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

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