Manual on sea level measurement and ... - unesdoc - Unesco

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Sea Level Measurement and Interpretationinstrument connected into this supply tube at thelandward end records the changes in water level aschanging pressures, according to the law:h=(p-p a )/(ρg)where hpp aρg= height of sea level above the bleed hole= measured pressure= atmospheric pressure= seawater density= gravitational accelerationMost pneumatic instruments use a pressure sensoras part of the recording equipment to monitor thechanges in pressure and hence sea level. It is commonto use a sensor operating in the differential mode, sensorsbeing so constructed that the system pressure isopposed by atmospheric pressure. Hence, the resultantpressure experienced by the sensor becomes (p–p a ),making the measured pressure directly proportional tothe required sea level.A knowledge of the seawater density (ρ) is important.This is normally obtained from separate watersampling, and, where the water is well mixed, can beconsidered constant. In estuarine locations, the densitymay change during a tidal cycle or seasonally, anddensity corrections will have to be included in the dataprocessing.Several other effects on the measured pressure haveto be considered. These include a ‘static’ effect, whichis a function of the height of the gauge above sealevel, and a ‘dynamic’ effect, which results from thedynamics of gas flow. The latter can be calculated interms of tube length and radius and the minimumair-flow consistent with preventing water from enteringthe system (Pugh, 1972). The effect of waves onthe system is to introduce a positive bias during stormconditions (i.e. sea level is measured too high). Theseeffects can perturb the sea level measurements at thesub-centimetre level during average conditions, butmeasurements may be incorrect by several centimetresunder extreme waves.In common with all pressure measuring systems, thereis a need to establish a datum for the observed timeseries. This can be achieved in several ways: (a) froma knowledge of the exact depth of the pressure pointbleed hole during installation, combined with accuratecalibration of the pressure transducer; (b) using datumlevel switches similar to those described above forstilling wells which trigger at a known sea level; (c) byhaving a parallel system (called a ‘B’ gauge; section3.3.4) with a second and more accessible pressurepoint fixed near mean sea level. Comparison of the differencesbetween the two bubbling systems when bothare submerged gives an accurate measure of the datum;method ‘c’ is the most accurate.Air is normally supplied to a bubbler from a compressorto afford continuous operation of the installation. In theevent of electrical supply failure, a reserve air capacitycapable of sustaining the system for at least several daysis necessary. For sustained operation under fault conditions,an alternative low power backup system in theform of a pressure transducer mounted directly in thesea is necessary. Transducers, compressors, data loggersetc. can be purchased from the major gauge manufacturerswith ready-to-go packages. An all-bubbler systemhas an advantage that most components are underwater,and that all components are both robust and, ifdamaged, relatively inexpensive to replace.3.3.2 Pressure Sensor GaugesPressure sensors can be fixed directly in the sea tomonitor sub-surface pressure in a similar fashion to thebubbler gauge. The sensor is connected by a cable thatcarries power and signal lines to an onshore controland logging unit. In the sea, the active sensor is usuallycontained within a copper or titanium housing withthe cable entering through a watertight gland. Materialused for the housing is chosen to limit marine growth.The assembly is contained in an outer protective tubeto provide a stable fixation to a sea wall or rock outcrop.Where this is not possible, the pressure sensormay be placed securely on the sea bed, but this methodhas disadvantages, as deployment and maintenanceusually require a diving team.Pressure-based instruments can be operated from batteriesfor periods of a year or more, as they consumea very small amount of power. This can be advantageouseven where electrical supplies are available butsubject to long periods of failure. Therefore, they havebeen used extensively in remote areas, such as oceanicislands, where access is limited. In polar regions, theyoffer the best alternative if the area is ice covered or ifthe gauge is to be left unattended for long periods. Themain disadvantage is the lack of a fixed datum level,which has to be found by alternative means.Pressure sensors are available in two varieties thatprovide either an absolute or differential signal. If anabsolute transducer is