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SCM 330 Ocean Discovery through Technology Area F GE

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Sensors - Physical Physical Sensors: Temperature Salinity Pressure Acoustics Active Passive Radar Theory Application Sensor

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Temperature Thermistors: The term "Thermistor" is used to describe a range of electronic components whose principle characteristic is that their electrical resistance changes in response to changes in their temperature. The word "Thermistor" derives from the description "thermally sensitive resistor". Thermistors are further classified as "Positive Temperature Coefficient" devices (PTC devices) or "Negative Temperature Coefficient" devices (NTC devices). PTC devices are devices whose resistance increases as their temperature increases. NTC devices are devices whose resistance decreases as their temperature increases.

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Resistance: Thermistors are devices that obey Ohms Law, which relates the current through a resistor to the voltage across it. Ohms law is usually stated as: V=IR, where V is the voltage across the resistor in units of Volts, I is the current through the resistor in units of Amps, R is the resistance of the resistor in units of Ohms. The resistance of the thermistor depends on the temperature of the thermistor, and at temperature points in its useful range, the thermistor obeys Ohms Law.

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Volume resistivity formula: r = L x W x R25 (ohm-cm) T Where: r = volume resistivity (ohm-cm) L = length of chip element cm W = width of chip element cm T = thickness of chip element (cm) R25 = measured resistance @ 25 o C (ohms) For instance in the thermistor industry it is common to express resistivity in units of ohm-cm, but to give chip dimensions in inches. Example: Calculate the volume resistivity for Curve 3 Material with dimensions of 0.04" x 0.04" and thickness 0.01" with measured resistance value 8120 ohms at 25 o C. r = 0.04(inches) x2.54(cm/inch) x 0.04(inches)x2.54(cm/inch)x 8120 (ohms) 0.01(inches) x 2.54 (cm/inch) r = 3299.96 ohm-cm.

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Density Heat Budgets Circulation Physiological Tolerances Chemical Speciation Hydrothermal Vents

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Salinity CONDUCTIVITY THEORY All aqueous solutions conduct electricity to various degrees. Conductivity is the measurement of a solution's ability to conduct electric current. Adding electrolytes such as acids, bases or salts to water increases its conductance. The basic unit of conductance is the Siemens (S). Conductance is the reciprocal of resistance with its basic unit of ohms. In practice, the units of Siemens and ohms are too large to use in measuring solution conductivity, so the typical units used are microSiemens or milliSiemens and the reciprocal, Megohms. Conductivity is measured by an electronic meter or controller which applies an alternating voltage on the Conductivity Sensor and measures the resulting signal. The Conductivity Sensors consists of two or more electrodes of a certain area (A) separated by a predetermined distance (d). The sensor's cell constant (expressed in units of centimeters) is defined by: K = d / A The measured conductivity are multiplied by the cell constant to arrive at the specific conductivity (giving microSiemens per centimeter).

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Conductivity Sensors are available in a variety of cell constants, typically 0.01, 0.1, 1.0, and 10. Cell constants should be matched to the meter or controller for the expected range of operation. The different cell constants act like mechanical multipliers for the solution's resistance. By using different cell constants, the same meter or controller can operate over the same resistance range for both very low conductivity solutions (like ultra-pure water) and high conductivity solutions (like sea water). Review the following table for typical conductivity values for various solutions at 25 ° C: ApplicationsIndustriesTypical RangesCell Constant µ S1/cm Ultra Pure Watersemi-conductor0.2 to 10.01 Pure Waterpower, boiler water0.3 to 20000.1 Ground waterpower, environmental, laboratory2 to 20,0001.0 Sea water, brine, acids, basesAquariums, laboratory, chemical processing20 to 200,00010.0

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Solution conductivity is very dependent upon temperature and the solution constitutes. For example, ultra pure water's Resistivity varies by 5.5% / ° C near 25 ° C. Ionic salts have a temperature coefficient of 2% / ° C, while acids and bases typically vary 1.5% / ° C. The conductivity of solutions increases with temperature as shown by: CT = CTCAL (1 + µ (T - TCAL)) where µ is the temperature coefficient of the solution and TCAL is typically 25 ° C. Consequently, conductivity sensors are frequently purchased with temperature compensation, such as 10K or 30K ohm thermistors, 1K or 100 ohm RTD. Salt ConcentrationWeight per literConductivity Normalitymg/lµS/cm.001N KCl74.55146.93.01N KCl745.51408.8.1N KCl745512,856.034N NaCl20003860.026N NaCl15002930.017N NaCl10001990.008N NaCl5001020.003N NaCl200400

