TI 101: MARINE INSTRUMENTATION
LECTURE 11-12: PHYSICAL OCEANOGRAPHY RESEARCH EQUIPMENT
Dr Y.W. Shaghude
Lecture Outline
❖Temperature
❖MBT
❖XBT
❖Corer & types of corers
❖Sieves
❖SCUBA
❖Research Submersibles/Underwater Vehicles
➢Remotely Operated Vehicles (ROV)
➢Autonomous Underwater Vehicles (AUV)
Measurements of Temperature
What is temperature?
❖Temperature is a physical quantity expressing
the degree or extent of hotness or coldness.
❖It is measured with a thermometer calibrated
in one or more temperature scales.
❖The most commonly used scales are the
Celsius scale (formerly called centigrade)
(denoted °C) and Kelvin scale (denoted K).
❖The kelvin (the word is spelled with a lowercase k) is the unit of temperature in the
International System of Units (SI). The Kelvin
scale is widely used in science and technology.
Temperature Scales
❖The Celsius scale (°C) is used for
common temperature measurements in
most of the world.
❖It is an empirical scale that was
developed by a historical progress,
which led to its zero point 0 °C being
defined by the freezing point of water,
and additional degrees defined so that
100 °C was the boiling point of water,
both at sea-level atmospheric pressure.
❖Because of the 100-degree interval, it
was called a centigrade scale.
Temperature Scale
❖Kelvin temperature scale is designed so that zero degrees K
is defined as absolute zero (at absolute zero, a hypothetical
temperature, all molecular movement stops - all actual
temperatures are above absolute zero) and the size of one
unit is the same as the size of one degree Celsius.
❖The Kelvin temperature scale is an absolute temperature
scale with zero at absolute zero.
❖ Because it is an absolute scale, measurements made using
the Kelvin scale do not have degrees. The kelvin (note the
lowercase letter) is the base unit of temperature in the
International System of Units (SI).
Instruments for Measurements of Temperature in the Ocean
Historically, two instruments had evolved:
❖Mechanical Bathythemograph (MBT)
❖Expendable Bathythermograph (XBT)
Mechanical Bathythermograph (MBT)
❖The Mechanical Bathythermograph, or MBT is a small
device that holds a temperature sensor and a transducer
that detect changes in water temperature versus depth
down to a depth of approximately 285 meters.
❖Lowered by a small winch on the ship into the water, the
BT records pressure and temperature changes on a
coated glass slide as it is dropped nearly freely through
the water.
❖ 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.
❖ Because the pressure is a function of depth (see
Pascal's law), temperature measurements can be
correlated with the depth at which they are recorded
Limitations of MBT
❖Mechanical bathythermograph (MBT) have several limitations:
❖They require slowing down of the ship for their operations.
❖Required trained personnel to operate the instruments.
❖Very sensitive to rough weather conditions.
❖Large time required from launching of the equipment to transmission and
reception of the data by the user is another major limitation.
❖Error likely to be induced by the readout equipment
❖Mechanically operated and therefore cannot be used at high depths
(maximum depth is about 280 m).
❖Since the arrival of the XBTs, the MBTs have been less common
Expendable Bathythermograph
❖What is an Expendable
Bathythermograph, or
“XBT”?
❖An Expendable
Bathythermograph (XBT) is
a probe used to measure
temperature throughout
the water column.
XBT
❖Uses a thermistor probe, as temperaturesensitive element.
❖The thermistor is in a small streamlined
weighted casing which is simply dropped over
the ship's side.
❖It is connected by a fine wire to a recorder on
the ship, which traces the temperature of the
water in a graphical plot against depth.
❖The latter is not sensed directly but is
estimated from the time elapsed since release,
using the known rate of sink of the freely falling
thermistor casing. These XBTs are available for
depth ranges from 200m to 1800m.
How Does an XBT Work?
❖An XBT is a small probe that is dropped
over the side of a ship.
❖As it falls through the water, it measures
temperature.
❖Small wires transmit the temperate data
back to the ship where it is recorded for
further analysis.
