cage
housing the capillary tubing of the thermal element.
The thermal element of traditional, older bathythermographs is a temperaturecompensated Bourdon tube, actuated by a xylene-filled capillary about IS-m long, that
is exposed to uniform flow of water over its surface. A pen and stylus are attached to the
Bourdon tube. The pressure element consists of an evacuated bellows, movable at one
end and fixed at the other, which compresses with increasing water pressure. An
accurately wound calibration spring resists this compressing action and is attached to a
slide holder at the end of the bellows.
In operation, a glass slide (smoked or coated with an extremely thin layer of a gold
compound) is placed into the slide holder on the free end of the bellows assembly, and
the stylus is positioned to make contact with the slide. As the bathythermograph is
lowered in the water, the slide moves vertically as increasing water pressure compresses
the bellows assembly. The stylus is fixed in the vertical direction. Horizontal movement
of the stylus with respect to the slide, which is fixed in position in this direction, is
produced by movement of the Bourdon tube caused by temperature change. The
resulting trace is a composite of depth on the vertical ordinate and temperature on the
horizontal ordinate. The accuracy ofthe temperature and depth values is about ± 1°C
and ± 1 % of instrument depth range. New oceanographic bathythermographs are
"disposable" in that data are transmitted electronically to the ship as the instrument
descends through the water column to the sediments.