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

Sea Level Measurement and InterpretationAcoustic Gauges without Sounding Tube inside Protective Tube or WellEquipment A ready-to-go package can be purchased from SRD that includes the sensor, the data logger and configuration unit, andthe communications system.Operation The device measures the time of flight of an acoustic pulse from a transducer to the sea surface. A bar is fixed at a knowndistance from the transducer, which is used for self-calibration and computation of the velocity of sound before each measurement.The time is converted to a sea level height using the value of the velocity of sound in air previously computedby means of the fixed bar.Installation This type of acoustic sensor has proved to be accurate enough if placed over an existing well, or inside a protective PVCRequirements tube of 300 mm diameter. The transducer must be located at a minimum distance of around 2–3 metres from the watersurface at any moment. As with many tide gauges, all ancillary equipment (data logger, modem, battery backup), needsto be housed in an adjacent building.Location Requires a sea wall or vertical structure for installation.Calibration Calibration of the reference is performed during manufacture, prior to delivery.The calibration of the velocity of sound is made by means of the reflective bar at a known position in the acoustic beam.Accuracy 1 cm of sea level.Cost Band 2.Record of Use They have been used successfully in the REDMAR network, the Spanish Harbour Authority’s sea level network, for nearly16 years. The long-term means seem to be as accurate or better than the standard float gauges operating in Spain.CommentsIn areas of large tidal range a long protective tube is needed which may result in magnified temperature and/or temperature-gradienteffects. Very sensitive to the careful design of the installation.Single Transducer Pressure GaugesEquipment Complete ready-to-go package (sub-sea pressure sensor, cabling and data logger) can be purchased from several manufacturers.Operation Two different options are available: (a) an absolute pressure sensor measuring the total pressure due to sea level and atmosphere;(b) a differential sensor which has a vented cable measuring pressure changes due to sea level alone. Conversionof pressure to sea level height requires knowing seawater density. Generally, an average value can be used unless thereare significant seasonal or tidal variations. Pressure sensors are also temperature sensitive, but, since sea temperature variesmuch less than atmospheric temperature, this normally has a small effect.Sensors vary in cost by up to a factor of 20. Relatively inexpensive sensors use strain gauge technology. Top-of-the-rangesensors are constructed using quartz crystals. For the latter, the temperature sensitivity of low-pressure sensors is around1 mm/°C. Instrumental drift of the same sensor is about 1 mm per year.Many pressure sensors produce a frequency-modulated output. This can be counted (integrated) by relatively simple electronicsto produce the required measurements. Resolution therefore depends on the integration period, which is typically15 or 6 minutes, but can be as short as 1 minute and still provide sufficient accuracy. Some manufacturers provide equipmentthat does not integrate over the full sampling interval, in order to conserve battery power.Installation These devices can be used virtually anywhere, even on shelving beaches. They are normally mounted in an outer protectivetube fastened to a sea wall but can be fixed directly on the sea bed and connected to the shore by armoured cable.RequirementsPressure sensors require very little power and can be run for periods of 1–2 years on non-rechargeable batteries.Location Pressure sensors can be used at virtually any site, even in hostile environments, such as the polar regions. Regions with largevariations in seawater density may cause significant errors.Calibration Calibrations traceable to National Physical Laboratory (UK) standards can be obtained from pressure sensor manufacturersand have been shown to remain stable over many years. However, drift in the datum value of a sensor may cause changesto its ‘zero’ value. Re-calibration at intervals may be necessary. Alternatively, the difficulty of establishing a datum can berectified by using alternative means (e.g. from annual tide pole measurements). These have proved adequate, since thedrift is normally linear with time.(Continued on next page)24IOC <strong>Manual</strong>s and Guides No 14 vol IV