employed, the sensor provides ameasurement of the total pressure including sea leveland atmosphere. Therefore, a separate barometer isrequired usually in the form of an identical transduceropen to the atmosphere. Both sensors are synchronizedto the same clock so they can readily be subtracted toyield sea level (with subsequent correction for densityand acceleration due to gravity). Differential pressuretransducers have a vented cable in which the referenceside of the transducer is open to the atmosphere.Vented systems are known to suffer from occasionalblockage and are used less frequently in hazardousenvironments. In addition, a record of barometric pressureis valuable for oceanographic studies, so the twotransduceroption is most frequently employed.14IOC <strong>Manual</strong>s and Guides No 14 vol IV
Sea Level Measurement and InterpretationRelatively inexpensive pressure sensors use strain gaugeor ceramic technology in which changes in water pressurecause changes in resistance or capacitance in thepressure element. The most accurate, but expensive,sensors use a quartz element, the resonant frequencyof which varies with the strain applied to it. The resultingsignal, which is normally a frequency proportionalto the applied pressure, is carried down the signalcable to the control electronics where it is convertedinto physical units and can be displayed and stored bya data logger.All pressure transducers are sensitive to temperature. Somehave an in-built temperature sensor to allow compensationof the pressure signal. If this is not the case, then itis important that temperature is monitored independentlyand used as a correction. In general, sea temperaturevaries much less than atmospheric temperature and compensationby either of the above methods is effective.Users with access to a test facility can also subject theinstruments to a range of temperatures and pressures toensure that calibration values are correct. Experience hasshown that the calibration coefficients supplied by leadingmanufacturers are accurate and constant over periods ofseveral years. Drift in the various properties of pressuresensors is confined to changes in its datum value (i.e.there is usually no change in scale). However, even for ahigh-quality low-pressure sensor suitable for coastal work,instrumental drift can be an important issue (of the orderof 1 mm per year) which has to be addressed throughregular checks of some kind.Single transducer systems can be deployed in environmentallyhostile areas where other forms of gauge willnot work. For example, they can be safely positionedon the sea bed under the winter ice at polar sites withthe signal cable to the tide gauge hut on the shoreprotected by a steel pipe. They can be operated atsites with harsh weather conditions where the exposedstructures of a stilling well or acoustic gauge maybe subject to extreme forces of winds and waves. Intropical locations, where equipment may be prone tomechanical damage by falling trees etc., single transducersystems can be deployed safely below the seasurface. Even in locations with excessive marine growthor silt deposits, pressure systems appear to work correctlyfor long periods of time.Pressure sensors have a fast response time and havebeen used to measure wave heights at periods of a fewseconds. In tide gauge applications, the signal is usuallyaveraged by the control electronics to a more relevantperiod, such as 1, 6 or 15 minutes. This method ofaveraging allows a great deal of flexibility, since thesampling period can be easily altered to suit the application.Changes can be made remotely if an installationis connected by a telephone link or to a two-way communicationnetwork.abFigure 3.3 Pressure gauge.(a) The pressure sensor is mounted directly in the sea.(b) In this case, it is fastened to a pier in Port Stanleyharbour.As with the bubbler gauge, seawater density is neededto convert measured pressures into heights. The commentsmade in section 3.3.1 are equally valid.3.3.3 The Datum of a Pressure SystemThe major problem with a single pressure transduceris establishing a datum for its measurements. A goodapproximation can be obtained with differential transducersby careful calibration within a test facility. Itis less accurate with absolute sensors because atmosphericpressure introduces an offset that may preventa sufficiently low pressure being reached during thecalibration. In general, other means of fixing the datumare preferred.A method frequently adopted is to make visual measurementsagainst a tide staff over a period of one dayand repeat this at regular intervals. Individual measurementsshould be accurate to 2–3 cm and on averageIOC <strong>Manual</strong>s and Guides No 14 vol IV15
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