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S =0.0080 - 0.1692 R t 1/2 + 25.3851 R t + 14.0941 R t 3/2 -7.0261 R t 2 + 2.7081 R t 5/2 + ΔS R t =C (S,t,0) / C (KCl,t,0) ΔS =[(t - 15) / (1 + 0.0162(t - 15))] (0.0005 - 0.0056 R t 1/2 - 0.0066 R t - 0.0375 R t 3/2 + 0.636 R t 2 - 0.0144 R t 5/2 ) 2°C≤T≤35°C Calculation of Salinity from Conductivity

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(PSU) (ºC) (m) (cm) DecJanFebMarAprilMay

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Pressure Definition. Pressure is the force per unit area exerted by water (or air in the atmosphere) on either side of the unit area. Units. The units of force are (mass length / time^2) which you can remember from Newton's Law F = ma. The units of pressure are (force / length^2) or (mass /[length time^2]). 1 Pascal = 1 Newton/m^2. Atmospheric pressure is usually measured in bars. 1 bar = 10^5 Pascal. Ocean pressure is usually measured in decibars. 1 dbar = 10^4 Pascal.

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Basic Types of Transducers for Measuring Pressure A transducer is a device that converts energy from one form to another. Electrical pressure transducers, which measure changes in pressure, consist of a mechanical-transduction element or force-summing device coupled to an electrical-transduction element, which is connected to a display or recording device, or both. There are two types of electrical transduction elements —active and passive. Electrical-transduction elements that convert pressure-induced mechanical changes directly to an electrical signal are referred to as active transducers. Passive transducers require an external excitation that causes the transducers to respond to pressure-induced mechanical changes. The electrical-transduction element converts mechanical energy into electrical energy and the force-summing device or mechanical- transduction element converts gas or liquid energy into mechanical energy.

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force-summing devices

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Piezoelectric Transducer The piezoelectric transducer is an example of a self-generating or active pressure transducer. The design of this type of transducer is based on the ability of certain crystals (quartz, tourmaline, Rochelle salt, or ammonium dihydrogen phosphate) and ceramic materials (barium-titanate, or lead-zirconate-titanite) to generate an electrical charge or voltage when mechanically stressed. The crystal geometry of these materials is oriented to provide maximum piezoelectric response in one direction and minimal response in other directions. The transducer develops a voltage proportional to the change in pressure. These transducers cannot be calibrated using normal static- pressure calibration techniques. This type of transducer is used to measure rapidly fluctuating pressures.

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Strain-Gage Transducer The strain-gage transducer, sometimes referred to as a resistive transducer, is by far the most widely used type of pressure transducer. Its electrical transduction elements operate on the principle that the electrical resistance of a wire is proportional to its strain-induced length.

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Before computers were used, water level data was recorded on a continuously running pen and ink strip chart. These records were collected by observers once a month and mailed to headquarters for manual processing. In the 1960s, data were recorded onto mechanically punched paper tape that were read into a computer for processing. Water levels were recorded at 6-minute intervals. Observers maintained and adjusted the clocks, and calibrated the gauges with the tide readings. Tide stations were visited annually to maintain the tide houses and clean biological fouling from the underwater surfaces. During these annual visits, the components and support structures also were checked for stability.

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Microprocessor-based technologies allow for customized data collection and have improved measurement accuracy. While older tidal measuring stations used mechanical floats and recorders, a new generation of monitoring stations uses advanced acoustics and electronics. Today's recorders send an audio signal down a half-inch-wide sounding tube and measure the time it takes for the reflected signal to travel back from the water's surface. The sounding tube is mounted inside a 6- inch diameter protective well, which is similar to the old stilling well. In addition to measuring tidal heights more accurately, the new system also records 11 different oceanographic and meteorological parameters. These include wind speed and direction, water current speed and direction, air and water temperature, and barometric pressure.

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