Physical Oceanographic Sensors

Physical Oceanographic Sensors are used to measure the basic physical properties of seawater. The most common are conductivity, temperature, and pressure (depth). Together these three sensors form a CTD , a common oceanographic instrument (described under its own listing in this glossary).
Conductivity is the measurement of the water’s ability to conduct electric current. It is often used as a proxy for measuring salinity. The basic unit of conductance is Siemens (represented by an S), formally called the mho (omh spelled backwards). The principle by which instruments measure conductivity is simple. After an electrode consisting of two conductive plates is placed in the water, a sinewave voltage is applied across the plates and the current is measured. Electrodes are prone to corrosion , and the pumps that supply them with water can foul.
Electrodeless conductivity sensors use inductive coils. The inductive conductivity sensor consists of two high-grade toroids (coils) that are incorporated concentrically and adjacent to one another in a polymer or ceramic body. These coils form a current transformer. The sensor is designed so part of the liquid media forms a closed conductive current path passing through the toroids. The primary coil is activated with a sinusoidal alternating voltage, which induces an alternating voltage in the liquid loop (sample medium). In liquids that conduct electricity, this causes a current flow that is proportional to the conductivity of the sample medium. The liquid loop is also acting as the primary winding of the secondary coil, which functions as a current transformer. This current is rectified to the correct phase and amplified.
Temperature is typically measured by one of two types of devices:
Resistance temperature detectors (RTDs) are made of coils or films of metals (usually platinum). When heated, the resistance of the metal increases; when cooled, the resistance decreases. Passing current through an RTD generates a voltage across the RTD. By measuring this voltage, you determine its resistance, and thus its temperature. RTDs are popular because of their excellent stability, and because they exhibit the most linear signal with respect to temperature of any electronic temperature sensor. They are generally more expensive than alternatives, however, because of the careful construction and use of platinum. RTDs are also characterized by a slow response time and low sensitivity.
Thermistors (thermally sensitive resistors) are similar to RTDs in that they are electrical resistors whose resistance changes with temperature. Thermistors are manufactured from metal oxide semiconductor material which is encapsulated in a glass or epoxy bead. Thermistors have a very high sensitivity, making them extremely responsive to changes in temperature. Thermistors are generally less expensive and less accurate then RTDs.
There are many ways to sense pressure (depth). Most pressure sensors require the transduction of pressure information into a physical displacement. Measurement of pressure requires techniques for producing the displacement and means for converting such displacement into a proportional electrical signal.
One common element used to convert pressure information into a physical displacement is the diaphragm. A diaphragm is like a spring, and therefore extends or contracts until a force is developed that balances the pressure difference force. The bellows is another device much like the diaphragm that converts a pressure differential into a physical displacement, except that here the displacement is much more a straight-line expansion. Other pressure sensor transducers consist of a ceramic disk that changes its capacitance linearly with applied pressure. This variation is measured by an electronic circuit and is converted to a voltage output, to give an accurate pressure measurement reading. Integrated circuit manufacturers have developed composite pressure sensors that are particularly easy to use. These devices commonly employ a semiconductor diaphragm onto which a semiconductor strain gauge and temperature compensation sensor have been grown. Appropriate signal conditioning is included in integrated circuit form, providing a DC voltage or current linearly proportional to pressure over a specified range.
The most accurate measurements of pressure use a transducer made of a single quartz crystal whose frequency of oscillation varies with pressure-induced stress. Quartz crystals were chosen for the sensing elements because of their remarkable repeatability, low hysteresis and excellent stability. Accuracies of ±0.01% of full scale are readily achieved. When operating in deep water (i.e., >10,000 feet), a small difference in accuracy can equate to a large difference in depth measured.