❖Because the probe falls through the water at
a known rate, the depth of the probe can be
inferred from the time of launch.
❖Scientists then plot temperature as a
function of depth to create a temperature
profile of the water.
CTD
• CTD is an acronym for Conductivity Temperature Depth
(actually pressure).
• T is measured using a thermistor mounted close to the
conductivity sensor.
CTD
❖CTD is a primary tool for determining
physical properties of sea water for
comprehensive charting of the
vertical distribution of temperature,
salinity and density that helps to
understand how the oceans affect
life.
❖A CTD Can measure temperature to
1/1000 of a degree, and salinity to
1/1000 parts per thousand.
CTD
❖A CTD is Made up of a set of small probes
attached to a metal body, and measurements
can be done at up to 30 readings per second.
❖It is lowered on a cable down to the seafloor
collecting data on the way.
❖A CTD cast, depending on water depth,
requires 30 minutes to 2 hours in one station.
❖Apart from the temperature and salinity
measurements, water sampling is also often
collected done at specific depths, allowing the
scientists to learn the physical properties of
the water column at that particular place and
time.
ROSETTE SAMPLER
❖Rosette sampler are sets of water sampling bottles that
are attached to a CTD.
❖Each bottle of a rosette sampler has a volume of about
5 – 20 litres.
❖The samples can be triggered from the vessel at
preselected depths.
How Does the Rosette Sampler Work?
❖The rosette is slowly lowered by a winch
to the seabed.
❖Scientists observes the water properties
in real time via a conducting cable
connecting the CTD to a computer.
❖A remotely operated device allows the
water bottles to selectively collect water
samples at different depths as the
instrument ascend to the sea floor.
❖The water samples collected may latter
be analyzed for biological and chemical
properties (Ch-a, nutrients, dissolved
oxygen, etc.
CTD Vertical Profiles
CURRENT METERS
❖An important concern in Physical oceanography is “
How fast does the water flow?” and “Where is the
water flowing to?”
❖Current meters are oceanographic equipment
designed to address these fundamental questions.
❖Current meters are used to measure the water
velocities (speed and direction).
❖Current meters use the Eulerian approach to examine
the motion of fluid parcels.
❖Eulerian approach: One describe the speed and
direction of fluid parcels as they pass through a fixed
point
CURRENT METERS
❖Most current meters have electromagnetic sensors
which uses Faraday Principle to measure water
speeds and direction.
❖The sensor generate an Electromagnetic Field
❖Water being a conductor, when it moves past the
electromagnetic field induces voltage and the
amount of voltage induced is proportional to the
water speeds.
❖The induced voltage is therefore ultimately
converted to water speed.
Types of Current meters
❖ Current sensors are normally divided into 2
groups,
1. Single-point Current Meters and
2. Current Profilers.
Acoustic Doppler Current Profiler (ADCP)
❖The ADCP measures water currents, using a principle of
sound waves known as the Doppler effect.
➢The ADCP transmits "pings" of sound at a constant frequency
into the water.
➢As the sound waves travel, they encounter particles
suspended in the moving water, and are reflected back to the
instrument.
➢Due to the Doppler effect, sound waves bounced back from a
particle moving away from the profiler have a slightly lowered
frequency when they return.
➢The difference in frequency between the waves the profiler
sends out and the echoes it receives is called the Doppler
shift.
➢The instrument uses this shift to calculate how fast the
particle and the water around it are moving.
Drifting Buoys
• Float in water to measure current
speed and direction using
Lagrangian principle, that is
observing a water parcel as it
moves in space through time.
• With this approach, one follows
the drifting buoy with a GPS and
takes the position of the drifting
buoy with time.
• Alternatively one can use a
satellite tracked drifter
Important Considerations
❖When planning field operations with
current meter device (whether single
point current meter or current profiler
to the field think about the following:
➢Do you have a stable platform to keep
the device at the bottom of the sea?
➢Do you have divers for deploying the
device and retrieving the device
to/from the sea bed?
➢How long will the device stay in the
sea?
➢What about the security of the device
(How can you get rid of vandalism?)?
➢Are there any physical markers for easy
location of the device
END