SensorsKey DefinitionsKey IssuesProperties of the ocean we want to measureProperties of the ocean we want to measureMeasuring TemperatureMeasuring SalinityMeasuring Pressure/DepthCTD – Conductivity/Temperature/DepthCurrent VelocityCurrent Velocity – EulerianCurrent Velocity - ADCPCurrent Velocity – LagrangianChemical SensorsChemical SensorsBiological SensorsBiological Sensors – Imaging Sensors12.097 – Overview of basic sensorsSensors• Sensing vs. Sampling– Some samples are altered by changes in temperature and pressure– More efficient for exploration and mapping• What is a sensor?– A device that converts a physical property or a change in a physical property into a more easily manipulated form (e.g., voltage, displacement, resistance)12.097 – Overview of basic sensorsKey Definitions• Accuracy– difference between a measured value and the true value•Precision– difference between individual measurements of the same quantity• Error– Systematic• results from a basic fault in the measurement (affects accuracy)– Random• results from basic limitations in the method (affects precision)• Resolution– The smallest increment that can be measured12.097 – Overview of basic sensorsKey Issues• Response time• Calibration•Drift• Bio-fouling• Small form factor• Low power• Low cost• Ease of use12.097 – Overview of basic sensorsProperties of the ocean we want to measureimage credit: NASA image credit: WHOI12.097 – Overview of basic sensorsProperties of the ocean we want to measure•Temperature– measured in °C• Salinity– total concentration of dissolved salts– measured in PSU (practical salinity units)•Density– ρ = ρ(T,S,p)– measured in kg/m3•Depth– measured in m• Currents– direction the current is moving from 0-360°– speed measured in m/s• Chemical constituents– dissolved gases– nutrients– measured in ppm, ppt, mg/l, moles/kg• Biological organisms–biomass– numbers of organisms– Types of organisms12.097 – Overview of basic sensorsMeasuring Temperature• Bathythermograph (BT)• Expendable BT (XBT)• Mercury thermometer– bucket measurements– reversing thermometers• Platinum resistance thermometers• Thermistor• Thermocoupleimage credits: USCG12.097 – Overview of basic sensorsMeasuring Salinity• About 85% of total dissolved solids are NaCl• Silver nitrate (AgNO3) titration– ship- or shore-based– S = 0.03+1.804*chlorinity• Electrical conductivity– Inductive cell– Electrode cell– is temperature dependentAg+(aq) + Cl-(aq) ⌫ AgCl(s) (white)2Ag+(aq) + CrO42-⌫ Ag2CrO4 (s)(red-brown)S = 3.55 + 10.2·C - 0.73·Thttp://www.seabird.com/Image removed due to copyright considerations. Please see:12.097 – Overview of basic sensorsMeasuring Pressure/Depth• Hydrostatic pressure– 10 m of water = 1 atm• Absolute pressure• Gauge pressure– referenced to atmospheric pressure• Strain gauge– measuring the electrical resistance of a metal •Vibratron– measuring the natural frequency of a vibrating tungsten wire• Quartz crystal– measuring the natural frequency of a quartz crystalP =ρgdzP =ρ(z)gdzoh∫12.097 – Overview of basic sensorsCTD – Conductivity/Temperature/Depth• Primary tool for determining the physical properties of seawater• Water samples can collected at different depths with a rosette of Niskin bottlesimage credit: WHOIhttp://www.seabird.com/Image removed due to copyright considerations. Please see:12.097 – Overview of basic sensorsCurrent VelocityFranklin-Folger map of the Gulf StreamImage removed due to copyright considerations.12.097 – Overview of basic sensorsCurrent Velocity – Eulerian• Measurements made at a fixed point• Rotors/vanes• ADCP– Acoustic Doppler Current ProfilerPickard, George L., and W. J. Emery, eds. Descriptive Physical Oceanography: An Introduction. Woburn, MA: Butterworth-Heinemann, 1990. ISBN: 075062759X.Images removed due to copyright considerations. Please see:12.097 – Overview of basic sensorsCurrent Velocity - ADCP• Acoustic Doppler Current Profiler– Measures a current profile up to 1000 m long– Sends out “pings” at a certain velocity• Measures time of return and change in frequencyimage credit: WHOI12.097 – Overview of basic sensorsCurrent Velocity – Lagrangian• “Where does the water go?”• Follow the water parcel– Measure current and other water properties• Surface Drifters• Floats– PALACE–SOFAR–RAFOSimage credit: WHOI12.097 – Overview of basic sensorsChemical Sensors• Salinity– Electrode cell• Dissolved gases– Electrochemical reactions– Fluorescence quenching– Gas tension device• Gas permeable membrane• Measure pressure in cell• Slow response time• Subject to bio-fouling• Concentrations are often calculated from other measurements• Carbon in the ocean– Particulate organic carbon– Particulate inorganic carbon– Dissolved organic carbon– Dissolved inorganic carbon–CO2in the oceanDalton’s LawPT= ∑piHenry’s Lawci= si·piCO2(aq) + H2O ⌦ H2CO3H2CO3⌦ H++ HCO3-HCO3-⌦ H++ CO32-12.097 – Overview of basic sensorsChemical Sensors• Sensors now exist for– Nitrate– Nitrite– Total nitrogen– Total phosporous– Phosphate– Ammonia–FeII/FeIII• Most current in situ chemical sensors measure the dissolved state• Gases and solids are also a part of the chemistry of the ocean• New types of in situspectroscopic instrumentation are now being developed12.097 – Overview of basic sensorsBiological Sensors• Secchi disk– Measures attenuation and thus material in the water• Optical Backscatter Sensor (OBS)– Measures concentration of particles • Laser In Situ Scattering and Transmissometry (LISST)– Determines size distribution of particles• Fluorometer– Fluorescence can be used to determine amount of chlorophyll or distinguish chlorophyll from other material• Flow Cytometer– Uses fluorescence to look at one cell at a timehttp://home.hiroshima-u.ac.jpImage removed due to copyright considerations. Please see:12.097 – Overview of basic sensorsBiological Sensors – Imaging Sensors• Video Plankton Recorder (VPR)– Underwater video microscope• FlowCytobot– Measures light scattering and fluorescence• Environmental Sampling Processor (ESP)– Uses DNA probes and fluorescent tagsimage credit: